Teach SkyShell.apk to stop posting frames when not visible

sky/shell/ui/animator.cc

 // 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 "sky/shell/ui/animator.h"
 
 #include "base/bind.h"
 #include "base/message_loop/message_loop.h"
 #include "base/trace_event/trace_event.h"
 
 namespace sky {
 namespace shell {
 
 Animator::Animator(const Engine::Config& config, Engine* engine)
     : config_(config),
       engine_(engine),
       engine_requested_frame_(false),
       frame_in_progress_(false),
+      paused_(false),
       weak_factory_(this) {
 }
 
 Animator::~Animator() {
 }
 
 void Animator::RequestFrame() {
   if (engine_requested_frame_)
     return;
 
   TRACE_EVENT_ASYNC_BEGIN0("sky", "Frame request pending", this);
   engine_requested_frame_ = true;
 
   if (!frame_in_progress_) {
     frame_in_progress_ = true;
     base::MessageLoop::current()->PostTask(
         FROM_HERE,
         base::Bind(&Animator::BeginFrame, weak_factory_.GetWeakPtr()));
   }
 }
 
-void Animator::CancelFrameRequest() {
+void Animator::Stop() {
+  paused_ = true;
   engine_requested_frame_ = false;
 }
 
+void Animator::Start() {
+  paused_ = false;
+  RequestFrame();
+}
+
 void Animator::BeginFrame() {
   DCHECK(frame_in_progress_);
   // There could be a request in the message loop at time of cancel.
   if (!engine_requested_frame_) {
     frame_in_progress_ = false;
     return;
   }
 
   engine_requested_frame_ = false;
   TRACE_EVENT_ASYNC_END0("sky", "Frame request pending", this);
 
   engine_->BeginFrame(base::TimeTicks::Now());
   config_.gpu_task_runner->PostTaskAndReply(
       FROM_HERE,
       base::Bind(&GPUDelegate::Draw, config_.gpu_delegate, engine_->Paint()),
       base::Bind(&Animator::OnFrameComplete, weak_factory_.GetWeakPtr()));
 }
 
 void Animator::OnFrameComplete() {
   DCHECK(frame_in_progress_);
   frame_in_progress_ = false;
+  if (paused_)
+    return;
+
   if (engine_requested_frame_) {
     frame_in_progress_ = true;
     BeginFrame();
   }
 }
 
 }  // namespace shell
 }  // namespace sky