Listview 原理解析

上周四去分享销客面试，整个过程面试官就问了一个问题：listview；其中涉及获得 listview 中的父控件中的子控件、listview 的优化(卡顿、层级)、listview 的性能提升以及查看 listview 性能的工具；坦白来讲，对于最后一个问题，我表示面试官很坑啊，listview 的监测工具 ??? 大写的什么鬼？ 你怎么不直接问我检测内存的工具呢？我表示很懵逼啊；我表示不服，必须重新撸一遍 listview 的源码，以解我心头之恨。

Adapter 的作用

我们都知道 listview 中是没有数据的，所有的数据都是通过 adapter 这个接口来获得的，它可以实现各种各样的子类，每个子类都会通过自己的逻辑来适配不同的数据源；

RecycleBin 机制

它是 AbsListView 的内部类，ListView 和 GridView 都继承自 AbsListView类；RecycleBin 保证了 listview 加载上千条 item 都不会出现 OOM 的情况 ;源码如下：

/**
     * The RecycleBin facilitates reuse of views across layouts. The RecycleBin has two levels of
     * storage: ActiveViews and ScrapViews. ActiveViews are those views which were onscreen at the
     * start of a layout. By construction, they are displaying current information. At the end of
     * layout, all views in ActiveViews are demoted to ScrapViews. ScrapViews are old views that
     * could potentially be used by the adapter to avoid allocating views unnecessarily.
     *
     * @see android.widget.AbsListView#setRecyclerListener(android.widget.AbsListView.RecyclerListener)
     * @see android.widget.AbsListView.RecyclerListener
     */
    class RecycleBin {
        private RecyclerListener mRecyclerListener;

        /**
         * The position of the first view stored in mActiveViews.
         */
        private int mFirstActivePosition;

        /**
         * Views that were on screen at the start of layout. This array is populated at the start of
         * layout, and at the end of layout all view in mActiveViews are moved to mScrapViews.
         * Views in mActiveViews represent a contiguous range of Views, with position of the first
         * view store in mFirstActivePosition.
         */
        private View[] mActiveViews = new View[0];

        /**
         * Unsorted views that can be used by the adapter as a convert view.
         */
        private ArrayList<View>[] mScrapViews;

        private int mViewTypeCount;

        private ArrayList<View> mCurrentScrap;

        private ArrayList<View> mSkippedScrap;

        private SparseArray<View> mTransientStateViews;
        private LongSparseArray<View> mTransientStateViewsById;

        public void setViewTypeCount(int viewTypeCount) {
            if (viewTypeCount < 1) {
                throw new IllegalArgumentException("Can't have a viewTypeCount < 1");
            }
            //noinspection unchecked
            ArrayList<View>[] scrapViews = new ArrayList[viewTypeCount];
            for (int i = 0; i < viewTypeCount; i++) {
                scrapViews[i] = new ArrayList<View>();
            }
            mViewTypeCount = viewTypeCount;
            mCurrentScrap = scrapViews[0];
            mScrapViews = scrapViews;
        }

        public void markChildrenDirty() {
            if (mViewTypeCount == 1) {
                final ArrayList<View> scrap = mCurrentScrap;
                final int scrapCount = scrap.size();
                for (int i = 0; i < scrapCount; i++) {
                    scrap.get(i).forceLayout();
                }
            } else {
                final int typeCount = mViewTypeCount;
                for (int i = 0; i < typeCount; i++) {
                    final ArrayList<View> scrap = mScrapViews[i];
                    final int scrapCount = scrap.size();
                    for (int j = 0; j < scrapCount; j++) {
                        scrap.get(j).forceLayout();
                    }
                }
            }
            if (mTransientStateViews != null) {
                final int count = mTransientStateViews.size();
                for (int i = 0; i < count; i++) {
                    mTransientStateViews.valueAt(i).forceLayout();
                }
            }
            if (mTransientStateViewsById != null) {
                final int count = mTransientStateViewsById.size();
                for (int i = 0; i < count; i++) {
                    mTransientStateViewsById.valueAt(i).forceLayout();
                }
            }
        }

        public boolean shouldRecycleViewType(int viewType) {
            return viewType >= 0;
        }

        /**
         * Clears the scrap heap.
         */
        void clear() {
            if (mViewTypeCount == 1) {
                final ArrayList<View> scrap = mCurrentScrap;
                clearScrap(scrap);
            } else {
                final int typeCount = mViewTypeCount;
                for (int i = 0; i < typeCount; i++) {
                    final ArrayList<View> scrap = mScrapViews[i];
                    clearScrap(scrap);
                }
            }

            clearTransientStateViews();
        }

        /**
         * Fill ActiveViews with all of the children of the AbsListView.
         *
         * @param childCount The minimum number of views mActiveViews should hold
         * @param firstActivePosition The position of the first view that will be stored in
         *        mActiveViews
         */
        void fillActiveViews(int childCount, int firstActivePosition) {
            if (mActiveViews.length < childCount) {
                mActiveViews = new View[childCount];
            }
            mFirstActivePosition = firstActivePosition;

            //noinspection MismatchedReadAndWriteOfArray
            final View[] activeViews = mActiveViews;
            for (int i = 0; i < childCount; i++) {
                View child = getChildAt(i);
                AbsListView.LayoutParams lp = (AbsListView.LayoutParams) child.getLayoutParams();
                // Don't put header or footer views into the scrap heap
                if (lp != null && lp.viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
                    // Note:  We do place AdapterView.ITEM_VIEW_TYPE_IGNORE in active views.
                    //        However, we will NOT place them into scrap views.
                    activeViews[i] = child;
                    // Remember the position so that setupChild() doesn't reset state.
                    lp.scrappedFromPosition = firstActivePosition + i;
                }
            }
        }

        /**
         * Get the view corresponding to the specified position. The view will be removed from
         * mActiveViews if it is found.
         *
         * @param position The position to look up in mActiveViews
         * @return The view if it is found, null otherwise
         */
        View getActiveView(int position) {
            int index = position - mFirstActivePosition;
            final View[] activeViews = mActiveViews;
            if (index >=0 && index < activeViews.length) {
                final View match = activeViews[index];
                activeViews[index] = null;
                return match;
            }
            return null;
        }

        View getTransientStateView(int position) {
            if (mAdapter != null && mAdapterHasStableIds && mTransientStateViewsById != null) {
                long id = mAdapter.getItemId(position);
                View result = mTransientStateViewsById.get(id);
                mTransientStateViewsById.remove(id);
                return result;
            }
            if (mTransientStateViews != null) {
                final int index = mTransientStateViews.indexOfKey(position);
                if (index >= 0) {
                    View result = mTransientStateViews.valueAt(index);
                    mTransientStateViews.removeAt(index);
                    return result;
                }
            }
            return null;
        }

        /**
         * Dumps and fully detaches any currently saved views with transient
         * state.
         */
        void clearTransientStateViews() {
            final SparseArray<View> viewsByPos = mTransientStateViews;
            if (viewsByPos != null) {
                final int N = viewsByPos.size();
                for (int i = 0; i < N; i++) {
                    removeDetachedView(viewsByPos.valueAt(i), false);
                }
                viewsByPos.clear();
            }

            final LongSparseArray<View> viewsById = mTransientStateViewsById;
            if (viewsById != null) {
                final int N = viewsById.size();
                for (int i = 0; i < N; i++) {
                    removeDetachedView(viewsById.valueAt(i), false);
                }
                viewsById.clear();
            }
        }

        /**
         * @return A view from the ScrapViews collection. These are unordered.
         */
        View getScrapView(int position) {
            final int whichScrap = mAdapter.getItemViewType(position);
            if (whichScrap < 0) {
                return null;
            }
            if (mViewTypeCount == 1) {
                return retrieveFromScrap(mCurrentScrap, position);
            } else if (whichScrap < mScrapViews.length) {
                return retrieveFromScrap(mScrapViews[whichScrap], position);
            }
            return null;
        }

        /**
         * Puts a view into the list of scrap views.
         * <p>
         * If the list data hasn't changed or the adapter has stable IDs, views
         * with transient state will be preserved for later retrieval.
         *
         * @param scrap The view to add
         * @param position The view's position within its parent
         */
        void addScrapView(View scrap, int position) {
            final AbsListView.LayoutParams lp = (AbsListView.LayoutParams) scrap.getLayoutParams();
            if (lp == null) {
                // Can't recycle, but we don't know anything about the view.
                // Ignore it completely.
                return;
            }

            lp.scrappedFromPosition = position;

            // Remove but don't scrap header or footer views, or views that
            // should otherwise not be recycled.
            final int viewType = lp.viewType;
            if (!shouldRecycleViewType(viewType)) {
                // Can't recycle. If it's not a header or footer, which have
                // special handling and should be ignored, then skip the scrap
                // heap and we'll fully detach the view later.
                if (viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
                    getSkippedScrap().add(scrap);
                }
                return;
            }

            scrap.dispatchStartTemporaryDetach();

            // The the accessibility state of the view may change while temporary
            // detached and we do not allow detached views to fire accessibility
            // events. So we are announcing that the subtree changed giving a chance
            // to clients holding on to a view in this subtree to refresh it.
            notifyViewAccessibilityStateChangedIfNeeded(
                    AccessibilityEvent.CONTENT_CHANGE_TYPE_SUBTREE);

            // Don't scrap views that have transient state.
            final boolean scrapHasTransientState = scrap.hasTransientState();
            if (scrapHasTransientState) {
                if (mAdapter != null && mAdapterHasStableIds) {
                    // If the adapter has stable IDs, we can reuse the view for
                    // the same data.
                    if (mTransientStateViewsById == null) {
                        mTransientStateViewsById = new LongSparseArray<>();
                    }
                    mTransientStateViewsById.put(lp.itemId, scrap);
                } else if (!mDataChanged) {
                    // If the data hasn't changed, we can reuse the views at
                    // their old positions.
                    if (mTransientStateViews == null) {
                        mTransientStateViews = new SparseArray<>();
                    }
                    mTransientStateViews.put(position, scrap);
                } else {
                    // Otherwise, we'll have to remove the view and start over.
                    getSkippedScrap().add(scrap);
                }
            } else {
                if (mViewTypeCount == 1) {
                    mCurrentScrap.add(scrap);
                } else {
                    mScrapViews[viewType].add(scrap);
                }

                if (mRecyclerListener != null) {
                    mRecyclerListener.onMovedToScrapHeap(scrap);
                }
            }
        }

        private ArrayList<View> getSkippedScrap() {
            if (mSkippedScrap == null) {
                mSkippedScrap = new ArrayList<>();
            }
            return mSkippedScrap;
        }

        /**
         * Finish the removal of any views that skipped the scrap heap.
         */
        void removeSkippedScrap() {
            if (mSkippedScrap == null) {
                return;
            }
            final int count = mSkippedScrap.size();
            for (int i = 0; i < count; i++) {
                removeDetachedView(mSkippedScrap.get(i), false);
            }
            mSkippedScrap.clear();
        }

        /**
         * Move all views remaining in mActiveViews to mScrapViews.
         */
        void scrapActiveViews() {
            final View[] activeViews = mActiveViews;
            final boolean hasListener = mRecyclerListener != null;
            final boolean multipleScraps = mViewTypeCount > 1;

            ArrayList<View> scrapViews = mCurrentScrap;
            final int count = activeViews.length;
            for (int i = count - 1; i >= 0; i--) {
                final View victim = activeViews[i];
                if (victim != null) {
                    final AbsListView.LayoutParams lp
                            = (AbsListView.LayoutParams) victim.getLayoutParams();
                    final int whichScrap = lp.viewType;

                    activeViews[i] = null;

                    if (victim.hasTransientState()) {
                        // Store views with transient state for later use.
                        victim.dispatchStartTemporaryDetach();

