requestLayout()的执行流程

每次更新View都会想到requestLayout()方法，所以想看看它的流程是怎样的。
我一直觉得，先理清楚步骤再去看源码会好很多，所以先给出requestLayout()的一个调用流程：

View#requestLayout()
ViewGroup#requestLayout()
ViewRootImpl#requestLayout()
ViewRootImpl#scheduleTraversals()
ViewRootImpl#doTraversal()
ViewRootImpl#performTraversals()

最后ViewRootImpl#performTraversals()会依次调用performMeasure() , performLayout() , performDraw()。

requestLayout()是在View中定义的，View#requestLayout()：

public void requestLayout() {
    if (mMeasureCache != null) mMeasureCache.clear();
    if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == null) {
        // Only trigger request-during-layout logic if this is the view requesting it,
        // not the views in its parent hierarchy
        ViewRootImpl viewRoot = getViewRootImpl();
        if (viewRoot != null && viewRoot.isInLayout()) {
            if (!viewRoot.requestLayoutDuringLayout(this)) {
                return;
            }
        }
        mAttachInfo.mViewRequestingLayout = this;
    }
    /**记住这里设置的标志位，非常重要...**/
    mPrivateFlags |= PFLAG_FORCE_LAYOUT;
    mPrivateFlags |= PFLAG_INVALIDATED;
    if (mParent != null && !mParent.isLayoutRequested()) {
        /**画重点，这里调用父类的requestLayout，一直往上循环...**/
        mParent.requestLayout();
    }
    if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == this) {
        mAttachInfo.mViewRequestingLayout = null;
    }
}

ViewParent是一个接口，定义了view的一些操作等方法。ViewGroup就是一个ViewParent，所以mParent.requestLayout会一直往上遍历，而终点是ViewRootImpl，ViewRootImpl#requestLayout()：

// 入口方法，接下来执行 scheduleTraversals();
public void requestLayout() {
    if (!mHandlingLayoutInLayoutRequest) {
        checkThread();
        mLayoutRequested = true;
        scheduleTraversals();
    }
}

// 会执行mTraversalRunnable，它是一个Runnable
void scheduleTraversals() {
    if (!mTraversalScheduled) {
        mTraversalScheduled = true;
        mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
        mChoreographer.postCallback(
                Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
        ......
    }
}

// 诺，TraversalRunnable 的定义。所以最后执行了doTraversal();
final class TraversalRunnable implements Runnable {
    @Override
    public void run() {
        doTraversal();
    }
}

// 重点还不在这，重点是 performTraversals();！！！！
void doTraversal() {
    if (mTraversalScheduled) {
        mTraversalScheduled = false;
        mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
        performTraversals();
        ......
    }
}

performTraversals()真的是一个很长很长的方法...，我平时写的一个类一般都不会有这么多。所以非常难去理解里面到底做了什么，所以我只是看了个大概的流程。

private void performTraversals() {
    final View host = mView;
    if (host == null || !mAdded)
        return;
        ...... // 此处省略了很多行。。。
       
        if (!mStopped || mReportNextDraw) {
            boolean focusChangedDueToTouchMode = ensureTouchModeLocally(
                    (relayoutResult&WindowManagerGlobal.RELAYOUT_RES_IN_TOUCH_MODE) != 0);
            if (focusChangedDueToTouchMode || mWidth != host.getMeasuredWidth()
                    || mHeight != host.getMeasuredHeight() || contentInsetsChanged ||
                    updatedConfiguration) {
                int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
                int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
                /**终于出现了：performMeasure**/
                // Ask host how big it wants to be
                performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
                ......
            }
        }
    } else {
        maybeHandleWindowMove(frame);
    }
    final boolean didLayout = layoutRequested && (!mStopped || mReportNextDraw);
    boolean triggerGlobalLayoutListener = didLayout
            || mAttachInfo.mRecomputeGlobalAttributes;
    if (didLayout) {
        /**这里是performLayout**/
        performLayout(lp, mWidth, mHeight);
        ......
    }
    ...... // 此处省略了很多行。。。
    if (!cancelDraw && !newSurface) {
        if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
            for (int i = 0; i < mPendingTransitions.size(); ++i) {
                mPendingTransitions.get(i).startChangingAnimations();
            }
            mPendingTransitions.clear();
        }
        /**这里是performDraw()**/
        performDraw();
    } else {
        ......
    }
    mIsInTraversal = false;
}

