cc: Introduce BeginFrame sequence numbers and acknowledgements.

cc/surfaces/display_begin_frame_source.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 CC_SURFACES_DISPLAY_BEGIN_FRAME_SOURCE_H_
+#define CC_SURFACES_DISPLAY_BEGIN_FRAME_SOURCE_H_
+
+#include <memory>
+#include <set>
+#include <unordered_map>
+
+#include "base/macros.h"
+#include "cc/output/begin_frame_args.h"
+#include "cc/scheduler/begin_frame_source.h"
+#include "cc/surfaces/surfaces_export.h"
+
+namespace cc {
+
+class CC_SURFACES_EXPORT DisplayBeginFrameSourceClient {
+ public:
+  virtual void BeginFrameObserverStatusChanged() = 0;
+  virtual void OnBeginFrame(const BeginFrameArgs& args) = 0;
+  virtual const BeginFrameArgs& LastUsedBeginFrameArgs() const = 0;
+};
+
+// Wraps a target BeginFrameSource and keeps track of observers that have
+// finished a BeginFrame. Supports swapping the target source underneath it.
+class CC_SURFACES_EXPORT DisplayBeginFrameSource : public BeginFrameSource,
+                                                   public BeginFrameObserver {
+ public:
+  explicit DisplayBeginFrameSource(
+      std::unique_ptr<BeginFrameSource> target_source);

[Sami, 2016/12/06 12:41:07]

Sorry for the bikeshed but "target source" is breaking my brain :) How about
wrapped_source, underlying_source, main_source, original_source or
upstream_source or something like that? Feel free to tell me to get used to
target_source too.

[Eric Seckler, 2016/12/06 17:34:00]

Done.

+  ~DisplayBeginFrameSource() override;
+
+  BeginFrameSource* GetTargetSource() { return target_source_.get(); }
+  void SetClient(DisplayBeginFrameSourceClient* client) { client_ = client; }
+
+  void SetClientNeedsBeginFrames(bool needs_begin_frames);
+
+  // Returns |true| if all the source's observers completed the current frame.
+  bool ObserversFinishedFrame();

[Sami, 2016/12/06 12:41:07]

ditto: AllObservers...?

[Eric Seckler, 2016/12/06 17:34:00]

Done.

+
+  // Completes the currently active frame. The client calls this to signal
+  // that it has executed the deadline for the BeginFrame. |had_updates|
+  // indicates whether the client produced an update (draw) as a result of the
+  // frame, and |has_pending_updates| whether it still has updates pending that
+  // it could not produce during this frame.
+  void FinishFrame(bool had_updates, bool has_pending_updates);

[Sami, 2016/12/06 12:41:07]

Should we call this FinishClientFrame()? I'd like to be explicit since we're
mixing several types of frames here :)

[Eric Seckler, 2016/12/06 17:34:00]

Done.

+
+  // Swaps the current target source with the one pointed to by
+  // |begin_frame_source|. The old source is returned through the provided
+  // pointer, transferring its ownership to the caller.
+  void SwapTargetSource(std::unique_ptr<BeginFrameSource>* begin_frame_source);
+
+  // BeginFrameSource implementation.
+  void DidFinishFrame(BeginFrameObserver* obs,
+                      const BeginFrameAck& ack) override;
+  void AddObserver(BeginFrameObserver* obs) override;
+  void RemoveObserver(BeginFrameObserver* obs) override;
+  bool IsThrottled() const override;
+
+  // BeginFrameObserver implementation.
+  void OnBeginFrame(const BeginFrameArgs& args) override;
+  const BeginFrameArgs& LastUsedBeginFrameArgs() const override;
+  void OnBeginFrameSourcePausedChanged(bool paused) override;
+
+ private:
+  void ObserverStatusChanged();
+
+  std::unique_ptr<BeginFrameSource> target_source_;
+  std::set<BeginFrameObserver*> observers_;
+  BeginFrameObserverAckTracker ack_tracker_;
+  BeginFrameArgs current_begin_frame_args_;
+  bool observing_begin_frames_;
+  bool frame_active_;
+  uint64_t current_source_id_;
+  uint64_t oldest_incorporated_frame_;
+
+  DisplayBeginFrameSourceClient* client_;  // Not owned.
+  bool client_needs_begin_frames_;
+
+  DISALLOW_COPY_AND_ASSIGN(DisplayBeginFrameSource);
+};
+
+}  // namespace cc
+
+#endif  // CC_SURFACES_DISPLAY_BEGIN_FRAME_SOURCE_H_