                        if (mAdapter != null && mAdapterHasStableIds) {
                            if (mTransientStateViewsById == null) {
                                mTransientStateViewsById = new LongSparseArray<View>();
                            }
                            long id = mAdapter.getItemId(mFirstActivePosition + i);
                            mTransientStateViewsById.put(id, victim);
                        } else if (!mDataChanged) {
                            if (mTransientStateViews == null) {
                                mTransientStateViews = new SparseArray<View>();
                            }
                            mTransientStateViews.put(mFirstActivePosition + i, victim);
                        } else if (whichScrap != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
                            // The data has changed, we can't keep this view.
                            removeDetachedView(victim, false);
                        }
                    } else if (!shouldRecycleViewType(whichScrap)) {
                        // Discard non-recyclable views except headers/footers.
                        if (whichScrap != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
                            removeDetachedView(victim, false);
                        }
                    } else {
                        // Store everything else on the appropriate scrap heap.
                        if (multipleScraps) {
                            scrapViews = mScrapViews[whichScrap];
                        }

                        lp.scrappedFromPosition = mFirstActivePosition + i;
                        removeDetachedView(victim, false);
                        scrapViews.add(victim);

                        if (hasListener) {
                            mRecyclerListener.onMovedToScrapHeap(victim);
                        }
                    }
                }
            }
            pruneScrapViews();
        }

        /**
         * At the end of a layout pass, all temp detached views should either be re-attached or
         * completely detached. This method ensures that any remaining view in the scrap list is
         * fully detached.
         */
        void fullyDetachScrapViews() {
            final int viewTypeCount = mViewTypeCount;
            final ArrayList<View>[] scrapViews = mScrapViews;
            for (int i = 0; i < viewTypeCount; ++i) {
                final ArrayList<View> scrapPile = scrapViews[i];
                for (int j = scrapPile.size() - 1; j >= 0; j--) {
                    final View view = scrapPile.get(j);
                    if (view.isTemporarilyDetached()) {
                        removeDetachedView(view, false);
                    }
                }
            }
        }

        /**
         * Makes sure that the size of mScrapViews does not exceed the size of
         * mActiveViews, which can happen if an adapter does not recycle its
         * views. Removes cached transient state views that no longer have
         * transient state.
         */
        private void pruneScrapViews() {
            final int maxViews = mActiveViews.length;
            final int viewTypeCount = mViewTypeCount;
            final ArrayList<View>[] scrapViews = mScrapViews;
            for (int i = 0; i < viewTypeCount; ++i) {
                final ArrayList<View> scrapPile = scrapViews[i];
                int size = scrapPile.size();
                while (size > maxViews) {
                    scrapPile.remove(--size);
                }
            }

            final SparseArray<View> transViewsByPos = mTransientStateViews;
            if (transViewsByPos != null) {
                for (int i = 0; i < transViewsByPos.size(); i++) {
                    final View v = transViewsByPos.valueAt(i);
                    if (!v.hasTransientState()) {
                        removeDetachedView(v, false);
                        transViewsByPos.removeAt(i);
                        i--;
                    }
                }
            }

            final LongSparseArray<View> transViewsById = mTransientStateViewsById;
            if (transViewsById != null) {
                for (int i = 0; i < transViewsById.size(); i++) {
                    final View v = transViewsById.valueAt(i);
                    if (!v.hasTransientState()) {
                        removeDetachedView(v, false);
                        transViewsById.removeAt(i);
                        i--;
                    }
                }
            }
        }

        /**
         * Puts all views in the scrap heap into the supplied list.
         */
        void reclaimScrapViews(List<View> views) {
            if (mViewTypeCount == 1) {
                views.addAll(mCurrentScrap);
            } else {
                final int viewTypeCount = mViewTypeCount;
                final ArrayList<View>[] scrapViews = mScrapViews;
                for (int i = 0; i < viewTypeCount; ++i) {
                    final ArrayList<View> scrapPile = scrapViews[i];
                    views.addAll(scrapPile);
                }
            }
        }

        /**
         * Updates the cache color hint of all known views.
         *
         * @param color The new cache color hint.
         */
        void setCacheColorHint(int color) {
            if (mViewTypeCount == 1) {
                final ArrayList<View> scrap = mCurrentScrap;
                final int scrapCount = scrap.size();
                for (int i = 0; i < scrapCount; i++) {
                    scrap.get(i).setDrawingCacheBackgroundColor(color);
                }
            } else {
                final int typeCount = mViewTypeCount;
                for (int i = 0; i < typeCount; i++) {
                    final ArrayList<View> scrap = mScrapViews[i];
                    final int scrapCount = scrap.size();
                    for (int j = 0; j < scrapCount; j++) {
                        scrap.get(j).setDrawingCacheBackgroundColor(color);
                    }
                }
            }
            // Just in case this is called during a layout pass
            final View[] activeViews = mActiveViews;
            final int count = activeViews.length;
            for (int i = 0; i < count; ++i) {
                final View victim = activeViews[i];
                if (victim != null) {
                    victim.setDrawingCacheBackgroundColor(color);
                }
            }
        }

        private View retrieveFromScrap(ArrayList<View> scrapViews, int position) {
            final int size = scrapViews.size();
            if (size > 0) {
                // See if we still have a view for this position or ID.
                for (int i = 0; i < size; i++) {
                    final View view = scrapViews.get(i);
                    final AbsListView.LayoutParams params =
                            (AbsListView.LayoutParams) view.getLayoutParams();

                    if (mAdapterHasStableIds) {
                        final long id = mAdapter.getItemId(position);
                        if (id == params.itemId) {
                            return scrapViews.remove(i);
                        }
                    } else if (params.scrappedFromPosition == position) {
                        final View scrap = scrapViews.remove(i);
                        clearAccessibilityFromScrap(scrap);
                        return scrap;
                    }
                }
                final View scrap = scrapViews.remove(size - 1);
                clearAccessibilityFromScrap(scrap);
                return scrap;
            } else {
                return null;
            }
        }

        private void clearScrap(final ArrayList<View> scrap) {
            final int scrapCount = scrap.size();
            for (int j = 0; j < scrapCount; j++) {
                removeDetachedView(scrap.remove(scrapCount - 1 - j), false);
            }
        }

        private void clearAccessibilityFromScrap(View view) {
            view.clearAccessibilityFocus();
            view.setAccessibilityDelegate(null);
        }

        private void removeDetachedView(View child, boolean animate) {
            child.setAccessibilityDelegate(null);
            AbsListView.this.removeDetachedView(child, animate);
        }
    }

在 RecycleBin 中有两个数组：mActiveViews 和 mScrapViews; mActiveViews 表示 listview 首屏展示的元素集合,mScrapViews 表示屏幕外被废弃的元素集合，它是无序的；这里面有几个非常重要的方法，我们现在逐一看看：

• fillActiveViews() 这个方法接收两个参数，第一个参数是要存储的 view 的数量，第二个参数是 listview 中第一个可见元素的 position;RecycleBin 中使用 mActiveViews 这个数组来存储 view；调用这个方法是将 AbsListView 中所有的子元素全部添加到 mActiveViews 中。
• getActiveView() 用于从mActiveViews 中获取元素。传入的 position 参数表示元素在 listview 中的位置，方法内部会自动转化成mActiveViews 数组中的下标值。 有一点需要注意：mActiveViews 中的 view 一旦被获取之后就会从 mActiveViews 中移除，下次在获取同样位置的 view 将会返回 null；也就是说 mActiveViews 不能被重复利用。
• addScrapView() 用于将一个废弃的 View 进行缓存；该方法接收两个参数，第一个参数表示要回收的 View ，第二个参数表示在父布局上的位置。RecycleBin 使用了 mScrapView 和 mCurrentScrap 两个 list 来存储废弃的 View。
• getScrapView() 用于从废弃缓存中取出一个 View ,该方法接收一个 position 参数，表示取出父布局上这个位置的 View；由于缓存中的 View 是无序的，因此这里面有一个判断，
  • 如果只有一个元素，那就从 mCurrentScrap 中取；如果这个 position中的 View 还存在（ID ／Position），那么就从 mCurrentScrap 中取出，如果不存在就取 mCurrentScrap 中的最后一个；
  • 如果这个 position 小于 mScrapViews 的长度，那么就从 mScrapViews 中取，同样的，如果这个 position中的 View 还存在（ID ／Position），那么就从 mScrapViews 中取出，如果不存在就取 mScrapViews 中的最后一个；
• setViewTypeCount() 作用是为每种类型的数据项单独启用一个 RecycleBin 缓存机制；

第一次 Layout

在 ListView 中我们并没有找到 onLayout() 方法，那一定是在父类中实现该方法，我们在AbsListView 中找到了这个方法，代码如下：

/**
     * Subclasses should NOT override this method but
     *  {@link #layoutChildren()} instead.
     */
    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        super.onLayout(changed, l, t, r, b);

        mInLayout = true;

        final int childCount = getChildCount();
        if (changed) {
            for (int i = 0; i < childCount; i++) {
                getChildAt(i).forceLayout();
            }
            mRecycler.markChildrenDirty();
        }

        layoutChildren();

        mOverscrollMax = (b - t) / OVERSCROLL_LIMIT_DIVISOR;

        // TODO: Move somewhere sane. This doesn't belong in onLayout().
        if (mFastScroll != null) {
            mFastScroll.onItemCountChanged(getChildCount(), mItemCount);
        }
        mInLayout = false;
    }

从这个方法的声明中我们知道，子类不需要重写这个方法，相应的子类应该重写 LayoutChildren() 方法；这也不难理解，因为子元素的布局应该由具体的实现类来完成，而不是父类来完成。

Listview 中的 LayoutChildren() 方法如下：

@Override
    protected void layoutChildren() {
        final boolean blockLayoutRequests = mBlockLayoutRequests;
        if (blockLayoutRequests) {
            return;
        }

        mBlockLayoutRequests = true;

        try {
            super.layoutChildren();

            invalidate();

            if (mAdapter == null) {
                resetList();
                invokeOnItemScrollListener();
                return;
            }

            final int childrenTop = mListPadding.top;
            final int childrenBottom = mBottom - mTop - mListPadding.bottom;
            final int childCount = getChildCount();

            int index = 0;
            int delta = 0;

            View sel;
            View oldSel = null;
            View oldFirst = null;
            View newSel = null;

            // Remember stuff we will need down below
            switch (mLayoutMode) {
            case LAYOUT_SET_SELECTION:
                index = mNextSelectedPosition - mFirstPosition;
                if (index >= 0 && index < childCount) {
                    newSel = getChildAt(index);
                }
                break;
            case LAYOUT_FORCE_TOP:
            case LAYOUT_FORCE_BOTTOM:
            case LAYOUT_SPECIFIC:
            case LAYOUT_SYNC:
                break;
            case LAYOUT_MOVE_SELECTION:
            default:
                // Remember the previously selected view
                index = mSelectedPosition - mFirstPosition;
                if (index >= 0 && index < childCount) {
                    oldSel = getChildAt(index);
                }

                // Remember the previous first child
                oldFirst = getChildAt(0);

                if (mNextSelectedPosition >= 0) {
                    delta = mNextSelectedPosition - mSelectedPosition;
                }

                // Caution: newSel might be null
                newSel = getChildAt(index + delta);
            }


            boolean dataChanged = mDataChanged;
            if (dataChanged) {
                handleDataChanged();
            }

            // Handle the empty set by removing all views that are visible
            // and calling it a day
            if (mItemCount == 0) {
                resetList();
                invokeOnItemScrollListener();
                return;
            } else if (mItemCount != mAdapter.getCount()) {
                throw new IllegalStateException("The content of the adapter has changed but "
                        + "ListView did not receive a notification. Make sure the content of "
                        + "your adapter is not modified from a background thread, but only from "
                        + "the UI thread. Make sure your adapter calls notifyDataSetChanged() "
                        + "when its content changes. [in ListView(" + getId() + ", " + getClass()
                        + ") with Adapter(" + mAdapter.getClass() + ")]");
            }

            setSelectedPositionInt(mNextSelectedPosition);

            AccessibilityNodeInfo accessibilityFocusLayoutRestoreNode = null;
            View accessibilityFocusLayoutRestoreView = null;
            int accessibilityFocusPosition = INVALID_POSITION;

