深入源码理解View的绘制过程

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我们都知道 View 的绘制过程包括三个步骤，即 onMeasure()、onLayout()、onDraw()。我们要逐一对三个过程进行解析。

onMeasure()

作用是用于测量视图的大小。在 View 的 onMeasure() 方法中会接收两个参数 widthMeasureSpce 和 heightMeasureSpec，这两个值用于确定视图的宽度和高度的规格和大小。

MeasureSpec 的值由 specSize 和 specMode 共同组成，其中 specSize 记录的是大小，specMode 记录的是规格，specMode 由三种规格，如下：

  • EXACTLY

    表示子视图的大小由 specSize 值来决定，系统默认会按照这个规则来设置子视图的大小，我们也可以按照自己的要求来设置成任意的大小

  • AT_MOST

    表示子视图最多只能是 specSize 指定的大小，

  • UNSPECIFIED

    表示我们可以按照自己的意愿设置成任意的大小，没有限制。但这种情况平时开发中很少用到。

widthMeasureSpec 和 heightMeasureSpec 这两个值是父视图经过计算传递给子视图的，那么父视图的数据又是从哪里来的呢？我们有必要去看下源码啦。

View 的绘制过程是从 ViewRootImpl 的 performTraversals()方法开始,代码如下：

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private void performTraversals() {
  ...
  if (!mStopped || mReportNextDraw) {
                boolean focusChangedDueToTouchMode = ensureTouchModeLocally(
                        (relayoutResult&WindowManagerGlobal.RELAYOUT_RES_IN_TOUCH_MODE) != 0);
                if (focusChangedDueToTouchMode || mWidth != host.getMeasuredWidth()
                        || mHeight != host.getMeasuredHeight() || contentInsetsChanged ||
                        updatedConfiguration) {
                    // 重点地方
                    int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
                    int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
                    if (DEBUG_LAYOUT) Log.v(mTag, "Ooops, something changed!  mWidth="
                            + mWidth + " measuredWidth=" + host.getMeasuredWidth()
                            + " mHeight=" + mHeight
                            + " measuredHeight=" + host.getMeasuredHeight()
                            + " coveredInsetsChanged=" + contentInsetsChanged);
                     // Ask host how big it wants to be
                    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
                    // Implementation of weights from WindowManager.LayoutParams
                    // We just grow the dimensions as needed and re-measure if
                    // needs be
                    int width = host.getMeasuredWidth();
                    int height = host.getMeasuredHeight();
                    boolean measureAgain = false;
                    if (lp.horizontalWeight > 0.0f) {
                        width += (int) ((mWidth - width) * lp.horizontalWeight);
                        childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(width,
                                MeasureSpec.EXACTLY);
                        measureAgain = true;
                    }
                    if (lp.verticalWeight > 0.0f) {
                        height += (int) ((mHeight - height) * lp.verticalWeight);
                        childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(height,
                                MeasureSpec.EXACTLY);
                        measureAgain = true;
                    }
                    if (measureAgain) {
                        if (DEBUG_LAYOUT) Log.v(mTag,
                                "And hey let's measure once more: width=" + width
                                + " height=" + height);
                        //执行绘制过程
                        performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
                    }
                    layoutRequested = true;
                }
            }
            ...
}

在上述代码中，我们可以发现这里调用了 getRootMeasureSpec() 方法，这个方法获取 widthMeasureSpec 和 heightMeasureSpec 的值，这个方法接收两个参数，第一个参数是 window 的可使用值。第二个参数是 创建 ViewGroup 实例时布局文件中的值，它们都等于 MATCH_PARENT。getRootMeasureSpec() 方法得代码如下：

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/**
 * Figures out the measure spec for the root view in a window based on it's
 * layout params.
 *
 * @param windowSize
 *            The available width or height of the window
 *
 * @param rootDimension
 *            The layout params for one dimension (width or height) of the
 *            window.
 *
 * @return The measure spec to use to measure the root view.
 */
private static int getRootMeasureSpec(int windowSize, int rootDimension) {
    int measureSpec;
    switch (rootDimension) {
    case ViewGroup.LayoutParams.MATCH_PARENT:
        // Window can't resize. Force root view to be windowSize.
        measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
        break;
    case ViewGroup.LayoutParams.WRAP_CONTENT:
        // Window can resize. Set max size for root view.
        measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
        break;
    default:
        // Window wants to be an exact size. Force root view to be that size.
        measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
        break;
    }
    return measureSpec;
}

