Make OffscreenCanvas animation in sync with its placeholder canvas's parent frame rate

third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/offscreencanvas/OffscreenCanvas.h"
 
 #include "core/dom/ExceptionCode.h"
 #include "core/fileapi/Blob.h"
 #include "core/frame/ImageBitmap.h"
 #include "core/html/ImageData.h"
@@ skipping 218 matching lines @@
     m_frameDispatcher = WTF::wrapUnique(new OffscreenCanvasFrameDispatcherImpl(
         this, m_clientId, m_sinkId, m_placeholderCanvasId, m_size.width(),
         m_size.height()));
   }
   return m_frameDispatcher.get();
 }
 
 ScriptPromise OffscreenCanvas::commit(RefPtr<StaticBitmapImage> image,
                                       bool isWebGLSoftwareRendering,
                                       ScriptState* scriptState) {
+  getOrCreateFrameDispatcher()->setNeedsBeginFrame(true);
   if (m_commitPromiseResolver) {
     if (image) {
       m_overdrawFrame = std::move(image);
       m_overdrawFrameIsWebGLSoftwareRendering = isWebGLSoftwareRendering;
     }
   } else {
     m_overdrawFrame = nullptr;
     m_commitPromiseResolver = ScriptPromiseResolver::create(scriptState);
     m_commitPromiseResolver->keepAliveWhilePending();
     doCommit(std::move(image), isWebGLSoftwareRendering);
@@ skipping 10 matching lines @@
 
 void OffscreenCanvas::beginFrame() {
   if (m_overdrawFrame) {
     // if we have an overdraw backlog, push the frame from the backlog
     // first and save the promise resolution for later.
     doCommit(std::move(m_overdrawFrame),
              m_overdrawFrameIsWebGLSoftwareRendering);
   } else if (m_commitPromiseResolver) {
     m_commitPromiseResolver->resolve();
     m_commitPromiseResolver.clear();
+    getOrCreateFrameDispatcher()->setNeedsBeginFrame(false);

[Justin Novosad, 2017/01/19 18:06:49]

Kudos for putting this in the right place.  A comment would help to explain why
we don't need this in the case where we have an overdrawFrame.

[xlai (Olivia), 2017/01/19 19:14:34]

Done.

   }
 }
 
 ScriptPromise OffscreenCanvas::convertToBlob(ScriptState* scriptState,
                                              const ImageEncodeOptions& options,
                                              ExceptionState& exceptionState) {
   if (this->isNeutered()) {
     exceptionState.throwDOMException(InvalidStateError,
                                      "OffscreenCanvas object is detached.");
     return exceptionState.reject(scriptState);
@@ skipping 40 matching lines @@
 }
 
 DEFINE_TRACE(OffscreenCanvas) {
   visitor->trace(m_context);
   visitor->trace(m_executionContext);
   visitor->trace(m_commitPromiseResolver);
   EventTargetWithInlineData::trace(visitor);
 }
 
 }  // namespace blink