            // Remember which child, if any, had accessibility focus. This must
            // occur before recycling any views, since that will clear
            // accessibility focus.
            final ViewRootImpl viewRootImpl = getViewRootImpl();
            if (viewRootImpl != null) {
                final View focusHost = viewRootImpl.getAccessibilityFocusedHost();
                if (focusHost != null) {
                    final View focusChild = getAccessibilityFocusedChild(focusHost);
                    if (focusChild != null) {
                        if (!dataChanged || isDirectChildHeaderOrFooter(focusChild)
                                || focusChild.hasTransientState() || mAdapterHasStableIds) {
                            // The views won't be changing, so try to maintain
                            // focus on the current host and virtual view.
                            accessibilityFocusLayoutRestoreView = focusHost;
                            accessibilityFocusLayoutRestoreNode = viewRootImpl
                                    .getAccessibilityFocusedVirtualView();
                        }

                        // If all else fails, maintain focus at the same
                        // position.
                        accessibilityFocusPosition = getPositionForView(focusChild);
                    }
                }
            }

            View focusLayoutRestoreDirectChild = null;
            View focusLayoutRestoreView = null;

            // Take focus back to us temporarily to avoid the eventual call to
            // clear focus when removing the focused child below from messing
            // things up when ViewAncestor assigns focus back to someone else.
            final View focusedChild = getFocusedChild();
            if (focusedChild != null) {
                // TODO: in some cases focusedChild.getParent() == null

                // We can remember the focused view to restore after re-layout
                // if the data hasn't changed, or if the focused position is a
                // header or footer.
                if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild)
                        || focusedChild.hasTransientState() || mAdapterHasStableIds) {
                    focusLayoutRestoreDirectChild = focusedChild;
                    // Remember the specific view that had focus.
                    focusLayoutRestoreView = findFocus();
                    if (focusLayoutRestoreView != null) {
                        // Tell it we are going to mess with it.
                        focusLayoutRestoreView.dispatchStartTemporaryDetach();
                    }
                }
                requestFocus();
            }

            // Pull all children into the RecycleBin.
            // These views will be reused if possible
            final int firstPosition = mFirstPosition;
            final RecycleBin recycleBin = mRecycler;
            if (dataChanged) {
                for (int i = 0; i < childCount; i++) {
                    recycleBin.addScrapView(getChildAt(i), firstPosition+i);
                }
            } else {
                recycleBin.fillActiveViews(childCount, firstPosition);
            }

            // Clear out old views
            detachAllViewsFromParent();
            recycleBin.removeSkippedScrap();

            switch (mLayoutMode) {
            case LAYOUT_SET_SELECTION:
                if (newSel != null) {
                    sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom);
                } else {
                    sel = fillFromMiddle(childrenTop, childrenBottom);
                }
                break;
            case LAYOUT_SYNC:
                sel = fillSpecific(mSyncPosition, mSpecificTop);
                break;
            case LAYOUT_FORCE_BOTTOM:
                sel = fillUp(mItemCount - 1, childrenBottom);
                adjustViewsUpOrDown();
                break;
            case LAYOUT_FORCE_TOP:
                mFirstPosition = 0;
                sel = fillFromTop(childrenTop);
                adjustViewsUpOrDown();
                break;
            case LAYOUT_SPECIFIC:
                final int selectedPosition = reconcileSelectedPosition();
                sel = fillSpecific(selectedPosition, mSpecificTop);
                /**
                 * When ListView is resized, FocusSelector requests an async selection for the
                 * previously focused item to make sure it is still visible. If the item is not
                 * selectable, it won't regain focus so instead we call FocusSelector
                 * to directly request focus on the view after it is visible.
                 */
                if (sel == null && mFocusSelector != null) {
                    final Runnable focusRunnable = mFocusSelector
                            .setupFocusIfValid(selectedPosition);
                    if (focusRunnable != null) {
                        post(focusRunnable);
                    }
                }
                break;
            case LAYOUT_MOVE_SELECTION:
                sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom);
                break;
            default:
                if (childCount == 0) {
                    if (!mStackFromBottom) {
                        final int position = lookForSelectablePosition(0, true);
                        setSelectedPositionInt(position);
                        sel = fillFromTop(childrenTop);
                    } else {
                        final int position = lookForSelectablePosition(mItemCount - 1, false);
                        setSelectedPositionInt(position);
                        sel = fillUp(mItemCount - 1, childrenBottom);
                    }
                } else {
                    if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {
                        sel = fillSpecific(mSelectedPosition,
                                oldSel == null ? childrenTop : oldSel.getTop());
                    } else if (mFirstPosition < mItemCount) {
                        sel = fillSpecific(mFirstPosition,
                                oldFirst == null ? childrenTop : oldFirst.getTop());
                    } else {
                        sel = fillSpecific(0, childrenTop);
                    }
                }
                break;
            }

            // Flush any cached views that did not get reused above
            recycleBin.scrapActiveViews();

            // remove any header/footer that has been temp detached and not re-attached
            removeUnusedFixedViews(mHeaderViewInfos);
            removeUnusedFixedViews(mFooterViewInfos);

            if (sel != null) {
                // The current selected item should get focus if items are
                // focusable.
                if (mItemsCanFocus && hasFocus() && !sel.hasFocus()) {
                    final boolean focusWasTaken = (sel == focusLayoutRestoreDirectChild &&
                            focusLayoutRestoreView != null &&
                            focusLayoutRestoreView.requestFocus()) || sel.requestFocus();
                    if (!focusWasTaken) {
                        // Selected item didn't take focus, but we still want to
                        // make sure something else outside of the selected view
                        // has focus.
                        final View focused = getFocusedChild();
                        if (focused != null) {
                            focused.clearFocus();
                        }
                        positionSelector(INVALID_POSITION, sel);
                    } else {
                        sel.setSelected(false);
                        mSelectorRect.setEmpty();
                    }
                } else {
                    positionSelector(INVALID_POSITION, sel);
                }
                mSelectedTop = sel.getTop();
            } else {
                final boolean inTouchMode = mTouchMode == TOUCH_MODE_TAP
                        || mTouchMode == TOUCH_MODE_DONE_WAITING;
                if (inTouchMode) {
                    // If the user's finger is down, select the motion position.
                    final View child = getChildAt(mMotionPosition - mFirstPosition);
                    if (child != null) {
                        positionSelector(mMotionPosition, child);
                    }
                } else if (mSelectorPosition != INVALID_POSITION) {
                    // If we had previously positioned the selector somewhere,
                    // put it back there. It might not match up with the data,
                    // but it's transitioning out so it's not a big deal.
                    final View child = getChildAt(mSelectorPosition - mFirstPosition);
                    if (child != null) {
                        positionSelector(mSelectorPosition, child);
                    }
                } else {
                    // Otherwise, clear selection.
                    mSelectedTop = 0;
                    mSelectorRect.setEmpty();
                }

                // Even if there is not selected position, we may need to
                // restore focus (i.e. something focusable in touch mode).
                if (hasFocus() && focusLayoutRestoreView != null) {
                    focusLayoutRestoreView.requestFocus();
                }
            }

            // Attempt to restore accessibility focus, if necessary.
            if (viewRootImpl != null) {
                final View newAccessibilityFocusedView = viewRootImpl.getAccessibilityFocusedHost();
                if (newAccessibilityFocusedView == null) {
                    if (accessibilityFocusLayoutRestoreView != null
                            && accessibilityFocusLayoutRestoreView.isAttachedToWindow()) {
                        final AccessibilityNodeProvider provider =
                                accessibilityFocusLayoutRestoreView.getAccessibilityNodeProvider();
                        if (accessibilityFocusLayoutRestoreNode != null && provider != null) {
                            final int virtualViewId = AccessibilityNodeInfo.getVirtualDescendantId(
                                    accessibilityFocusLayoutRestoreNode.getSourceNodeId());
                            provider.performAction(virtualViewId,
                                    AccessibilityNodeInfo.ACTION_ACCESSIBILITY_FOCUS, null);
                        } else {
                            accessibilityFocusLayoutRestoreView.requestAccessibilityFocus();
                        }
                    } else if (accessibilityFocusPosition != INVALID_POSITION) {
                        // Bound the position within the visible children.
                        final int position = MathUtils.constrain(
                                accessibilityFocusPosition - mFirstPosition, 0,
                                getChildCount() - 1);
                        final View restoreView = getChildAt(position);
                        if (restoreView != null) {
                            restoreView.requestAccessibilityFocus();
                        }
                    }
                }
            }

            // Tell focus view we are done mucking with it, if it is still in
            // our view hierarchy.
            if (focusLayoutRestoreView != null
                    && focusLayoutRestoreView.getWindowToken() != null) {
                focusLayoutRestoreView.dispatchFinishTemporaryDetach();
            }
            
            mLayoutMode = LAYOUT_NORMAL;
            mDataChanged = false;
            if (mPositionScrollAfterLayout != null) {
                post(mPositionScrollAfterLayout);
                mPositionScrollAfterLayout = null;
            }
            mNeedSync = false;
            setNextSelectedPositionInt(mSelectedPosition);

            updateScrollIndicators();

            if (mItemCount > 0) {
                checkSelectionChanged();
            }

            invokeOnItemScrollListener();
        } finally {
            if (mFocusSelector != null) {
                mFocusSelector.onLayoutComplete();
            }
            if (!blockLayoutRequests) {
                mBlockLayoutRequests = false;
            }
        }
    }

这段代码较长，我们挑重点看，首先可以确定，ListView 中没有任何子 View ，因此 getChildCount() 方法得到的值肯定是0，接着会根据 dataChanged 这个布尔值来判断执行逻辑，dataChanged 只有在数据源发生改变的时候才会变成 ture,其他情况下默认是 false,因此会调用 RecycleBin 的 fillActiveViews() 方法，调用 fillActiveViews() 方法的目的是将 ListView 中 的子 View 进行缓存，但是目前 ListView 此时并没有任何数据，因此这一行暂时不起作用。

接下来会根据 mLayoutMode 的值来决定布局模式，默认情况下都是普通模式 LAYOUT_NORMAL， 因此会进入 default 语句中。而下面会紧接着进行两次 if 判断，childCount 目前等于0，并且默认的布局顺序是从上而下的，因此会进入到 fillFromTop() 方法里，跟进去瞧一瞧：

/**
     * Fills the list from top to bottom, starting with mFirstPosition
     *
     * @param nextTop The location where the top of the first item should be
     *        drawn
     *
     * @return The view that is currently selected
     */
    private View fillFromTop(int nextTop) {
        mFirstPosition = Math.min(mFirstPosition, mSelectedPosition);
        mFirstPosition = Math.min(mFirstPosition, mItemCount - 1);
        if (mFirstPosition < 0) {
            mFirstPosition = 0;
        }
        return fillDown(mFirstPosition, nextTop);
    }

这个方法的作用是从 mFirstPosition 开始，自顶至低取填充数据；从这个方法中，它只是简单的做了一个起始位置的判断并没有执行填充操作，因此我们有理由相信填充 ListView 的操作是在 fillDown() 方法中。跟进去瞧瞧：

/**
     * Fills the list from pos down to the end of the list view.
     *
     * @param pos The first position to put in the list
     *
     * @param nextTop The location where the top of the item associated with pos
     *        should be drawn
     *
     * @return The view that is currently selected, if it happens to be in the
     *         range that we draw.
     */
    private View fillDown(int pos, int nextTop) {
        View selectedView = null;

        int end = (mBottom - mTop);
        if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
            end -= mListPadding.bottom;
        }

        while (nextTop < end && pos < mItemCount) {
            // is this the selected item?
            boolean selected = pos == mSelectedPosition;
            View child = makeAndAddView(pos, nextTop, true, mListPadding.left, selected);

            nextTop = child.getBottom() + mDividerHeight;
            if (selected) {
                selectedView = child;
            }
            pos++;
        }

        setVisibleRangeHint(mFirstPosition, mFirstPosition + getChildCount() - 1);
        return selectedView;
    }

可以看到，这里有一个 while 循环来执行重复逻辑，这里面有几个相关值需要说明一下：

• nextTop 表示第一个子元素顶部距离整个 ListView 顶部的像素值。
• pos 是传入的 mFirstPosition 值。
• end 是 ListView 底部减去顶部所得的像素值。
• mItemCount 是 Adapter 中的元素数量。

