Refactoring the way begin frame sources inside scheduler work.

cc/scheduler/begin_frame_source.h

+// Copyright 2014 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_SCHEDULER_BEGIN_FRAME_SOURCE_H_
+#define CC_SCHEDULER_BEGIN_FRAME_SOURCE_H_
+
+#include <set>
+#include <string>
+
+#include "base/debug/trace_event.h"
+#include "base/logging.h"
+#include "cc/output/begin_frame_args.h"
+#include "cc/output/vsync_parameter_observer.h"
+#include "cc/scheduler/delay_based_time_source.h"
+
+namespace cc {
+
+// (Pure) Interface for observing BeginFrame messages from BeginFrameSource
+// objects.
+class CC_EXPORT BeginFrameObserver {
+ public:
+  // The |args| given to OnBeginFrame is guaranteed to have
+  // |args|.IsValid()==true and have |args|.frame_time
+  // field be strictly greater than the previous call.
+  //
+  // Side effects: This function can (and most of the time *will*) change the
+  // return value of the LastUsedBeginFrameArgs method. See the documentation
+  // on that method for more information.
+  virtual void OnBeginFrame(const BeginFrameArgs& args) = 0;
+
+  // Some frame sources *may* provide the last missed begin frame when
+  // SetNeedsBeginFrames is set true (such as the SyntheticBeginFrameSource).
+  //
+  // **Don't** depend on this function ever being called.
+  virtual void OnMissedBeginFrame(const BeginFrameArgs& args) = 0;
+
+  // Returns the last BeginFrameArgs used by the observer. This method's return
+  // value is affected by the OnBeginFrame method!
+  //
+  //  - Before the first call of OnBeginFrame, this method should return a
+  //    BeginFrameArgs on which IsValid() returns false.
+  //
+  //  - If the |args| passed to OnBeginFrame is (or *will be*) used, then
+  //    LastUsedBeginFrameArgs return value should become the |args| given to
+  //    OnBeginFrame.
+  //
+  //  - If the |args| passed to OnBeginFrame is dropped, then
+  //    LastUsedBeginFrameArgs return value should *not* change.
+  //
+  // These requirements are designed to allow chaining and nesting of
+  // BeginFrameObservers which filter the incoming BeginFrame messages while
+  // preventing "double dropping" and other bad side effects.
+  virtual const BeginFrameArgs LastUsedBeginFrameArgs() const = 0;
+
+  // Tracing support
+  virtual void AsValueInto(base::debug::TracedValue* dict) const = 0;
+};
+
+// Simple mix in which implements a BeginFrameObserver which checks the
+// incoming values meet the BeginFrameObserver requirements and implements the
+// required LastUsedBeginFrameArgs behaviour.
+//
+// Users of this mix in should;
+//  - Implement the OnBeginFrameMixInDelegate function.
+//  - Recommended (but not required) to call
+//    BeginFrameObserverMixIn::OnValueInto in their overridden OnValueInto
+//    function.
+//
+// Advance observers may also wish to override the OnMissedBeginFrame method.

[simonhong, 2014/09/25 06:19:46]

Advance -> Advanced?

[mithro-old, 2014/09/25 13:32:19]

Done.

+class CC_EXPORT BeginFrameObserverMixIn : public BeginFrameObserver {
+ public:
+  // BeginFrameObserver

[simonhong, 2014/09/25 06:19:46]

wrong comment.

[mithro-old, 2014/09/25 13:32:19]

Done.