可以看到，这是使用 MeasureSpec.makeMeasureSpe() 方法返回一个 MeasureSpec，当 rootDimension 等于 MATCH_PARENT 时，MeasureSpec 的 specMode 是 EXACTLY。当 rootDimension 等于 WRAP_CONTENT 时，MeasureSpec 的specMode 是AT_MOST。并且 MATCH_PARENT 和 WRAP_CONTENT 时的 specSize 都是 windowSize,也就意味着根视图总是充满全屏的。

我们回到 performTraversals() 方法中，在这个方法中会调用 performMeasure() 方法，代码如下：

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private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
        try {
            mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
    }

在这个方法中会调用 View 的 measure() 方法来测量视图的大小，View 的 measure() 方法代码如下：

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/**
     * <p>
     * This is called to find out how big a view should be. The parent
     * supplies constraint information in the width and height parameters.
     * </p>
     *
     * <p>
     * The actual measurement work of a view is performed in
     * {@link #onMeasure(int, int)}, called by this method. Therefore, only
     * {@link #onMeasure(int, int)} can and must be overridden by subclasses.
     * </p>
     *
     *
     * @param widthMeasureSpec Horizontal space requirements as imposed by the
     *        parent
     * @param heightMeasureSpec Vertical space requirements as imposed by the
     *        parent
     *
     * @see #onMeasure(int, int)
     */
    public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
        boolean optical = isLayoutModeOptical(this);
        if (optical != isLayoutModeOptical(mParent)) {
            Insets insets = getOpticalInsets();
            int oWidth  = insets.left + insets.right;
            int oHeight = insets.top  + insets.bottom;
            widthMeasureSpec  = MeasureSpec.adjust(widthMeasureSpec,  optical ? -oWidth  : oWidth);
            heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
        }
        // Suppress sign extension for the low bytes
        long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
        if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);
        final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;
        // Optimize layout by avoiding an extra EXACTLY pass when the view is
        // already measured as the correct size. In API 23 and below, this
        // extra pass is required to make LinearLayout re-distribute weight.
        final boolean specChanged = widthMeasureSpec != mOldWidthMeasureSpec
                || heightMeasureSpec != mOldHeightMeasureSpec;
        final boolean isSpecExactly = MeasureSpec.getMode(widthMeasureSpec) == MeasureSpec.EXACTLY
                && MeasureSpec.getMode(heightMeasureSpec) == MeasureSpec.EXACTLY;
        final boolean matchesSpecSize = getMeasuredWidth() == MeasureSpec.getSize(widthMeasureSpec)
                && getMeasuredHeight() == MeasureSpec.getSize(heightMeasureSpec);
        final boolean needsLayout = specChanged
                && (sAlwaysRemeasureExactly || !isSpecExactly || !matchesSpecSize);
        if (forceLayout || needsLayout) {
            // first clears the measured dimension flag
            mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
            resolveRtlPropertiesIfNeeded();
            int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
            if (cacheIndex < 0 || sIgnoreMeasureCache) {
                // measure ourselves, this should set the measured dimension flag back
                // 重点来了，我们必须重写这个方法
                onMeasure(widthMeasureSpec, heightMeasureSpec);
                mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
            } else {
                long value = mMeasureCache.valueAt(cacheIndex);
                // Casting a long to int drops the high 32 bits, no mask needed
                setMeasuredDimensionRaw((int) (value >> 32), (int) value);
                mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
            }
            // flag not set, setMeasuredDimension() was not invoked, we raise
            // an exception to warn the developer
            if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
                throw new IllegalStateException("View with id " + getId() + ": "
                        + getClass().getName() + "#onMeasure() did not set the"
                        + " measured dimension by calling"
                        + " setMeasuredDimension()");
            }
            mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
        }
        mOldWidthMeasureSpec = widthMeasureSpec;
        mOldHeightMeasureSpec = heightMeasureSpec;
        mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
                (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
    }

上述代码中，measure() 方法是被final 修饰的，所以这个方法我们是不能重写的，然而 上面调用了 onMeasure() 方法，这个就是我们经常使用的方法，如下：