因此在开始的时候，nextTop 必定小于 end 并且 pos 小于 mItemCount值，没执行一次 while 循环，pos 的值自动加1，并且 nextTop 也会增加，当 nextTop 大约等于 end 时，也就是子元素超出了当前屏幕，或者 pos 大于等于 mItemCount 时候，也就是说 Adapter 中所有的元素都已遍历完成。之后就会结束 while 循环。

在 while 循环中，真正有意义的是 makeAndAddView() 方法，追进去看一下，代码如下：

/**
     * Obtains the view and adds it to our list of children. The view can be
     * made fresh, converted from an unused view, or used as is if it was in
     * the recycle bin.
     *
     * @param position logical position in the list
     * @param y top or bottom edge of the view to add
     * @param flow {@code true} to align top edge to y, {@code false} to align
     *             bottom edge to y
     * @param childrenLeft left edge where children should be positioned
     * @param selected {@code true} if the position is selected, {@code false}
     *                 otherwise
     * @return the view that was added
     */
    private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
            boolean selected) {
        if (!mDataChanged) {
            // Try to use an existing view for this position.
            final View activeView = mRecycler.getActiveView(position);
            if (activeView != null) {
                // Found it. We're reusing an existing child, so it just needs
                // to be positioned like a scrap view.
                setupChild(activeView, position, y, flow, childrenLeft, selected, true);
                return activeView;
            }
        }

        // Make a new view for this position, or convert an unused view if
        // possible.
        final View child = obtainView(position, mIsScrap);

        // This needs to be positioned and measured.
        setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);

        return child;
    }

在这个方法中首先尝试从 RecycleBin 中快速获取一个 activeView ，但是目前 RecycleBin 中还没有任何一个 View，所以这里得到的值肯定是null，那就跳出去继续执行下面语句，接下来调用 obtainView() 方法来重新获取一个 View ，这个方法会返回一个 View ，并将这个 View 传入 setupChild() 方法中；下面我们追到 obtainView() 方法中去看看，代码如下：

/**
     * Gets a view and have it show the data associated with the specified
     * position. This is called when we have already discovered that the view
     * is not available for reuse in the recycle bin. The only choices left are
     * converting an old view or making a new one.
     *
     * @param position the position to display
     * @param outMetadata an array of at least 1 boolean where the first entry
     *                    will be set {@code true} if the view is currently
     *                    attached to the window, {@code false} otherwise (e.g.
     *                    newly-inflated or remained scrap for multiple layout
     *                    passes)
     *
     * @return A view displaying the data associated with the specified position
     */
    View obtainView(int position, boolean[] outMetadata) {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "obtainView");

        outMetadata[0] = false;

        // Check whether we have a transient state view. Attempt to re-bind the
        // data and discard the view if we fail.
        final View transientView = mRecycler.getTransientStateView(position);
        if (transientView != null) {
            final LayoutParams params = (LayoutParams) transientView.getLayoutParams();

            // If the view type hasn't changed, attempt to re-bind the data.
            if (params.viewType == mAdapter.getItemViewType(position)) {
                final View updatedView = mAdapter.getView(position, transientView, this);

                // If we failed to re-bind the data, scrap the obtained view.
                if (updatedView != transientView) {
                    setItemViewLayoutParams(updatedView, position);
                    mRecycler.addScrapView(updatedView, position);
                }
            }

            outMetadata[0] = true;

            // Finish the temporary detach started in addScrapView().
            transientView.dispatchFinishTemporaryDetach();
            return transientView;
        }

        final View scrapView = mRecycler.getScrapView(position);
        final View child = mAdapter.getView(position, scrapView, this);
        if (scrapView != null) {
            if (child != scrapView) {
                // Failed to re-bind the data, return scrap to the heap.
                mRecycler.addScrapView(scrapView, position);
            } else if (child.isTemporarilyDetached()) {
                outMetadata[0] = true;

                // Finish the temporary detach started in addScrapView().
                child.dispatchFinishTemporaryDetach();
            }
        }

        if (mCacheColorHint != 0) {
            child.setDrawingCacheBackgroundColor(mCacheColorHint);
        }

        if (child.getImportantForAccessibility() == IMPORTANT_FOR_ACCESSIBILITY_AUTO) {
            child.setImportantForAccessibility(IMPORTANT_FOR_ACCESSIBILITY_YES);
        }

        setItemViewLayoutParams(child, position);

        if (AccessibilityManager.getInstance(mContext).isEnabled()) {
            if (mAccessibilityDelegate == null) {
                mAccessibilityDelegate = new ListItemAccessibilityDelegate();
            }
            if (child.getAccessibilityDelegate() == null) {
                child.setAccessibilityDelegate(mAccessibilityDelegate);
            }
        }

        Trace.traceEnd(Trace.TRACE_TAG_VIEW);

        return child;
    }

obtainView() 方法是整个 ListView 中的最重要的内容，首先调用 RecycleBin 的 getScrapView() 方法来获取一个废弃缓存的 View，因为 RecycleBin 中没有 View ，因此返回值为null，接着代码会执行 mAdapter 的 getView() 方法，是不是很熟悉，没错就是我们经常重写的方法，传入的三个参数分别为 position、 null 、this。

getView() 方法接收三个参数，第一个参数 position 代表当前子元素的位置，第二个元素是 convertView，由于我们传入的是null，因此 convertView 并没有用，我们会调用 LayoutInflater 的 inflate() 方法来加载一个布局，接下来会对这个 View 进行一下属性和值的设定，最后将 View 返回。

因此，这个 View 作为 ObtainView() 的结果返回，并传入 setupChild() 中。也就是说，第一次 layout 过程中，所有的子 View 都是通过 LayoutInflater 来加载进来的，这样相对比较耗时，但后面就不会出现这个情况啦，我们继续往下看：

/**
     * Adds a view as a child and make sure it is measured (if necessary) and
     * positioned properly.
     *
     * @param child the view to add
     * @param position the position of this child
     * @param y the y position relative to which this view will be positioned
     * @param flowDown {@code true} to align top edge to y, {@code false} to
     *                 align bottom edge to y
     * @param childrenLeft left edge where children should be positioned
     * @param selected {@code true} if the position is selected, {@code false}
     *                 otherwise
     * @param isAttachedToWindow {@code true} if the view is already attached
     *                           to the window, e.g. whether it was reused, or
     *                           {@code false} otherwise
     */
    private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,
            boolean selected, boolean isAttachedToWindow) {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "setupListItem");

        final boolean isSelected = selected && shouldShowSelector();
        final boolean updateChildSelected = isSelected != child.isSelected();
        final int mode = mTouchMode;
        final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL
                && mMotionPosition == position;
        final boolean updateChildPressed = isPressed != child.isPressed();
        final boolean needToMeasure = !isAttachedToWindow || updateChildSelected
                || child.isLayoutRequested();

        // Respect layout params that are already in the view. Otherwise make
        // some up...
        AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
        if (p == null) {
            p = (AbsListView.LayoutParams) generateDefaultLayoutParams();
        }
        p.viewType = mAdapter.getItemViewType(position);
        p.isEnabled = mAdapter.isEnabled(position);

        // Set up view state before attaching the view, since we may need to
        // rely on the jumpDrawablesToCurrentState() call that occurs as part
        // of view attachment.
        if (updateChildSelected) {
            child.setSelected(isSelected);
        }

        if (updateChildPressed) {
            child.setPressed(isPressed);
        }

        if (mChoiceMode != CHOICE_MODE_NONE && mCheckStates != null) {
            if (child instanceof Checkable) {
                ((Checkable) child).setChecked(mCheckStates.get(position));
            } else if (getContext().getApplicationInfo().targetSdkVersion
                    >= android.os.Build.VERSION_CODES.HONEYCOMB) {
                child.setActivated(mCheckStates.get(position));
            }
        }

        if ((isAttachedToWindow && !p.forceAdd) || (p.recycledHeaderFooter
                && p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {
            attachViewToParent(child, flowDown ? -1 : 0, p);

            // If the view was previously attached for a different position,
            // then manually jump the drawables.
            if (isAttachedToWindow
                    && (((AbsListView.LayoutParams) child.getLayoutParams()).scrappedFromPosition)
                            != position) {
                child.jumpDrawablesToCurrentState();
            }
        } else {
            p.forceAdd = false;
            if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
                p.recycledHeaderFooter = true;
            }
            addViewInLayout(child, flowDown ? -1 : 0, p, true);
            // add view in layout will reset the RTL properties. We have to re-resolve them
            child.resolveRtlPropertiesIfNeeded();
        }

        if (needToMeasure) {
            final int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,
                    mListPadding.left + mListPadding.right, p.width);
            final int lpHeight = p.height;
            final int childHeightSpec;
            if (lpHeight > 0) {
                childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);
            } else {
                childHeightSpec = MeasureSpec.makeSafeMeasureSpec(getMeasuredHeight(),
                        MeasureSpec.UNSPECIFIED);
            }
            child.measure(childWidthSpec, childHeightSpec);
        } else {
            cleanupLayoutState(child);
        }

        final int w = child.getMeasuredWidth();
        final int h = child.getMeasuredHeight();
        final int childTop = flowDown ? y : y - h;

        if (needToMeasure) {
            final int childRight = childrenLeft + w;
            final int childBottom = childTop + h;
            child.layout(childrenLeft, childTop, childRight, childBottom);
        } else {
            child.offsetLeftAndRight(childrenLeft - child.getLeft());
            child.offsetTopAndBottom(childTop - child.getTop());
        }

        if (mCachingStarted && !child.isDrawingCacheEnabled()) {
            child.setDrawingCacheEnabled(true);
        }

        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }

在这个方法中将 ObtainView() 方法获取的子元素 View ，通过调用 addViewInLayout() 方法将它添加到 ListView 当中,addViewInLayout() 方法属于 ViewGroup，作用是在 layout 中添加 View，代码如下：

/**
     * Adds a view during layout. This is useful if in your onLayout() method,
     * you need to add more views (as does the list view for example).
     *
     * If index is negative, it means put it at the end of the list.
     *
     * @param child the view to add to the group
     * @param index the index at which the child must be added or -1 to add last
     * @param params the layout parameters to associate with the child
     * @param preventRequestLayout if true, calling this method will not trigger a
     *        layout request on child
     * @return true if the child was added, false otherwise
     */
    protected boolean addViewInLayout(View child, int index, LayoutParams params,
            boolean preventRequestLayout) {
        if (child == null) {
            throw new IllegalArgumentException("Cannot add a null child view to a ViewGroup");
        }
        child.mParent = null;
        addViewInner(child, index, params, preventRequestLayout);
        child.mPrivateFlags = (child.mPrivateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;
        return true;
    }

那么根据 fillDown() 方法中的 while 循环，会让子元素将整个 Listview 填满。 也就是说无论我们的 Adatper 中有多少条数据，ListView 只会加载第一屏的数据，剩下的数据完全不会加载，所以不会有多余的加载工作，这样可以保证 ListView 中的内容可以迅速的展示在屏幕上。

到此为止，第一阶段的 onLayout 过程结束。

第二次 Layout

我们还是从 LayoutChildren() 方法开始：

@Override
    protected void layoutChildren() {
        final boolean blockLayoutRequests = mBlockLayoutRequests;
        if (blockLayoutRequests) {
            return;
        }

        mBlockLayoutRequests = true;

        try {
            super.layoutChildren();

            invalidate();

            if (mAdapter == null) {
                resetList();
                invokeOnItemScrollListener();
                return;
            }

            final int childrenTop = mListPadding.top;
            final int childrenBottom = mBottom - mTop - mListPadding.bottom;
            final int childCount = getChildCount();

            int index = 0;
            int delta = 0;

            View sel;
            View oldSel = null;
            View oldFirst = null;
            View newSel = null;