本来想看看performTraversals()具体执行了些什么，后来我放弃了...，我只是找出了跟View绘制相关的三个方法：performMeasure() performLayout() performDraw()。
ViewRootImpl#performMeasure()：

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

直接就调用了view的measure方法，从ViewRootImpl回到了View。View#measure()：

public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
    ......
    /**还记得requestLayout方法里有把mPrivateFlags  = PFLAG_FORCE_LAYOUT**/
    final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;
    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);
    /**依据forceLayout 和needsLayout决定是否要执行onMeasure**/
    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
            /**出现了onMeasure**/
            onMeasure(widthMeasureSpec, heightMeasureSpec);
            mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
        } 
        /**记住这里设置的标识位**/
        mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
        ......
}

调用requestLayout时，设置了标识位mPrivateFlags = PFLAG_FORCE_LAYOUT，最终这个view会根据mPrivateFlags 来判断是否要执行onMeasure方法。
那么最后还mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;，这个接下来onLayout会需要。
ViewRootImpl#performLayout()

private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
        int desiredWindowHeight) {
    ......
    final View host = mView;
    if (host == null) {
        return;
    }
    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "layout");
    try {
        host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
        ......
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
    mInLayout = false;
}

host是view，那么performLayout就执行了view的layout()，View#layout()：

public void layout(int l, int t, int r, int b) {
    ......
    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);
       ......
    }
    ......
}

这里根据条件执行onLayout，上一步执行measure时，mPrivateFlags |= PFLAG_LAYOUT_REQUIRED。
ViewRootImpl#performDraw()：

private void performDraw() {
    ......
    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "draw");
    try {
        draw(fullRedrawNeeded);
    } finally {
        mIsDrawing = false;
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
    ......
}

// 省略了很多行的代码，心好累。。。
private void draw(boolean fullRedrawNeeded) {
    ......
    mAttachInfo.mDrawingTime =
            mChoreographer.getFrameTimeNanos() / TimeUtils.NANOS_PER_MS;
    if (!dirty.isEmpty() || mIsAnimating || accessibilityFocusDirty) {
        ......
        } else {
            ......
            /**你敢信onDraw的逻辑竟然隐藏在这里？**/
            if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
                return;
            }
        }
    }
    ......
}

private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
        boolean scalingRequired, Rect dirty) {
    final Canvas canvas;
    try {
        .....
        // 意外找到了Canvas的赋值地点
        canvas = mSurface.lockCanvas(dirty);
        .....
    } catch (Surface.OutOfResourcesException e) {
        handleOutOfResourcesException(e);
        return false;
    } catch (IllegalArgumentException e) {
        mLayoutRequested = true;    // ask wm for a new surface next time.
        return false;
    }
    try {
        ......
        try {
            ......
            /**终于找到你**/
            mView.draw(canvas);
            drawAccessibilityFocusedDrawableIfNeeded(canvas);
        } finally {
           ......
        }
    } finally {
       ......
    }
    return true;
}

因此，逻辑是：performDraw()->draw()->drawSoftware()->view.draw()。
View#draw()：

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:
    * 好贴心的注释
    *      1. Draw the background （绘制背景）
    *      2. If necessary, save the canvas' layers to prepare for fading
    *      3. Draw view's content （绘制自身）
    *      4. Draw children （绘制子view）
    *      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);
        drawAutofilledHighlight(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);
        // Step 7, draw the default focus highlight
        drawDefaultFocusHighlight(canvas);
        if (debugDraw()) {
            debugDrawFocus(canvas);
        }
        // we're done...
        return;
    }

注释真好，这个方法的逻辑一下子就看懂了。

总结

当我们调用requestLayout时，会执行parent的requestLayout，最终执行到ViewRootImpl的requestLayout。
ViewRootImpl经过一系列方法的调用执行performTraversals()方法。
performTraversals()会依次执行performMeasure() performLayout() performDraw()方法。

看起来很简单，但真的会被源码转晕。而且实际调试过程中发现，View的draw()方法不一定是drawSoftware()执行的，可以看看ThreadedRenderer#draw()。
mPrivateFlags 挺重要的，会根据它的值来决定一些方法是否要调用。

其实忽略了的代码中，隐藏着很重要的逻辑，主要是对draw()方法的执行判断。因为代码通篇看下来执行requestLayout后，onMeasure(),onLayout(),onDraw()方法会依次执行，onMeasure()和onLayout()执行是可以肯定的。但是onDraw()就不一定了。这部分我还没看懂，果然源码不简单。。。

requestLayout()的执行流程

总结

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