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Commit 1e619aa8 authored by Adam Powell's avatar Adam Powell
Browse files

Describe API behavior guarantees provided by MessageQueue sync barriers 

Add a comment in ViewRootImpl.java's usage of
MessageQueue.postSyncBarrier describing the public API behavior
guarantees it supports.

Bug: 428051877

Flag: DOCS_ONLY

Change-Id: I4f9ea99be4e65142ebfe9573d08b9258fc09a546
parent 709df92c

core/java/android/view/ViewRootImpl.java

+18 −0
Original line number Original line Diff line number Diff line
@@ -3040,6 +3040,24 @@ public final class ViewRootImpl implements ViewParent, 
    void scheduleTraversals() {

    void scheduleTraversals() {

        if (!mTraversalScheduled) {

        if (!mTraversalScheduled) {

            mTraversalScheduled = true;

            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();

            mTraversalBarrier = mQueue.postSyncBarrier();

            mChoreographer.postCallback(

            mChoreographer.postCallback(

                    Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);

                    Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);