            // Remember stuff we will need down below
            switch (mLayoutMode) {
            case LAYOUT_SET_SELECTION:
                index = mNextSelectedPosition - mFirstPosition;
                if (index >= 0 && index < childCount) {
                    newSel = getChildAt(index);
                }
                break;
            case LAYOUT_FORCE_TOP:
            case LAYOUT_FORCE_BOTTOM:
            case LAYOUT_SPECIFIC:
            case LAYOUT_SYNC:
                break;
            case LAYOUT_MOVE_SELECTION:
            default:
                // Remember the previously selected view
                index = mSelectedPosition - mFirstPosition;
                if (index >= 0 && index < childCount) {
                    oldSel = getChildAt(index);
                }

                // Remember the previous first child
                oldFirst = getChildAt(0);

                if (mNextSelectedPosition >= 0) {
                    delta = mNextSelectedPosition - mSelectedPosition;
                }

                // Caution: newSel might be null
                newSel = getChildAt(index + delta);
            }


            boolean dataChanged = mDataChanged;
            if (dataChanged) {
                handleDataChanged();
            }

            // Handle the empty set by removing all views that are visible
            // and calling it a day
            if (mItemCount == 0) {
                resetList();
                invokeOnItemScrollListener();
                return;
            } else if (mItemCount != mAdapter.getCount()) {
                throw new IllegalStateException("The content of the adapter has changed but "
                        + "ListView did not receive a notification. Make sure the content of "
                        + "your adapter is not modified from a background thread, but only from "
                        + "the UI thread. Make sure your adapter calls notifyDataSetChanged() "
                        + "when its content changes. [in ListView(" + getId() + ", " + getClass()
                        + ") with Adapter(" + mAdapter.getClass() + ")]");
            }

            setSelectedPositionInt(mNextSelectedPosition);

            AccessibilityNodeInfo accessibilityFocusLayoutRestoreNode = null;
            View accessibilityFocusLayoutRestoreView = null;
            int accessibilityFocusPosition = INVALID_POSITION;

            // Remember which child, if any, had accessibility focus. This must
            // occur before recycling any views, since that will clear
            // accessibility focus.
            final ViewRootImpl viewRootImpl = getViewRootImpl();
            if (viewRootImpl != null) {
                final View focusHost = viewRootImpl.getAccessibilityFocusedHost();
                if (focusHost != null) {
                    final View focusChild = getAccessibilityFocusedChild(focusHost);
                    if (focusChild != null) {
                        if (!dataChanged || isDirectChildHeaderOrFooter(focusChild)
                                || focusChild.hasTransientState() || mAdapterHasStableIds) {
                            // The views won't be changing, so try to maintain
                            // focus on the current host and virtual view.
                            accessibilityFocusLayoutRestoreView = focusHost;
                            accessibilityFocusLayoutRestoreNode = viewRootImpl
                                    .getAccessibilityFocusedVirtualView();
                        }

                        // If all else fails, maintain focus at the same
                        // position.
                        accessibilityFocusPosition = getPositionForView(focusChild);
                    }
                }
            }

            View focusLayoutRestoreDirectChild = null;
            View focusLayoutRestoreView = null;

            // Take focus back to us temporarily to avoid the eventual call to
            // clear focus when removing the focused child below from messing
            // things up when ViewAncestor assigns focus back to someone else.
            final View focusedChild = getFocusedChild();
            if (focusedChild != null) {
                // TODO: in some cases focusedChild.getParent() == null

                // We can remember the focused view to restore after re-layout
                // if the data hasn't changed, or if the focused position is a
                // header or footer.
                if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild)
                        || focusedChild.hasTransientState() || mAdapterHasStableIds) {
                    focusLayoutRestoreDirectChild = focusedChild;
                    // Remember the specific view that had focus.
                    focusLayoutRestoreView = findFocus();
                    if (focusLayoutRestoreView != null) {
                        // Tell it we are going to mess with it.
                        focusLayoutRestoreView.dispatchStartTemporaryDetach();
                    }
                }
                requestFocus();
            }

            // Pull all children into the RecycleBin.
            // These views will be reused if possible
            final int firstPosition = mFirstPosition;
            final RecycleBin recycleBin = mRecycler;
            if (dataChanged) {
                for (int i = 0; i < childCount; i++) {
                    recycleBin.addScrapView(getChildAt(i), firstPosition+i);
                }
            } else {
                recycleBin.fillActiveViews(childCount, firstPosition);
            }

            // Clear out old views
            detachAllViewsFromParent();
            recycleBin.removeSkippedScrap();

            switch (mLayoutMode) {
            case LAYOUT_SET_SELECTION:
                if (newSel != null) {
                    sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom);
                } else {
                    sel = fillFromMiddle(childrenTop, childrenBottom);
                }
                break;
            case LAYOUT_SYNC:
                sel = fillSpecific(mSyncPosition, mSpecificTop);
                break;
            case LAYOUT_FORCE_BOTTOM:
                sel = fillUp(mItemCount - 1, childrenBottom);
                adjustViewsUpOrDown();
                break;
            case LAYOUT_FORCE_TOP:
                mFirstPosition = 0;
                sel = fillFromTop(childrenTop);
                adjustViewsUpOrDown();
                break;
            case LAYOUT_SPECIFIC:
                final int selectedPosition = reconcileSelectedPosition();
                sel = fillSpecific(selectedPosition, mSpecificTop);
                /**
                 * When ListView is resized, FocusSelector requests an async selection for the
                 * previously focused item to make sure it is still visible. If the item is not
                 * selectable, it won't regain focus so instead we call FocusSelector
                 * to directly request focus on the view after it is visible.
                 */
                if (sel == null && mFocusSelector != null) {
                    final Runnable focusRunnable = mFocusSelector
                            .setupFocusIfValid(selectedPosition);
                    if (focusRunnable != null) {
                        post(focusRunnable);
                    }
                }
                break;
            case LAYOUT_MOVE_SELECTION:
                sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom);
                break;
            default:
                if (childCount == 0) {
                    if (!mStackFromBottom) {
                        final int position = lookForSelectablePosition(0, true);
                        setSelectedPositionInt(position);
                        sel = fillFromTop(childrenTop);
                    } else {
                        final int position = lookForSelectablePosition(mItemCount - 1, false);
                        setSelectedPositionInt(position);
                        sel = fillUp(mItemCount - 1, childrenBottom);
                    }
                } else {
                    if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {
                        sel = fillSpecific(mSelectedPosition,
                                oldSel == null ? childrenTop : oldSel.getTop());
                    } else if (mFirstPosition < mItemCount) {
                        sel = fillSpecific(mFirstPosition,
                                oldFirst == null ? childrenTop : oldFirst.getTop());
                    } else {
                        sel = fillSpecific(0, childrenTop);
                    }
                }
                break;
            }

            // Flush any cached views that did not get reused above
            recycleBin.scrapActiveViews();

            // remove any header/footer that has been temp detached and not re-attached
            removeUnusedFixedViews(mHeaderViewInfos);
            removeUnusedFixedViews(mFooterViewInfos);

            if (sel != null) {
                // The current selected item should get focus if items are
                // focusable.
                if (mItemsCanFocus && hasFocus() && !sel.hasFocus()) {
                    final boolean focusWasTaken = (sel == focusLayoutRestoreDirectChild &&
                            focusLayoutRestoreView != null &&
                            focusLayoutRestoreView.requestFocus()) || sel.requestFocus();
                    if (!focusWasTaken) {
                        // Selected item didn't take focus, but we still want to
                        // make sure something else outside of the selected view
                        // has focus.
                        final View focused = getFocusedChild();
                        if (focused != null) {
                            focused.clearFocus();
                        }
                        positionSelector(INVALID_POSITION, sel);
                    } else {
                        sel.setSelected(false);
                        mSelectorRect.setEmpty();
                    }
                } else {
                    positionSelector(INVALID_POSITION, sel);
                }
                mSelectedTop = sel.getTop();
            } else {
                final boolean inTouchMode = mTouchMode == TOUCH_MODE_TAP
                        || mTouchMode == TOUCH_MODE_DONE_WAITING;
                if (inTouchMode) {
                    // If the user's finger is down, select the motion position.
                    final View child = getChildAt(mMotionPosition - mFirstPosition);
                    if (child != null) {
                        positionSelector(mMotionPosition, child);
                    }
                } else if (mSelectorPosition != INVALID_POSITION) {
                    // If we had previously positioned the selector somewhere,
                    // put it back there. It might not match up with the data,
                    // but it's transitioning out so it's not a big deal.
                    final View child = getChildAt(mSelectorPosition - mFirstPosition);
                    if (child != null) {
                        positionSelector(mSelectorPosition, child);
                    }
                } else {
                    // Otherwise, clear selection.
                    mSelectedTop = 0;
                    mSelectorRect.setEmpty();
                }

                // Even if there is not selected position, we may need to
                // restore focus (i.e. something focusable in touch mode).
                if (hasFocus() && focusLayoutRestoreView != null) {
                    focusLayoutRestoreView.requestFocus();
                }
            }

            // Attempt to restore accessibility focus, if necessary.
            if (viewRootImpl != null) {
                final View newAccessibilityFocusedView = viewRootImpl.getAccessibilityFocusedHost();
                if (newAccessibilityFocusedView == null) {
                    if (accessibilityFocusLayoutRestoreView != null
                            && accessibilityFocusLayoutRestoreView.isAttachedToWindow()) {
                        final AccessibilityNodeProvider provider =
                                accessibilityFocusLayoutRestoreView.getAccessibilityNodeProvider();
                        if (accessibilityFocusLayoutRestoreNode != null && provider != null) {
                            final int virtualViewId = AccessibilityNodeInfo.getVirtualDescendantId(
                                    accessibilityFocusLayoutRestoreNode.getSourceNodeId());
                            provider.performAction(virtualViewId,
                                    AccessibilityNodeInfo.ACTION_ACCESSIBILITY_FOCUS, null);
                        } else {
                            accessibilityFocusLayoutRestoreView.requestAccessibilityFocus();
                        }
                    } else if (accessibilityFocusPosition != INVALID_POSITION) {
                        // Bound the position within the visible children.
                        final int position = MathUtils.constrain(
                                accessibilityFocusPosition - mFirstPosition, 0,
                                getChildCount() - 1);
                        final View restoreView = getChildAt(position);
                        if (restoreView != null) {
                            restoreView.requestAccessibilityFocus();
                        }
                    }
                }
            }

            // Tell focus view we are done mucking with it, if it is still in
            // our view hierarchy.
            if (focusLayoutRestoreView != null
                    && focusLayoutRestoreView.getWindowToken() != null) {
                focusLayoutRestoreView.dispatchFinishTemporaryDetach();
            }
            
            mLayoutMode = LAYOUT_NORMAL;
            mDataChanged = false;
            if (mPositionScrollAfterLayout != null) {
                post(mPositionScrollAfterLayout);
                mPositionScrollAfterLayout = null;
            }
            mNeedSync = false;
            setNextSelectedPositionInt(mSelectedPosition);

            updateScrollIndicators();

            if (mItemCount > 0) {
                checkSelectionChanged();
            }

            invokeOnItemScrollListener();
        } finally {
            if (mFocusSelector != null) {
                mFocusSelector.onLayoutComplete();
            }
            if (!blockLayoutRequests) {
                mBlockLayoutRequests = false;
            }
        }
    }

同样的调用 getChildCount() 方法来获取子 View 的数量，此时得到的值已经不是0了，而是一屏的 Listview 中子 view 的数量；调用 RecycleBin 的fillActiveViews() 方法，会将所有的子 view 都缓存到 RecycleBin 的 mActiveViews 数组中，后面将会用到。

接下来一个很重要的操作，调用 detachAllViewsFromParent() 方法，这个方法的作用是将 ListView 中所有的子 View 全部清除掉，从而保证第二次 layout 过程不会产生重复的数据。以后所有的 View 都会从 RecycleBin 的 mActiveViews 中获取。

接下来会进入到 mLayoutMode 的switch 判断中，走 default 情况，此时 childCount 不为0，会走到 else 里，里面有三个逻辑判断，第一个不成立，因为默认情况下我们没有选择任何子元素，mSeletedPosition 应该等于-1；第二个逻辑判断成立，那么进入到fillSpecific()方法当中，代码如下所示：

