调用流程：ActivityThread.handleResumeActivity-->WindowManager.addView(DecorView,LayoutParams)-->WindowManagerGlobal.addView-->ViewRootImpl.setView-->requestLayout()-->scheduleTraversals()

void scheduleTraversals() {
    if (!mTraversalScheduled) {
        mTraversalScheduled = true;
        // 发送同步屏障
        mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
        // 监听，等待下一次vsync信号到来
        mChoreographer.postCallback(
                Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
        notifyRendererOfFramePending();
        pokeDrawLockIfNeeded();
    }
}

postSyncBarrier()方法用于在消息队列中插入一个同步屏障。这个屏障会暂时暂停MessageQueue中的消息处理，直到屏障被清除。确保doTraversal()优先执行

final class TraversalRunnable implements Runnable {
    @Override
    public void run() {
        doTraversal();
    }
}

void doTraversal() {
    if (mTraversalScheduled) {
        mTraversalScheduled = false;
        // 执行异步消息时，取消同步屏障
        mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);

        if (mProfile) {
            Debug.startMethodTracing("ViewAncestor");
        }
        
        // 这里面主要干了三件事，也就是View三大流程measure、layout、draw
        performTraversals();

        if (mProfile) {
            Debug.stopMethodTracing();
            mProfile = false;
        }
    }
}

看一些draw的细节

private boolean draw(boolean fullRedrawNeeded, boolean forceDraw) {
    ...
    if (!dirty.isEmpty() || mIsAnimating || accessibilityFocusDirty) {
        if (isHardwareEnabled()) {
            ...
            // 硬件draw
            mAttachInfo.mThreadedRenderer.draw(mView, mAttachInfo, this);
        } else {
            ...
            // 软件draw
            if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset,
                    scalingRequired, dirty, surfaceInsets)) {
                return false;
            }
        }
    }
    ...
}