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protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
    }

这个方法中默认会调用 getDefaultSize() 方法来获取视图的大小，如下：

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/**
     * Utility to return a default size. Uses the supplied size if the
     * MeasureSpec imposed no constraints. Will get larger if allowed
     * by the MeasureSpec.
     *
     * @param size Default size for this view
     * @param measureSpec Constraints imposed by the parent
     * @return The size this view should be.
     */
    public static int getDefaultSize(int size, int measureSpec) {
        int result = size;
        int specMode = MeasureSpec.getMode(measureSpec);
        int specSize = MeasureSpec.getSize(measureSpec);
        switch (specMode) {
        case MeasureSpec.UNSPECIFIED:
            result = size;
            break;
        case MeasureSpec.AT_MOST:
        case MeasureSpec.EXACTLY:
            result = specSize;
            break;
        }
        return result;
    }

这里的 measureSpec 是一直从 measure() 方法中传递过来的，然后调用 MeasureSpec.getMode() 方法来获取 specMode,调用 MeasureSpec.getSize() 方法来获取 specSize。接下来进行判断，如果 specMode 是 AT_MOST 和 EXACTLY,那么返回 specSize，如果 specMode 是 UNSPECIFIED ，那么返回的是 size。之后会在 onMeasure() 方法中调用 setMeasureDimension() 方法来设定测量的大小，到此为止，一次测量过程就结束啦。

当然，如果一个布局中包含了多个子视图，每个子视图的绘制都会经历一个 measure 过程，ViewGroup 中定义了一个 measureChildren() 方法来测量子视图的大小，如下：

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/**
     * Ask all of the children of this view to measure themselves, taking into
     * account both the MeasureSpec requirements for this view and its padding.
     * We skip children that are in the GONE state The heavy lifting is done in
     * getChildMeasureSpec.
     *
     * @param widthMeasureSpec The width requirements for this view
     * @param heightMeasureSpec The height requirements for this view
     */
    protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
        final int size = mChildrenCount;
        final View[] children = mChildren;
        for (int i = 0; i < size; ++i) {
            final View child = children[i];
            if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
                measureChild(child, widthMeasureSpec, heightMeasureSpec);
            }
        }
    }

这个方法首先会遍历当前布局下的所有子视图，然后逐一调用 measureChild() 方法来测量相应视图的大小，如下：

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/**
 * Ask one of the children of this view to measure itself, taking into
 * account both the MeasureSpec requirements for this view and its padding.
 * The heavy lifting is done in getChildMeasureSpec.
 *
 * @param child The child to measure
 * @param parentWidthMeasureSpec The width requirements for this view
 * @param parentHeightMeasureSpec The height requirements for this view
 */
protected void measureChild(View child, int parentWidthMeasureSpec,
        int parentHeightMeasureSpec) {
    final LayoutParams lp = child.getLayoutParams();
    final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
            mPaddingLeft + mPaddingRight, lp.width);
    final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
            mPaddingTop + mPaddingBottom, lp.height);
    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

在这个方法中，调用 getChildMeasureSpec() 方法来计算子视图的 MeasureSpec 方法，计算的依据是布局文件中的 MATCH_PARENT、WRAP_CONTENT、paddingLeft、paddingRight 等值，在最后会调用 child.measure() 方法，这个方法就是我么上面分析的 View 的 measure()。也就是再把上面的流程再走一遍。

需要注意, 在 setMeasureDimension() 方法调用之后，我们才能使用 getMeasuredWidth() 和 getMeasuredHeight() 来获取视图测量出的宽高，以此之前调用这两个方法得到的值都会是0。

通过上面的分析，我们知道视图大小的控制是由父视图、布局文件以及视图本身共同决定的，父视图会根据 specMode 决定 自视图的大小，而我们可以在布局文件中指定视图的大小，然后视图本身会对最终的大小进行决定。

到此为止，视图绘制过程的第一个阶段就完成啦。

onLayout()

这个方法的作用是确定视图的位置。ViewRootImpl 的 performTravels() 方法在执行完 performMeasure() 方法之后，会调用 performLayout() 方法，如下：

