Android View Measure源码分析

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Android View Measure源码分析

在Android Activity启动流程文章中已经介绍了，在activity的onResume之后，ViewRootImpl#setView(View view, WindowManager.LayoutParams attrs, ...)，其中view就是DecorView，ViewRootImpl#setView中就会调用requestLayout，然后调用scheduleTraversals。这是首次调用，后续view的改动都会调用到scheduleTraversals。

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//android-12.1.0_r27\frameworks\base\core\java\android\view\ViewRootImpl.java
    void scheduleTraversals() {
        if (!mTraversalScheduled) {
            mTraversalScheduled = true;
            //添加同步屏障，主线程的同步消息都会进制执行，保证vsync信号到来时TraversalRunnable能够立刻被执行
            mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
            mChoreographer.postCallback(
                    Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
            notifyRendererOfFramePending();
            pokeDrawLockIfNeeded();
        }
    }

    final TraversalRunnable mTraversalRunnable = new TraversalRunnable();

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

    void doTraversal() {
        if (mTraversalScheduled) {
            mTraversalScheduled = false;
            //删除同步屏障
            mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);

            if (mProfile) {
                Debug.startMethodTracing("ViewAncestor");
            }

            performTraversals();

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