/**
     * Put a specific item at a specific location on the screen and then build
     * up and down from there.
     *
     * @param position The reference view to use as the starting point
     * @param top Pixel offset from the top of this view to the top of the
     *        reference view.
     *
     * @return The selected view, or null if the selected view is outside the
     *         visible area.
     */
    private View fillSpecific(int position, int top) {
        boolean tempIsSelected = position == mSelectedPosition;
        View temp = makeAndAddView(position, top, true, mListPadding.left, tempIsSelected);
        // Possibly changed again in fillUp if we add rows above this one.
        mFirstPosition = position;

        View above;
        View below;

        final int dividerHeight = mDividerHeight;
        if (!mStackFromBottom) {
            above = fillUp(position - 1, temp.getTop() - dividerHeight);
            // This will correct for the top of the first view not touching the top of the list
            adjustViewsUpOrDown();
            below = fillDown(position + 1, temp.getBottom() + dividerHeight);
            int childCount = getChildCount();
            if (childCount > 0) {
                correctTooHigh(childCount);
            }
        } else {
            below = fillDown(position + 1, temp.getBottom() + dividerHeight);
            // This will correct for the bottom of the last view not touching the bottom of the list
            adjustViewsUpOrDown();
            above = fillUp(position - 1, temp.getTop() - dividerHeight);
            int childCount = getChildCount();
            if (childCount > 0) {
                 correctTooLow(childCount);
            }
        }

        if (tempIsSelected) {
            return temp;
        } else if (above != null) {
            return above;
        } else {
            return below;
        }
    }

fillSpecific 主要作用是 优先将指定位置的子 View 先加载到屏幕上，然后再加载该子 view 往上以及往下的其他子 View。在这个方法中再次调用了 makeAndAddView() 方法，如下：

/**
     * Obtains the view and adds it to our list of children. The view can be
     * made fresh, converted from an unused view, or used as is if it was in
     * the recycle bin.
     *
     * @param position logical position in the list
     * @param y top or bottom edge of the view to add
     * @param flow {@code true} to align top edge to y, {@code false} to align
     *             bottom edge to y
     * @param childrenLeft left edge where children should be positioned
     * @param selected {@code true} if the position is selected, {@code false}
     *                 otherwise
     * @return the view that was added
     */
    private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
            boolean selected) {
        if (!mDataChanged) {
            // Try to use an existing view for this position.
            final View activeView = mRecycler.getActiveView(position);
            if (activeView != null) {
                // Found it. We're reusing an existing child, so it just needs
                // to be positioned like a scrap view.
                setupChild(activeView, position, y, flow, childrenLeft, selected, true);
                return activeView;
            }
        }

        // Make a new view for this position, or convert an unused view if
        // possible.
        final View child = obtainView(position, mIsScrap);

        // This needs to be positioned and measured.
        setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);

        return child;
    }

仍然是调用 RecycleBin 中的 Active View，这次active View 不为 null，因为前面我们调用了 RecycleBin 的 fillActiveViews() 方法来缓存子View。那么既然如此，就不会再进入到 obtainView() 方法，而是会直接进入 setupChild() 方法当中，这样也省去了很多时间，因为如果在 obtainView() 方法中又要去 infalte 布局的话，那么 ListView 的初始加载效率就大大降低了。

注意，在 setupChild() 方法中最后一个参数传入是 true，表明当前 View 是之前被回收过的，代码如下：

/**
     * Adds a view as a child and make sure it is measured (if necessary) and
     * positioned properly.
     *
     * @param child the view to add
     * @param position the position of this child
     * @param y the y position relative to which this view will be positioned
     * @param flowDown {@code true} to align top edge to y, {@code false} to
     *                 align bottom edge to y
     * @param childrenLeft left edge where children should be positioned
     * @param selected {@code true} if the position is selected, {@code false}
     *                 otherwise
     * @param isAttachedToWindow {@code true} if the view is already attached
     *                           to the window, e.g. whether it was reused, or
     *                           {@code false} otherwise
     */
    private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,
            boolean selected, boolean isAttachedToWindow) {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "setupListItem");

        final boolean isSelected = selected && shouldShowSelector();
        final boolean updateChildSelected = isSelected != child.isSelected();
        final int mode = mTouchMode;
        final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL
                && mMotionPosition == position;
        final boolean updateChildPressed = isPressed != child.isPressed();
        final boolean needToMeasure = !isAttachedToWindow || updateChildSelected
                || child.isLayoutRequested();

        // Respect layout params that are already in the view. Otherwise make
        // some up...
        AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
        if (p == null) {
            p = (AbsListView.LayoutParams) generateDefaultLayoutParams();
        }
        p.viewType = mAdapter.getItemViewType(position);
        p.isEnabled = mAdapter.isEnabled(position);

        // Set up view state before attaching the view, since we may need to
        // rely on the jumpDrawablesToCurrentState() call that occurs as part
        // of view attachment.
        if (updateChildSelected) {
            child.setSelected(isSelected);
        }

        if (updateChildPressed) {
            child.setPressed(isPressed);
        }

        if (mChoiceMode != CHOICE_MODE_NONE && mCheckStates != null) {
            if (child instanceof Checkable) {
                ((Checkable) child).setChecked(mCheckStates.get(position));
            } else if (getContext().getApplicationInfo().targetSdkVersion
                    >= android.os.Build.VERSION_CODES.HONEYCOMB) {
                child.setActivated(mCheckStates.get(position));
            }
        }

        if ((isAttachedToWindow && !p.forceAdd) || (p.recycledHeaderFooter
                && p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {
            attachViewToParent(child, flowDown ? -1 : 0, p);

            // If the view was previously attached for a different position,
            // then manually jump the drawables.
            if (isAttachedToWindow
                    && (((AbsListView.LayoutParams) child.getLayoutParams()).scrappedFromPosition)
                            != position) {
                child.jumpDrawablesToCurrentState();
            }
        } else {
            p.forceAdd = false;
            if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
                p.recycledHeaderFooter = true;
            }
            addViewInLayout(child, flowDown ? -1 : 0, p, true);
            // add view in layout will reset the RTL properties. We have to re-resolve them
            child.resolveRtlPropertiesIfNeeded();
        }

        if (needToMeasure) {
            final int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,
                    mListPadding.left + mListPadding.right, p.width);
            final int lpHeight = p.height;
            final int childHeightSpec;
            if (lpHeight > 0) {
                childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);
            } else {
                childHeightSpec = MeasureSpec.makeSafeMeasureSpec(getMeasuredHeight(),
                        MeasureSpec.UNSPECIFIED);
            }
            child.measure(childWidthSpec, childHeightSpec);
        } else {
            cleanupLayoutState(child);
        }

        final int w = child.getMeasuredWidth();
        final int h = child.getMeasuredHeight();
        final int childTop = flowDown ? y : y - h;

        if (needToMeasure) {
            final int childRight = childrenLeft + w;
            final int childBottom = childTop + h;
            child.layout(childrenLeft, childTop, childRight, childBottom);
        } else {
            child.offsetLeftAndRight(childrenLeft - child.getLeft());
            child.offsetTopAndBottom(childTop - child.getTop());
        }

        if (mCachingStarted && !child.isDrawingCacheEnabled()) {
            child.setDrawingCacheEnabled(true);
        }

        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }

可以看到 ，setupChild() 方法的最后一个参数是 isAttachedToWindow，在接下来的代码中，对这个变量进行判断，会调用 attchViewToParent()方法，我们第一次调用的方法是 addViewInLayout()。这个两个方法的最大区别在于：

• 如果向 ViewGroup 中添加一个新的子 View ，应该调用 addViewInLayout() 方法；
• 如果想要将一个之前的 detach 的View 重新 attach 到 ViewGroup 上，应该调用 attachViewToParent() 方法。

前面在 layoutChildren() 方法当中调用了 detachAllViewsFromParent() 方法，这样ListView中所有的子 View 都是处于 detach 状态的，所以这里 attachViewToParent() 方法是正确的选择。

到此为止，第二次 Layout 结束。

滑动加载更多数据

由于滑动机制属于通用的，我们去 AbsListView 的 onTouchEvent() 方法中去一探究竟，代码如下：

@Override
    public boolean onTouchEvent(MotionEvent ev) {
        if (!isEnabled()) {
            // A disabled view that is clickable still consumes the touch
            // events, it just doesn't respond to them.
            return isClickable() || isLongClickable();
        }

        if (mPositionScroller != null) {
            mPositionScroller.stop();
        }

        if (mIsDetaching || !isAttachedToWindow()) {
            // Something isn't right.
            // Since we rely on being attached to get data set change notifications,
            // don't risk doing anything where we might try to resync and find things
            // in a bogus state.
            return false;
        }

        startNestedScroll(SCROLL_AXIS_VERTICAL);

        if (mFastScroll != null && mFastScroll.onTouchEvent(ev)) {
            return true;
        }

        initVelocityTrackerIfNotExists();
        final MotionEvent vtev = MotionEvent.obtain(ev);

        final int actionMasked = ev.getActionMasked();
        if (actionMasked == MotionEvent.ACTION_DOWN) {
            mNestedYOffset = 0;
        }
        vtev.offsetLocation(0, mNestedYOffset);
        switch (actionMasked) {
            case MotionEvent.ACTION_DOWN: {
                onTouchDown(ev);
                break;
            }

            case MotionEvent.ACTION_MOVE: {
                onTouchMove(ev, vtev);
                break;
            }

            case MotionEvent.ACTION_UP: {
                onTouchUp(ev);
                break;
            }

            case MotionEvent.ACTION_CANCEL: {
                onTouchCancel();
                break;
            }

            case MotionEvent.ACTION_POINTER_UP: {
                onSecondaryPointerUp(ev);
                final int x = mMotionX;
                final int y = mMotionY;
                final int motionPosition = pointToPosition(x, y);
                if (motionPosition >= 0) {
                    // Remember where the motion event started
                    final View child = getChildAt(motionPosition - mFirstPosition);
                    mMotionViewOriginalTop = child.getTop();
                    mMotionPosition = motionPosition;
                }
                mLastY = y;
                break;
            }

            case MotionEvent.ACTION_POINTER_DOWN: {
                // New pointers take over dragging duties
                final int index = ev.getActionIndex();
                final int id = ev.getPointerId(index);
                final int x = (int) ev.getX(index);
                final int y = (int) ev.getY(index);
                mMotionCorrection = 0;
                mActivePointerId = id;
                mMotionX = x;
                mMotionY = y;
                final int motionPosition = pointToPosition(x, y);
                if (motionPosition >= 0) {
                    // Remember where the motion event started
                    final View child = getChildAt(motionPosition - mFirstPosition);
                    mMotionViewOriginalTop = child.getTop();
                    mMotionPosition = motionPosition;
                }
                mLastY = y;
                break;
            }
        }

        if (mVelocityTracker != null) {
            mVelocityTracker.addMovement(vtev);
        }
        vtev.recycle();
        return true;
    }

private void onTouchMove(MotionEvent ev, MotionEvent vtev) {
        if (mHasPerformedLongPress) {
            // Consume all move events following a successful long press.
            return;
        }

        int pointerIndex = ev.findPointerIndex(mActivePointerId);
        if (pointerIndex == -1) {
            pointerIndex = 0;
            mActivePointerId = ev.getPointerId(pointerIndex);
        }

        if (mDataChanged) {
            // Re-sync everything if data has been changed
            // since the scroll operation can query the adapter.
            layoutChildren();
        }

        final int y = (int) ev.getY(pointerIndex);

        switch (mTouchMode) {
            case TOUCH_MODE_DOWN:
            case TOUCH_MODE_TAP:
            case TOUCH_MODE_DONE_WAITING:
                // Check if we have moved far enough that it looks more like a
                // scroll than a tap. If so, we'll enter scrolling mode.
                if (startScrollIfNeeded((int) ev.getX(pointerIndex), y, vtev)) {
                    break;
                }
                // Otherwise, check containment within list bounds. If we're
                // outside bounds, cancel any active presses.
                final View motionView = getChildAt(mMotionPosition - mFirstPosition);
                final float x = ev.getX(pointerIndex);
                if (!pointInView(x, y, mTouchSlop)) {
                    setPressed(false);
                    if (motionView != null) {
                        motionView.setPressed(false);
                    }
                    removeCallbacks(mTouchMode == TOUCH_MODE_DOWN ?
                            mPendingCheckForTap : mPendingCheckForLongPress);
                    mTouchMode = TOUCH_MODE_DONE_WAITING;
                    updateSelectorState();
                } else if (motionView != null) {
                    // Still within bounds, update the hotspot.
                    final float[] point = mTmpPoint;
                    point[0] = x;
                    point[1] = y;
                    transformPointToViewLocal(point, motionView);
                    motionView.drawableHotspotChanged(point[0], point[1]);
                }
                break;
            case TOUCH_MODE_SCROLL:
            case TOUCH_MODE_OVERSCROLL:
                scrollIfNeeded((int) ev.getX(pointerIndex), y, vtev);
                break;
        }
    }