+  BeginFrameObserverMixIn();
+
+  // Traces |args| and DCHECK |args| satisfies pre-conditions then calls
+  // OnBeginFrameMixInDelegate and updates the last_begin_frame_args_ value on
+  // true.
+  virtual void OnBeginFrame(const BeginFrameArgs& args) OVERRIDE;
+
+  // Forwards call to the OnBeginFrame callback.
+  virtual void OnMissedBeginFrame(const BeginFrameArgs& args) OVERRIDE;
+
+  virtual const BeginFrameArgs LastUsedBeginFrameArgs() const OVERRIDE;
+
+  // Outputs last_begin_frame_args_
+  virtual void AsValueInto(base::debug::TracedValue* dict) const OVERRIDE;
+
+ protected:
+  // Subclasses should override this method!
+  // Return true if the given argument is (or will be) used.
+  virtual bool OnBeginFrameMixInDelegate(const BeginFrameArgs& args) = 0;
+  virtual bool OnMissedBeginFrameMixInDelegate(const BeginFrameArgs& args);
+
+  BeginFrameArgs last_begin_frame_args_;
+  int64_t dropped_begin_frame_args_;
+  int64_t dropped_missed_begin_frame_args_;
+};
+
+// Interface for a class which produces BeginFrame calls to a
+// BeginFrameObserver.
+//
+// BeginFrame calls *normally* occur just after a vsync interrupt when input
+// processing has been finished and provide information about the time values
+// of the vsync times. *However*, these values can be heavily modified or even
+// plain made up (when no vsync signal is available or vsync throttling is
+// turned off). See the BeginFrameObserver for information about the guarantees
+// all BeginFrameSources *must* provide.
+//
+// Implementation classes should:
+//  - Use the CallOn(Missed)BeginFrame method to call to the observer(s).
+//  - Implement the pure virtual methods.
+//  - Recommend (but not required) to call BeginFrameSource::AsValueInto in
+//    their own AsValueInto implementation.
+class CC_EXPORT BeginFrameSource {
+ public:
+  virtual ~BeginFrameSource() {}
+
+  // SetNeedsBeginFrames is the on/off "switch" for the BeginFrameSource. When
+  // set to false no more BeginFrame messages should be sent to observer.
+  virtual bool NeedsBeginFrames() const = 0;
+  virtual void SetNeedsBeginFrames(bool needs_begin_frames) = 0;
+
+  // DidFinishFrame provides back pressure to a frame source about frame
+  // processing (rather than toggling SetNeedsBeginFrames every frame). It is
+  // used by systems like the BackToBackFrameSource to make sure only one frame
+  // is pending at a time.
+  virtual void DidFinishFrame(size_t remaining_frames) = 0;

[simonhong, 2014/09/25 06:19:46]

Does we really need this function?
I'm not sure exposing scheduler's information(the number of remained frame) to
Source is good design.
I think we can provide back pressure with SetNeedsBeginFrames()?

[mithro-old, 2014/09/25 13:32:19]

We went back and forth on this problem and eventually settled on
SetNeedsBeginFrames being level triggered (IE it's on/off). This means that we
needed another back-pressure method.

This is required for the BackToBackFrameSource and also enabled a "dynamic
BeginFrame throttler" which takes into account how many frames are pending
before sending more.

+
+  // Add/Remove and observer from the source.

[simonhong, 2014/09/25 06:19:46]

and -> an.

[mithro-old, 2014/09/25 13:32:19]

Done.

