Merge "Describe API behavior guarantees provided by MessageQueue sync barriers" into main

    void scheduleTraversals() {

        if (!mTraversalScheduled) {

            mTraversalScheduled = true;

            // The following behavior is load-bearing for public API correctness.

            // For example, the following code is defined to be correct and the

            // MessageQueue sync barrier mechanism and its usage here is

            // responsible for ensuring it:

            //

            //   textView.setText("Hello, world!");

            //   textView.getHandler().post(new Runnable() {

            //     public void run() {

            //       // This code will run after traversals have happened

            //       // and the TextView has been measured with its new text.

            //       reportNewTextWidth(textView.getWidth());

            //     }

            //   });

            //

            // Any message posted after scheduling traversals (e.g. via

            // View#requestLayout or View#invalidate) is guaranteed to run after

            // the scheduled traversals have occurred unless the message is

            // specifically "asynchronous" - see Message#setAsynchronous

            mTraversalBarrier = mQueue.postSyncBarrier();

            mChoreographer.postCallback(

                    Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);