手指在屏幕上滑动时，TouchMode 是 TOUCH_HOME_SCROLL 这个值，因此继续寻找，进入 scrollIfNeeded() 方法，代码如下：

private void scrollIfNeeded(int x, int y, MotionEvent vtev) {
        int rawDeltaY = y - mMotionY;
        int scrollOffsetCorrection = 0;
        int scrollConsumedCorrection = 0;
        if (mLastY == Integer.MIN_VALUE) {
            rawDeltaY -= mMotionCorrection;
        }
        if (dispatchNestedPreScroll(0, mLastY != Integer.MIN_VALUE ? mLastY - y : -rawDeltaY,
                mScrollConsumed, mScrollOffset)) {
            rawDeltaY += mScrollConsumed[1];
            scrollOffsetCorrection = -mScrollOffset[1];
            scrollConsumedCorrection = mScrollConsumed[1];
            if (vtev != null) {
                vtev.offsetLocation(0, mScrollOffset[1]);
                mNestedYOffset += mScrollOffset[1];
            }
        }
        final int deltaY = rawDeltaY;
        int incrementalDeltaY =
                mLastY != Integer.MIN_VALUE ? y - mLastY + scrollConsumedCorrection : deltaY;
        int lastYCorrection = 0;

        if (mTouchMode == TOUCH_MODE_SCROLL) {
            if (PROFILE_SCROLLING) {
                if (!mScrollProfilingStarted) {
                    Debug.startMethodTracing("AbsListViewScroll");
                    mScrollProfilingStarted = true;
                }
            }

            if (mScrollStrictSpan == null) {
                // If it's non-null, we're already in a scroll.
                mScrollStrictSpan = StrictMode.enterCriticalSpan("AbsListView-scroll");
            }

            if (y != mLastY) {
                // We may be here after stopping a fling and continuing to scroll.
                // If so, we haven't disallowed intercepting touch events yet.
                // Make sure that we do so in case we're in a parent that can intercept.
                if ((mGroupFlags & FLAG_DISALLOW_INTERCEPT) == 0 &&
                        Math.abs(rawDeltaY) > mTouchSlop) {
                    final ViewParent parent = getParent();
                    if (parent != null) {
                        parent.requestDisallowInterceptTouchEvent(true);
                    }
                }

                final int motionIndex;
                if (mMotionPosition >= 0) {
                    motionIndex = mMotionPosition - mFirstPosition;
                } else {
                    // If we don't have a motion position that we can reliably track,
                    // pick something in the middle to make a best guess at things below.
                    motionIndex = getChildCount() / 2;
                }

                int motionViewPrevTop = 0;
                View motionView = this.getChildAt(motionIndex);
                if (motionView != null) {
                    motionViewPrevTop = motionView.getTop();
                }

                // No need to do all this work if we're not going to move anyway
                boolean atEdge = false;
                if (incrementalDeltaY != 0) {
                    atEdge = trackMotionScroll(deltaY, incrementalDeltaY);
                }

                // Check to see if we have bumped into the scroll limit
                motionView = this.getChildAt(motionIndex);
                if (motionView != null) {
                    // Check if the top of the motion view is where it is
                    // supposed to be
                    final int motionViewRealTop = motionView.getTop();
                    if (atEdge) {
                        // Apply overscroll

                        int overscroll = -incrementalDeltaY -
                                (motionViewRealTop - motionViewPrevTop);
                        if (dispatchNestedScroll(0, overscroll - incrementalDeltaY, 0, overscroll,
                                mScrollOffset)) {
                            lastYCorrection -= mScrollOffset[1];
                            if (vtev != null) {
                                vtev.offsetLocation(0, mScrollOffset[1]);
                                mNestedYOffset += mScrollOffset[1];
                            }
                        } else {
                            final boolean atOverscrollEdge = overScrollBy(0, overscroll,
                                    0, mScrollY, 0, 0, 0, mOverscrollDistance, true);

                            if (atOverscrollEdge && mVelocityTracker != null) {
                                // Don't allow overfling if we're at the edge
                                mVelocityTracker.clear();
                            }

                            final int overscrollMode = getOverScrollMode();
                            if (overscrollMode == OVER_SCROLL_ALWAYS ||
                                    (overscrollMode == OVER_SCROLL_IF_CONTENT_SCROLLS &&
                                            !contentFits())) {
                                if (!atOverscrollEdge) {
                                    mDirection = 0; // Reset when entering overscroll.
                                    mTouchMode = TOUCH_MODE_OVERSCROLL;
                                }
                                if (incrementalDeltaY > 0) {
                                    mEdgeGlowTop.onPull((float) -overscroll / getHeight(),
                                            (float) x / getWidth());
                                    if (!mEdgeGlowBottom.isFinished()) {
                                        mEdgeGlowBottom.onRelease();
                                    }
                                    invalidateTopGlow();
                                } else if (incrementalDeltaY < 0) {
                                    mEdgeGlowBottom.onPull((float) overscroll / getHeight(),
                                            1.f - (float) x / getWidth());
                                    if (!mEdgeGlowTop.isFinished()) {
                                        mEdgeGlowTop.onRelease();
                                    }
                                    invalidateBottomGlow();
                                }
                            }
                        }
                    }
                    mMotionY = y + lastYCorrection + scrollOffsetCorrection;
                }
                mLastY = y + lastYCorrection + scrollOffsetCorrection;
            }
        } else if (mTouchMode == TOUCH_MODE_OVERSCROLL) {
            if (y != mLastY) {
                final int oldScroll = mScrollY;
                final int newScroll = oldScroll - incrementalDeltaY;
                int newDirection = y > mLastY ? 1 : -1;

                if (mDirection == 0) {
                    mDirection = newDirection;
                }

                int overScrollDistance = -incrementalDeltaY;
                if ((newScroll < 0 && oldScroll >= 0) || (newScroll > 0 && oldScroll <= 0)) {
                    overScrollDistance = -oldScroll;
                    incrementalDeltaY += overScrollDistance;
                } else {
                    incrementalDeltaY = 0;
                }

                if (overScrollDistance != 0) {
                    overScrollBy(0, overScrollDistance, 0, mScrollY, 0, 0,
                            0, mOverscrollDistance, true);
                    final int overscrollMode = getOverScrollMode();
                    if (overscrollMode == OVER_SCROLL_ALWAYS ||
                            (overscrollMode == OVER_SCROLL_IF_CONTENT_SCROLLS &&
                                    !contentFits())) {
                        if (rawDeltaY > 0) {
                            mEdgeGlowTop.onPull((float) overScrollDistance / getHeight(),
                                    (float) x / getWidth());
                            if (!mEdgeGlowBottom.isFinished()) {
                                mEdgeGlowBottom.onRelease();
                            }
                            invalidateTopGlow();
                        } else if (rawDeltaY < 0) {
                            mEdgeGlowBottom.onPull((float) overScrollDistance / getHeight(),
                                    1.f - (float) x / getWidth());
                            if (!mEdgeGlowTop.isFinished()) {
                                mEdgeGlowTop.onRelease();
                            }
                            invalidateBottomGlow();
                        }
                    }
                }

                if (incrementalDeltaY != 0) {
                    // Coming back to 'real' list scrolling
                    if (mScrollY != 0) {
                        mScrollY = 0;
                        invalidateParentIfNeeded();
                    }

                    trackMotionScroll(incrementalDeltaY, incrementalDeltaY);

                    mTouchMode = TOUCH_MODE_SCROLL;

                    // We did not scroll the full amount. Treat this essentially like the
                    // start of a new touch scroll
                    final int motionPosition = findClosestMotionRow(y);

                    mMotionCorrection = 0;
                    View motionView = getChildAt(motionPosition - mFirstPosition);
                    mMotionViewOriginalTop = motionView != null ? motionView.getTop() : 0;
                    mMotionY =  y + scrollOffsetCorrection;
                    mMotionPosition = motionPosition;
                }
                mLastY = y + lastYCorrection + scrollOffsetCorrection;
                mDirection = newDirection;
            }
        }
    }

调用 trackMotionScroll() 方法，只要我们的手指在屏幕上稍微有一点点移动，这个方法就会被调用多次，代码如下：

/**
     * Track a motion scroll
     *
     * @param deltaY Amount to offset mMotionView. This is the accumulated delta since the motion
     *        began. Positive numbers mean the user's finger is moving down the screen.
     * @param incrementalDeltaY Change in deltaY from the previous event.
     * @return true if we're already at the beginning/end of the list and have nothing to do.
     */
    boolean trackMotionScroll(int deltaY, int incrementalDeltaY) {
        final int childCount = getChildCount();
        if (childCount == 0) {
            return true;
        }

        final int firstTop = getChildAt(0).getTop();
        final int lastBottom = getChildAt(childCount - 1).getBottom();

        final Rect listPadding = mListPadding;

        // "effective padding" In this case is the amount of padding that affects
        // how much space should not be filled by items. If we don't clip to padding
        // there is no effective padding.
        int effectivePaddingTop = 0;
        int effectivePaddingBottom = 0;
        if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
            effectivePaddingTop = listPadding.top;
            effectivePaddingBottom = listPadding.bottom;
        }

         // FIXME account for grid vertical spacing too?
        final int spaceAbove = effectivePaddingTop - firstTop;
        final int end = getHeight() - effectivePaddingBottom;
        final int spaceBelow = lastBottom - end;

        final int height = getHeight() - mPaddingBottom - mPaddingTop;
        if (deltaY < 0) {
            deltaY = Math.max(-(height - 1), deltaY);
        } else {
            deltaY = Math.min(height - 1, deltaY);
        }

        if (incrementalDeltaY < 0) {
            incrementalDeltaY = Math.max(-(height - 1), incrementalDeltaY);
        } else {
            incrementalDeltaY = Math.min(height - 1, incrementalDeltaY);
        }

        final int firstPosition = mFirstPosition;

        // Update our guesses for where the first and last views are
        if (firstPosition == 0) {
            mFirstPositionDistanceGuess = firstTop - listPadding.top;
        } else {
            mFirstPositionDistanceGuess += incrementalDeltaY;
        }
        if (firstPosition + childCount == mItemCount) {
            mLastPositionDistanceGuess = lastBottom + listPadding.bottom;
        } else {
            mLastPositionDistanceGuess += incrementalDeltaY;
        }

        final boolean cannotScrollDown = (firstPosition == 0 &&
                firstTop >= listPadding.top && incrementalDeltaY >= 0);
        final boolean cannotScrollUp = (firstPosition + childCount == mItemCount &&
                lastBottom <= getHeight() - listPadding.bottom && incrementalDeltaY <= 0);

        if (cannotScrollDown || cannotScrollUp) {
            return incrementalDeltaY != 0;
        }

        final boolean down = incrementalDeltaY < 0;

        final boolean inTouchMode = isInTouchMode();
        if (inTouchMode) {
            hideSelector();
        }

        final int headerViewsCount = getHeaderViewsCount();
        final int footerViewsStart = mItemCount - getFooterViewsCount();

        int start = 0;
        int count = 0;

        if (down) {
            int top = -incrementalDeltaY;
            if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
                top += listPadding.top;
            }
            for (int i = 0; i < childCount; i++) {
                final View child = getChildAt(i);
                if (child.getBottom() >= top) {
                    break;
                } else {
                    count++;
                    int position = firstPosition + i;
                    if (position >= headerViewsCount && position < footerViewsStart) {
                        // The view will be rebound to new data, clear any
                        // system-managed transient state.
                        child.clearAccessibilityFocus();
                        mRecycler.addScrapView(child, position);
                    }
                }
            }
        } else {
            int bottom = getHeight() - incrementalDeltaY;
            if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
                bottom -= listPadding.bottom;
            }
            for (int i = childCount - 1; i >= 0; i--) {
                final View child = getChildAt(i);
                if (child.getTop() <= bottom) {
                    break;
                } else {
                    start = i;
                    count++;
                    int position = firstPosition + i;
                    if (position >= headerViewsCount && position < footerViewsStart) {
                        // The view will be rebound to new data, clear any
                        // system-managed transient state.
                        child.clearAccessibilityFocus();
                        mRecycler.addScrapView(child, position);
                    }
                }
            }
        }

        mMotionViewNewTop = mMotionViewOriginalTop + deltaY;

        mBlockLayoutRequests = true;

        if (count > 0) {
            detachViewsFromParent(start, count);
            mRecycler.removeSkippedScrap();
        }