+  // *At the moment* only a single observer can be added to the source, however
+  // in the future this may be extended to allow multiple observers.
+  // If making this change, please use base::ObserverList to do so.
+  void AddObserver(BeginFrameObserver* obs);
+  void RemoveObserver(BeginFrameObserver* obs);
+
+  // Tracing support - Recommend (but not required) to call this implementation
+  // in any override.
+  virtual void AsValueInto(base::debug::TracedValue* dict) const;
+
+ protected:
+  BeginFrameSource();
+
+  // These methods should be used by subclasses to make the call to the
+  // observers.
+  void CallOnBeginFrame(const BeginFrameArgs& args);
+  void CallOnMissedBeginFrame(const BeginFrameArgs& args);
+
+  BeginFrameObserver* observer_;
+
+ private:
+  bool inside_as_value_into_;
+};
+
+// A frame source which calls BeginFrame (at the next possible time) as soon as
+// remaining frames reaches zero.
+class CC_EXPORT BackToBackBeginFrameSource : public BeginFrameSource {
+ public:
+  static scoped_ptr<BackToBackBeginFrameSource> Create(
+      base::SingleThreadTaskRunner* task_runner);
+  virtual ~BackToBackBeginFrameSource();
+
+  // BeginFrameSource
+  virtual bool NeedsBeginFrames() const OVERRIDE;
+  virtual void SetNeedsBeginFrames(bool needs_begin_frames) OVERRIDE;
+  virtual void DidFinishFrame(size_t remaining_frames) OVERRIDE;
+
+  // Tracing
+  virtual void AsValueInto(base::debug::TracedValue* dict) const OVERRIDE;
+
+ protected:
+  explicit BackToBackBeginFrameSource(
+      base::SingleThreadTaskRunner* task_runner);
+  virtual base::TimeTicks Now();  // Now overridable for testing
+
+  base::WeakPtrFactory<BackToBackBeginFrameSource> weak_factory_;
+  base::SingleThreadTaskRunner* task_runner_;
+
+  bool needs_begin_frames_;
+  bool send_begin_frame_posted_;
+
+  void ScheduleBeginFrame();
+  void BeginFrame();
+};
+
+// A frame source which is locked to an external parameters provides from a
+// vsync source and generates BeginFrameArgs for it.
+class CC_EXPORT SyntheticBeginFrameSource : public BeginFrameSource,
+                                            public VSyncParameterObserver,
+                                            public TimeSourceClient {
+ public:
+  static scoped_ptr<SyntheticBeginFrameSource> Create(
+      base::SingleThreadTaskRunner* task_runner,
+      base::TimeTicks initial_vsync_timebase,
+      base::TimeDelta initial_vsync_interval);
+  virtual ~SyntheticBeginFrameSource();
+
+  // BeginFrameSource
+  virtual bool NeedsBeginFrames() const OVERRIDE;
+  virtual void SetNeedsBeginFrames(bool needs_begin_frames) OVERRIDE;
+  virtual void DidFinishFrame(size_t remaining_frames) OVERRIDE{};
+
+  // Tracing
+  virtual void AsValueInto(base::debug::TracedValue* dict) const OVERRIDE;
+
+  // VSyncParameterObserver
+  virtual void OnUpdateVSyncParameters(
+      base::TimeTicks new_vsync_timebase,
+      base::TimeDelta new_vsync_interval) OVERRIDE;
+
+  // TimeSourceClient
+  virtual void OnTimerTick() OVERRIDE;
+
+ protected:
+  explicit SyntheticBeginFrameSource(
+      scoped_refptr<DelayBasedTimeSource> time_source);
+
+  void SendBeginFrameFromTick(base::TimeTicks frame_time);
+
+  scoped_refptr<DelayBasedTimeSource> time_source_;
+};
+
+// A "virtual" frame source which lets you switch between multiple other frame
+// sources while making sure the BeginFrameArgs stays increasing (possibly
+// enforcing minimum boundry between BeginFrameArgs messages).
+class CC_EXPORT BeginFrameSourceMultiplexer : public BeginFrameSource,
+                                              public BeginFrameObserver {
+ public:
+  static scoped_ptr<BeginFrameSourceMultiplexer> Create();
+  virtual ~BeginFrameSourceMultiplexer();
+
+  void SetMinimumInterval(base::TimeDelta new_minimum_interval);
+
+  void AddSource(BeginFrameSource* new_source);
+  void RemoveSource(BeginFrameSource* existing_source);
+  void SetActiveSource(BeginFrameSource* new_source);
+  const BeginFrameSource* ActiveSource();
+
+  // BeginFrameObserver
+  // The mux is an BeginFrameObserver as it needs to proxy the OnBeginFrame
+  // calls to preserve the monotonicity of the BeginFrameArgs when switching
+  // sources.
+  virtual void OnBeginFrame(const BeginFrameArgs& args) OVERRIDE;
+  virtual void OnMissedBeginFrame(const BeginFrameArgs& args) OVERRIDE;
+  virtual const BeginFrameArgs LastUsedBeginFrameArgs() const OVERRIDE;
+
+  // BeginFrameSource
+  virtual bool NeedsBeginFrames() const OVERRIDE;
+  virtual void SetNeedsBeginFrames(bool needs_begin_frames) OVERRIDE;
+  virtual void DidFinishFrame(size_t remaining_frames) OVERRIDE;
+
+  // Tracing
+  virtual void AsValueInto(base::debug::TracedValue* dict) const OVERRIDE;
+
+ protected:
+  BeginFrameSourceMultiplexer();
+  explicit BeginFrameSourceMultiplexer(base::TimeDelta minimum_interval);
+
+  bool HasSource(BeginFrameSource* source);
+  bool IsIncreasing(const BeginFrameArgs& args);
+
+  base::TimeDelta minimum_interval_;
+
+  BeginFrameSource* active_source_;
+  std::set<BeginFrameSource*> source_list_;
+};
+
+}  // namespace cc
+
+#endif  // CC_SCHEDULER_BEGIN_FRAME_SOURCE_H_