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/logging.h"
#include "base/trace_event/trace_event.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 BeginFrameObserver {
 public:
  virtual ~BeginFrameObserver() {}

  // 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;

  // 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::trace_event::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.
class BeginFrameObserverMixIn : public BeginFrameObserver {
 public:
  BeginFrameObserverMixIn();

  // BeginFrameObserver

  // Traces |args| and DCHECK |args| satisfies pre-conditions then calls
  // OnBeginFrameMixInDelegate and updates the last_begin_frame_args_ value on
  // true.
  void OnBeginFrame(const BeginFrameArgs& args) override;
  const BeginFrameArgs LastUsedBeginFrameArgs() const override;

  // Outputs last_begin_frame_args_
  void AsValueInto(base::trace_event::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;

  BeginFrameArgs last_begin_frame_args_;
  int64_t dropped_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.
class 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;

  // Add/Remove an observer from the source.
  // *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.
  virtual void AddObserver(BeginFrameObserver* obs) = 0;
  virtual void RemoveObserver(BeginFrameObserver* obs) = 0;

  // Tells the Source that client is ready to handle BeginFrames messages.
  virtual void SetClientReady() = 0;

  // Tracing support - Recommend (but not required) to call this implementation
  // in any override.
  virtual void AsValueInto(base::trace_event::TracedValue* dict) const = 0;
};

// Simple mix in which implements a BeginFrameSource.
// Implementation classes should:
//  - Implement the pure virtual (Set)NeedsBeginFrames methods from
//    BeginFrameSource.
//  - Use the CallOnBeginFrame method to call to the observer(s).
//  - Recommended (but not required) to call BeginFrameSourceMixIn::AsValueInto
//    in their own AsValueInto implementation.
class BeginFrameSourceMixIn : public BeginFrameSource {
 public:
  ~BeginFrameSourceMixIn() override {}

  // BeginFrameSource
  bool NeedsBeginFrames() const final;
  void SetNeedsBeginFrames(bool needs_begin_frames) final;
  void DidFinishFrame(size_t remaining_frames) override {}
  void AddObserver(BeginFrameObserver* obs) final;
  void RemoveObserver(BeginFrameObserver* obs) final;
  void SetClientReady() override {}

  // Tracing support - Recommend (but not required) to call this implementation
  // in any override.
  void AsValueInto(base::trace_event::TracedValue* dict) const override;

 protected:
  BeginFrameSourceMixIn();

  // These methods should be used by subclasses to make the call to the
  // observers.
  void CallOnBeginFrame(const BeginFrameArgs& args);

  // This method should be overridden if you want to change some behaviour on
  // needs_begin_frames change.
  virtual void OnNeedsBeginFramesChange(bool needs_begin_frames) {}

  BeginFrameObserver* observer_;
  bool needs_begin_frames_;

 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 BackToBackBeginFrameSource : public BeginFrameSourceMixIn {
 public:
  static scoped_ptr<BackToBackBeginFrameSource> Create(
      base::SingleThreadTaskRunner* task_runner);
  ~BackToBackBeginFrameSource() override;

  // BeginFrameSource
  void DidFinishFrame(size_t remaining_frames) override;

  // Tracing
  void AsValueInto(base::trace_event::TracedValue* dict) const override;

 protected:
  explicit BackToBackBeginFrameSource(
      base::SingleThreadTaskRunner* task_runner);
  virtual base::TimeTicks Now();  // Now overridable for testing

  base::SingleThreadTaskRunner* task_runner_;

  bool send_begin_frame_posted_;

  // BeginFrameSourceMixIn
  void OnNeedsBeginFramesChange(bool needs_begin_frames) override;

  void BeginFrame();

 private:
  base::WeakPtrFactory<BackToBackBeginFrameSource> weak_factory_;
};

// A frame source which is locked to an external parameters provides from a
// vsync source and generates BeginFrameArgs for it.
class SyntheticBeginFrameSource : public BeginFrameSourceMixIn,
                                  public VSyncParameterObserver,
                                  public TimeSourceClient {
 public:
  static scoped_ptr<SyntheticBeginFrameSource> Create(
      base::SingleThreadTaskRunner* task_runner,
      base::TimeTicks initial_vsync_timebase,
      base::TimeDelta initial_vsync_interval);
  ~SyntheticBeginFrameSource() override;

  // Tracing
  void AsValueInto(base::trace_event::TracedValue* dict) const override;

  // VSyncParameterObserver
  void OnUpdateVSyncParameters(base::TimeTicks new_vsync_timebase,
                               base::TimeDelta new_vsync_interval) override;

  // TimeSourceClient
  void OnTimerTick() override;

 protected:
  explicit SyntheticBeginFrameSource(
      scoped_refptr<DelayBasedTimeSource> time_source);

  BeginFrameArgs CreateBeginFrameArgs(base::TimeTicks frame_time,
                                      BeginFrameArgs::BeginFrameArgsType type);

  // BeginFrameSourceMixIn
  void OnNeedsBeginFramesChange(bool needs_begin_frames) override;

  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 BeginFrameSourceMultiplexer : public BeginFrameSourceMixIn,
                                    public BeginFrameObserver {
 public:
  static scoped_ptr<BeginFrameSourceMultiplexer> Create();
  ~BeginFrameSourceMultiplexer() override;

  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.
  void OnBeginFrame(const BeginFrameArgs& args) override;
  const BeginFrameArgs LastUsedBeginFrameArgs() const override;

  // BeginFrameSource
  void DidFinishFrame(size_t remaining_frames) override;

  // BeginFrameSourceMixIn
  void OnNeedsBeginFramesChange(bool needs_begin_frames) override;

  // Tracing
  void AsValueInto(base::trace_event::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_