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performLayout(lp, mWidth, mHeight);
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
            int desiredWindowHeight) {
        mLayoutRequested = false;
        mScrollMayChange = true;
        mInLayout = true;
        final View host = mView;
        if (DEBUG_ORIENTATION || DEBUG_LAYOUT) {
            Log.v(mTag, "Laying out " + host + " to (" +
                    host.getMeasuredWidth() + ", " + host.getMeasuredHeight() + ")");
        }
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "layout");
        try {
        //执行 view 的 layout方法
            host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
            mInLayout = false;
            int numViewsRequestingLayout = mLayoutRequesters.size();
            if (numViewsRequestingLayout > 0) {
                // requestLayout() was called during layout.
                // If no layout-request flags are set on the requesting views, there is no problem.
                // If some requests are still pending, then we need to clear those flags and do
                // a full request/measure/layout pass to handle this situation.
                ArrayList<View> validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters,
                        false);
                if (validLayoutRequesters != null) {
                    // Set this flag to indicate that any further requests are happening during
                    // the second pass, which may result in posting those requests to the next
                    // frame instead
                    mHandlingLayoutInLayoutRequest = true;
                    // Process fresh layout requests, then measure and layout
                    int numValidRequests = validLayoutRequesters.size();
                    for (int i = 0; i < numValidRequests; ++i) {
                        final View view = validLayoutRequesters.get(i);
                        Log.w("View", "requestLayout() improperly called by " + view +
                                " during layout: running second layout pass");
                        view.requestLayout();
                    }
                    measureHierarchy(host, lp, mView.getContext().getResources(),
                            desiredWindowWidth, desiredWindowHeight);
                    mInLayout = true;
                    host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
                    mHandlingLayoutInLayoutRequest = false;
                    // Check the valid requests again, this time without checking/clearing the
                    // layout flags, since requests happening during the second pass get noop'd
                    validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, true);
                    if (validLayoutRequesters != null) {
                        final ArrayList<View> finalRequesters = validLayoutRequesters;
                        // Post second-pass requests to the next frame
                        getRunQueue().post(new Runnable() {
                            @Override
                            public void run() {
                                int numValidRequests = finalRequesters.size();
                                for (int i = 0; i < numValidRequests; ++i) {
                                    final View view = finalRequesters.get(i);
                                    Log.w("View", "requestLayout() improperly called by " + view +
                                            " during second layout pass: posting in next frame");
                                    view.requestLayout();
                                }
                            }
                        });
                    }
                }
            }
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
        mInLayout = false;
    }

在这个方法里，调用了 View 的 layout() 方法，它的四个参数分别是左、上、右、下的坐标，那么我们来看一下 layout() 方法，如下：

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/**
     * Assign a size and position to a view and all of its
     * descendants
     *
     * <p>This is the second phase of the layout mechanism.
     * (The first is measuring). In this phase, each parent calls
     * layout on all of its children to position them.
     * This is typically done using the child measurements
     * that were stored in the measure pass().</p>
     *
     * <p>Derived classes should not override this method.
     * Derived classes with children should override
     * onLayout. In that method, they should
     * call layout on each of their children.</p>
     *
     * @param l Left position, relative to parent
     * @param t Top position, relative to parent
     * @param r Right position, relative to parent
     * @param b Bottom position, relative to parent
     */
    @SuppressWarnings({"unchecked"})
    public void layout(int l, int t, int r, int b) {
        if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
            onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
            mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
        }
        int oldL = mLeft;
        int oldT = mTop;
        int oldB = mBottom;
        int oldR = mRight;
        boolean changed = isLayoutModeOptical(mParent) ?
                setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
        if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
            onLayout(changed, l, t, r, b);
            if (shouldDrawRoundScrollbar()) {
                if(mRoundScrollbarRenderer == null) {
                    mRoundScrollbarRenderer = new RoundScrollbarRenderer(this);
                }
            } else {
                mRoundScrollbarRenderer = null;
            }
            mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
            ListenerInfo li = mListenerInfo;
            if (li != null && li.mOnLayoutChangeListeners != null) {
                ArrayList<OnLayoutChangeListener> listenersCopy =
                        (ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone();
                int numListeners = listenersCopy.size();
                for (int i = 0; i < numListeners; ++i) {
                    listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
                }
            }
        }
        mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
        mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
    }