        // invalidate before moving the children to avoid unnecessary invalidate
        // calls to bubble up from the children all the way to the top
        if (!awakenScrollBars()) {
           invalidate();
        }

        offsetChildrenTopAndBottom(incrementalDeltaY);

        if (down) {
            mFirstPosition += count;
        }

        final int absIncrementalDeltaY = Math.abs(incrementalDeltaY);
        if (spaceAbove < absIncrementalDeltaY || spaceBelow < absIncrementalDeltaY) {
            fillGap(down);
        }

        mRecycler.fullyDetachScrapViews();
        if (!inTouchMode && mSelectedPosition != INVALID_POSITION) {
            final int childIndex = mSelectedPosition - mFirstPosition;
            if (childIndex >= 0 && childIndex < getChildCount()) {
                positionSelector(mSelectedPosition, getChildAt(childIndex));
            }
        } else if (mSelectorPosition != INVALID_POSITION) {
            final int childIndex = mSelectorPosition - mFirstPosition;
            if (childIndex >= 0 && childIndex < getChildCount()) {
                positionSelector(INVALID_POSITION, getChildAt(childIndex));
            }
        } else {
            mSelectorRect.setEmpty();
        }

        mBlockLayoutRequests = false;

        invokeOnItemScrollListener();

        return false;
    }

这个方法接收两个参数，deltaY 表示从手指按下时的位置到当前手指位置的距离，incrementalDeltaY 表示据上次触发 event 事件手指在 Y 方向位置的改变量，这个值的正负表示用户是向上或者向下滑动，如果 incrementalDeltaY 小于0，说明是向下滑动，否者是向上滑动。

下面将会进行一个边界检测的过程，当 ListView 向下滑动的时候，就会进入一个 for 循环当中，从上而下依次获取子 View，如果子 View 的bottom 值已经小于 top 值，就说明这个子 View 已经移除屏幕，所以会调用 RecycleBin 的 addScrapView() 方法将这个 View 加入到废弃缓存当中，并将 count 计数器加1，计数器用于记录移出屏幕的子 View 的数量。那么如果是 ListView 向上滑动的话，其实过程是基本相同的，只不过变成了从下往上依次获取子 View ，然后判断该子 View 的 top 值是不是大于 bottom 值了，如果大于的话说明子 View 已经移出了屏幕，同样把它加入到废弃缓存中，并将计数器加1。

接下来根据当前计数器的值来调用 detachViewsFromParent() 方法，它的作用是把所有的移除屏幕的子 View 全部 detach 掉。紧接着调用了 offsetChildrenTopAndBottom() 方法，并将 incrementalDeltaY 作为参数，它的作用是让 ListView 中所有的子 View 都按照传入的参数值进行相应的偏移，这样就实现啦随着手指的移动，ListView的内容也会随着滚动的效果。

接下来进行判断，如果 ListView 中最后一个 View 的底部已经移入屏幕，或者 ListView 中的第一个 View 的顶部移入了屏幕，就会调用 fillGap() 方法，它的作用是用来加载屏幕外的数据。代码如下：

/**
     * Fills the gap left open by a touch-scroll. During a touch scroll, children that
     * remain on screen are shifted and the other ones are discarded. The role of this
     * method is to fill the gap thus created by performing a partial layout in the
     * empty space.
     *
     * @param down true if the scroll is going down, false if it is going up
     */
    abstract void fillGap(boolean down);

这是一个抽象的方法，我们需要去 ListView 中去寻找，代码如下：

/**
     * {@inheritDoc}
     */
    @Override
    void fillGap(boolean down) {
        final int count = getChildCount();
        if (down) {
            int paddingTop = 0;
            if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
                paddingTop = getListPaddingTop();
            }
            final int startOffset = count > 0 ? getChildAt(count - 1).getBottom() + mDividerHeight :
                    paddingTop;
            fillDown(mFirstPosition + count, startOffset);
            correctTooHigh(getChildCount());
        } else {
            int paddingBottom = 0;
            if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
                paddingBottom = getListPaddingBottom();
            }
            final int startOffset = count > 0 ? getChildAt(0).getTop() - mDividerHeight :
                    getHeight() - paddingBottom;
            fillUp(mFirstPosition - 1, startOffset);
            correctTooLow(getChildCount());
        }
    }

down 参数表示 ListView 是向下滑动还是向上滑动，如果向下滑动就调用 fillDown() 方法，如果向上滑动就调用 fillUp() 方法；我们太熟悉这两个方法，但是填充 ListView 是通过 调用 makeAndAddView() 方法来完成的，让我们仔细瞧瞧：

/**
     * Obtains the view and adds it to our list of children. The view can be
     * made fresh, converted from an unused view, or used as is if it was in
     * the recycle bin.
     *
     * @param position logical position in the list
     * @param y top or bottom edge of the view to add
     * @param flow {@code true} to align top edge to y, {@code false} to align
     *             bottom edge to y
     * @param childrenLeft left edge where children should be positioned
     * @param selected {@code true} if the position is selected, {@code false}
     *                 otherwise
     * @return the view that was added
     */
    private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
            boolean selected) {
        if (!mDataChanged) {
            // Try to use an existing view for this position.
            final View activeView = mRecycler.getActiveView(position);
            if (activeView != null) {
                // Found it. We're reusing an existing child, so it just needs
                // to be positioned like a scrap view.
                setupChild(activeView, position, y, flow, childrenLeft, selected, true);
                return activeView;
            }
        }

        // Make a new view for this position, or convert an unused view if
        // possible.
        final View child = obtainView(position, mIsScrap);

        // This needs to be positioned and measured.
        setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);

        return child;
    }

这里首先还是尝试调用 RecycleBin 的 getActiveView() 方法来获取子布局，只不过肯定是获取不到的了，因为在第二次 Layout 过程中我们已经从 mActiveViews 中获取过了数据，而根据 RecycleBin 的机制， mActiveViews 是不能够重复利用的，因此这里返回的值肯定是 null。

既然 getActiveView() 方法返回的值是 null，那么还是走到 obtainView() 方法中，代码如下：

/**
     * Gets a view and have it show the data associated with the specified
     * position. This is called when we have already discovered that the view
     * is not available for reuse in the recycle bin. The only choices left are
     * converting an old view or making a new one.
     *
     * @param position the position to display
     * @param outMetadata an array of at least 1 boolean where the first entry
     *                    will be set {@code true} if the view is currently
     *                    attached to the window, {@code false} otherwise (e.g.
     *                    newly-inflated or remained scrap for multiple layout
     *                    passes)
     *
     * @return A view displaying the data associated with the specified position
     */
    View obtainView(int position, boolean[] outMetadata) {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "obtainView");

        outMetadata[0] = false;

        // Check whether we have a transient state view. Attempt to re-bind the
        // data and discard the view if we fail.
        final View transientView = mRecycler.getTransientStateView(position);
        if (transientView != null) {
            final LayoutParams params = (LayoutParams) transientView.getLayoutParams();

            // If the view type hasn't changed, attempt to re-bind the data.
            if (params.viewType == mAdapter.getItemViewType(position)) {
                final View updatedView = mAdapter.getView(position, transientView, this);

                // If we failed to re-bind the data, scrap the obtained view.
                if (updatedView != transientView) {
                    setItemViewLayoutParams(updatedView, position);
                    mRecycler.addScrapView(updatedView, position);
                }
            }

            outMetadata[0] = true;

            // Finish the temporary detach started in addScrapView().
            transientView.dispatchFinishTemporaryDetach();
            return transientView;
        }

        final View scrapView = mRecycler.getScrapView(position);
        final View child = mAdapter.getView(position, scrapView, this);
        if (scrapView != null) {
            if (child != scrapView) {
                // Failed to re-bind the data, return scrap to the heap.
                mRecycler.addScrapView(scrapView, position);
            } else if (child.isTemporarilyDetached()) {
                outMetadata[0] = true;

                // Finish the temporary detach started in addScrapView().
                child.dispatchFinishTemporaryDetach();
            }
        }

        if (mCacheColorHint != 0) {
            child.setDrawingCacheBackgroundColor(mCacheColorHint);
        }

        if (child.getImportantForAccessibility() == IMPORTANT_FOR_ACCESSIBILITY_AUTO) {
            child.setImportantForAccessibility(IMPORTANT_FOR_ACCESSIBILITY_YES);
        }

        setItemViewLayoutParams(child, position);

        if (AccessibilityManager.getInstance(mContext).isEnabled()) {
            if (mAccessibilityDelegate == null) {
                mAccessibilityDelegate = new ListItemAccessibilityDelegate();
            }
            if (child.getAccessibilityDelegate() == null) {
                child.setAccessibilityDelegate(mAccessibilityDelegate);
            }
        }

        Trace.traceEnd(Trace.TRACE_TAG_VIEW);

        return child;
    }

这里会调用 RecycleBin 的 getScrapView() 方法来尝试从废弃缓存中获取一个 View，这个 View 是存在的，因为在 trackMotionScroll() 方法中，一旦有任何子 View 被移出屏幕，就会将它加入到废弃缓存中，而从obtainView() 方法中的逻辑来看，一旦有新的数据需要显示到屏幕上，就会尝试从废弃缓存中获取 View 。所以它们之间就形成了一个生产者和消费者的模式，那么 ListView 神奇的地方也就在这里体现出来了，不管你有任意多条数据需要显示，ListView 中的子 View 其实来来回回就那么几个，移出屏幕的子View会很快被移入屏幕的数据重新利用起来，因而不管我们加载多少数据都不会出现 OOM 的情况，甚至内存都不会有所增加。

这里获取的 scrapView,我们将它作为第二个参数放在了 Adapter 的 getView() 方法中，我们都知道 getView() 的第二个参数是 convertView，因此，第一次加载时，convertView 为 null ，需要调用 inflater 加载布局，不等于 null 就可以直接利用 convertView，因为 convertView 就是我们之间用过的 View,移出屏幕进入到废弃缓存中，现在重新拿出来使用。然后我们只需要把 convertView 中的数据更新成当前位置上应该显示的数据，那么看起来就好像是全新加载出来的一个布局一样，从缓存中拿到子View之后再调用 setupChild() 方法将它重新attach到ListView当中，因为缓存中的View也是之前从ListView中detach掉的，这部分代码就不再重复进行分析了。

到目前为止，我们终于把 ListView 的源码整个过程梳理了一遍，希望大家都能够得到帮助。

我在写这篇文章的时候，在想我们在用 BaseAdapter 的时候，为啥用 ViewHolder 呢？它到底有啥作用吗？最近在一次面试中，我想到这个问题：我们利用 ViewHolder 的目的就是减少 findViewById 的使用，减少查找次数。

还有一个问题：当我的 ListView 拥有不同类型的时候，当加载 item 时，前后两个 item 的类型不一致，那么由于 RecycleBin 机制，后面的 item 是否为空？个人的判断是 空。还没有验证。

ListView 还有很多可以考察的点啊，未完待续…

参考文章：

1.郭霖

[http://blog.csdn.net/guolin_blog/article/details/44996879]: