[ui/ws] Acknowledge BeginFrames in FrameGenerator + add tests.

services/ui/ws/frame_generator.h

 // Copyright 2016 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.
 
 #ifndef SERVICES_UI_WS_FRAME_GENERATOR_H_
 #define SERVICES_UI_WS_FRAME_GENERATOR_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 66 matching lines @@
 
   // SetNeedsBeginFrame sets observing_begin_frames_ and add/remove
   // FrameGenerator as an observer to/from begin_frame_source_ accordingly.
   void SetNeedsBeginFrame(bool needs_begin_frame);
 
   ServerWindow* const root_window_;
   float device_scale_factor_ = 1.f;
 
   std::unique_ptr<cc::CompositorFrameSink> compositor_frame_sink_;
   cc::BeginFrameArgs last_begin_frame_args_;
+  cc::BeginFrameAck current_begin_frame_ack_;

[msw, 2017/03/21 17:38:57]

what's the difference between last and current?

[Eric Seckler, 2017/03/22 09:16:50]

This is args vs. ack. The ack is set and used (in GenerateCompositorFrame) while
the current BeginFrame is being processed (thus "current ack"). The args are
stored to implement the LastUsedBeginFrameArgs() interface of BeginFrameObserver
and are intended to be valid after the BeginFrame was completed (thus "last
args").

   cc::BeginFrameSource* begin_frame_source_ = nullptr;
   bool observing_begin_frames_ = false;
   bool high_contrast_mode_enabled_ = false;
 
   cc::SurfaceInfo window_manager_surface_info_;
 
   DISALLOW_COPY_AND_ASSIGN(FrameGenerator);
 };
 
 }  // namespace ws
 
 }  // namespace ui
 
 #endif  // SERVICES_UI_WS_FRAME_GENERATOR_H_