在上述方法中，首先会调用 isLayoutModeOptical() 方法来判断父控件是 View 还是 ViewGroup，如果是 ViewGroup，那么就执行 setOpticalFrame() 方法，如果是 View，那么就执行 setFrame() 方法，通过观察我们发现 setOpticalFrame() 方法的也是调用 setFrame() 方法，因此是通过调用 setFrame() 方法来判断视图的大小是否发生过改变，来决定是否要对当前视图进行重绘。接下来会调用 onLayout() 方法，我们跟进去瞧瞧，原来 onLayout() 方法是空方法，如下：

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/**
 * Called from layout when this view should
 * assign a size and position to each of its children.
 *
 * Derived classes with children should override
 * this method and call layout on each of
 * their children.
 * @param changed This is a new size or position for this view
 * @param left Left position, relative to parent
 * @param top Top position, relative to parent
 * @param right Right position, relative to parent
 * @param bottom Bottom position, relative to parent
 */
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}

从上面的注视可以知道，onLayout() 方法是 View 的子类必须重写的一个方法，这是因为 onLayout() 方法的作用是确定视图在布局中的位置，那么这个工作应该由布局来完成，即父视图决定子视图的显示位置。

在 onLayout() 方法结束之后，我们就可以用 getWidth() 方法和 getHeight() 方法来获取视图的宽高啦，有的朋友可能分不清 getWidth() 方法和 getMeasureWidth() 方法，那么我么来详细说一下，

  • getMeasureWidth() 方法是在 measure() 过程结束后就可以获得到，它的值是经过 setMeasureDimension() 方法来设置的。
  • getWidth() 方法是在 Layout() 方法中获得的，它的值是通过视图右边的值减去左边的值计算出来的。

到此为止，View 绘制的第二阶段我们就分析完毕啦。

onDraw()

它的作用是对视图进行绘制。View 的 draw() 方法如下，

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**
     * Manually render this view (and all of its children) to the given Canvas.
     * The view must have already done a full layout before this function is
     * called.  When implementing a view, implement
     * {@link #onDraw(android.graphics.Canvas)} instead of overriding this method.
     * If you do need to override this method, call the superclass version.
     *
     * @param canvas The Canvas to which the View is rendered.
     */
    @CallSuper
    public void draw(Canvas canvas) {
        final int privateFlags = mPrivateFlags;
        final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
                (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
        mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;
        /*
         * Draw traversal performs several drawing steps which must be executed
         * in the appropriate order:
         *
         *      1. Draw the background
         *      2. If necessary, save the canvas' layers to prepare for fading
         *      3. Draw view's content
         *      4. Draw children
         *      5. If necessary, draw the fading edges and restore layers
         *      6. Draw decorations (scrollbars for instance)
         */
        // Step 1, draw the background, if needed
        int saveCount;
        if (!dirtyOpaque) {
            drawBackground(canvas);
        }
        // skip step 2 & 5 if possible (common case)
        final int viewFlags = mViewFlags;
        boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
        boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
        if (!verticalEdges && !horizontalEdges) {
            // Step 3, draw the content
            if (!dirtyOpaque) onDraw(canvas);
            // Step 4, draw the children
            dispatchDraw(canvas);
            // Overlay is part of the content and draws beneath Foreground
            if (mOverlay != null && !mOverlay.isEmpty()) {
                mOverlay.getOverlayView().dispatchDraw(canvas);
            }
            // Step 6, draw decorations (foreground, scrollbars)
            onDrawForeground(canvas);
            // we're done...
            return;
        }
        /*
         * Here we do the full fledged routine...
         * (this is an uncommon case where speed matters less,
         * this is why we repeat some of the tests that have been
         * done above)
         */
        boolean drawTop = false;
        boolean drawBottom = false;
        boolean drawLeft = false;
        boolean drawRight = false;
        float topFadeStrength = 0.0f;
        float bottomFadeStrength = 0.0f;
        float leftFadeStrength = 0.0f;
        float rightFadeStrength = 0.0f;
        // Step 2, save the canvas' layers
        int paddingLeft = mPaddingLeft;
        final boolean offsetRequired = isPaddingOffsetRequired();
        if (offsetRequired) {
            paddingLeft += getLeftPaddingOffset();
        }
        int left = mScrollX + paddingLeft;
        int right = left + mRight - mLeft - mPaddingRight - paddingLeft;
        int top = mScrollY + getFadeTop(offsetRequired);
        int bottom = top + getFadeHeight(offsetRequired);
        if (offsetRequired) {
            right += getRightPaddingOffset();
            bottom += getBottomPaddingOffset();
        }
        final ScrollabilityCache scrollabilityCache = mScrollCache;
        final float fadeHeight = scrollabilityCache.fadingEdgeLength;
        int length = (int) fadeHeight;
        // clip the fade length if top and bottom fades overlap
        // overlapping fades produce odd-looking artifacts
        if (verticalEdges && (top + length > bottom - length)) {
            length = (bottom - top) / 2;
        }
        // also clip horizontal fades if necessary
        if (horizontalEdges && (left + length > right - length)) {
            length = (right - left) / 2;
        }
        if (verticalEdges) {
            topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));
            drawTop = topFadeStrength * fadeHeight > 1.0f;
            bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));
            drawBottom = bottomFadeStrength * fadeHeight > 1.0f;
        }
        if (horizontalEdges) {
            leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));
            drawLeft = leftFadeStrength * fadeHeight > 1.0f;
            rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));
            drawRight = rightFadeStrength * fadeHeight > 1.0f;
        }
        saveCount = canvas.getSaveCount();
        int solidColor = getSolidColor();
        if (solidColor == 0) {
            final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;
            if (drawTop) {
                canvas.saveLayer(left, top, right, top + length, null, flags);
            }
            if (drawBottom) {
                canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
            }
            if (drawLeft) {
                canvas.saveLayer(left, top, left + length, bottom, null, flags);
            }
            if (drawRight) {
                canvas.saveLayer(right - length, top, right, bottom, null, flags);
            }
        } else {
            scrollabilityCache.setFadeColor(solidColor);
        }
        // Step 3, draw the content
        if (!dirtyOpaque) onDraw(canvas);
        // Step 4, draw the children
        dispatchDraw(canvas);
        // Step 5, draw the fade effect and restore layers
        final Paint p = scrollabilityCache.paint;
        final Matrix matrix = scrollabilityCache.matrix;
        final Shader fade = scrollabilityCache.shader;
        if (drawTop) {
            matrix.setScale(1, fadeHeight * topFadeStrength);
            matrix.postTranslate(left, top);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(left, top, right, top + length, p);
        }
        if (drawBottom) {
            matrix.setScale(1, fadeHeight * bottomFadeStrength);
            matrix.postRotate(180);
            matrix.postTranslate(left, bottom);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(left, bottom - length, right, bottom, p);
        }
        if (drawLeft) {
            matrix.setScale(1, fadeHeight * leftFadeStrength);
            matrix.postRotate(-90);
            matrix.postTranslate(left, top);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(left, top, left + length, bottom, p);
        }
        if (drawRight) {
            matrix.setScale(1, fadeHeight * rightFadeStrength);
            matrix.postRotate(90);
            matrix.postTranslate(right, top);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(right - length, top, right, bottom, p);
        }
        canvas.restoreToCount(saveCount);
        // Overlay is part of the content and draws beneath Foreground
        if (mOverlay != null && !mOverlay.isEmpty()) {
            mOverlay.getOverlayView().dispatchDraw(canvas);
        }
        // Step 6, draw decorations (foreground, scrollbars)
        onDrawForeground(canvas);
    }

可以看到，View 的绘制需要经过六步，但一般从第二步和第五步我们很少用到。首先，

  • 第一步是对视图的背景进行绘制。
  • 第三步是绘制内容，调用 onDraw() 方法，它的里面是一个空实现，我们必须重写该方法。
  • 第四步是对当前视图的所有子视图进行绘制，我们发现 dispatchDraw() 方法也是一个空方法，但 ViewGroup 的 dispatchDraw() 方法就会有具体的实现。
  • 第六步的作用是画滚动条，调用 onDrawForeground () 方法，在这个方法里会调用 onDrawScrollIndicators() 方法 和 onDrawScrollBars() 方法去画滚动条。

onDraw() 是我们根据具体的需求需要自己手动重写的，根据不同的视图绘制不同的内容。

到此为止，我们的 onDraw() 方法就分析完毕啦。