Refactoring the way begin frame sources inside scheduler work.

cc/scheduler/frame_source.cc

Index: cc/scheduler/frame_source.cc
diff --git a/cc/scheduler/frame_source.cc b/cc/scheduler/frame_source.cc
new file mode 100644
index 0000000000000000000000000000000000000000..c4cf412508f7475939b5bbafc53153430f47982b
--- /dev/null
+++ b/cc/scheduler/frame_source.cc
@@ -0,0 +1,318 @@
+// Copyright 2011 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 "cc/scheduler/scheduler.h"
+
+#include "base/debug/trace_event.h"
+#include "base/logging.h"
+#include "cc/scheduler/delay_based_time_source.h"
+#include "ui/gfx/frame_time.h"
+
+namespace cc {
+
+class FrameSink {
+ public:
+  virtual void BeginFrame(BeginFrameArgs* args) = 0;
+};
+
+class FrameSource {
+ public:
+  FrameSource(FrameSink* sink) : sink_(sink) {}
+  virtual ~FrameSource() {}
+
+  virtual void SetSink(FrameSink* sink) { sink_ = sink; }
+
+  virtual void SendBeginFrame(BeginFrameArgs* args) {
+    DCHECK(sink_);
+    DCHECK(args);
+    sink_->BeginFrame(args);
+  }
+
+  virtual void SetNeedsBeginFrame(bool needs_begin_frame) = 0;
+
+  virtual void UpdateFrameSource(base::TimeTicks timebase,
+                                 base::TimeDelta interval) {
+    if (!interval.is_null())
+      interval_ = interval;
+  }
+
+  virtual scoped_ptr<base::Value> AsValue() const {
+    scoped_ptr<base::DictionaryValue> state(new base::DictionaryValue);
+    state->SetString("type", GetType());
+    state->SetInteger("frame_source", this);
+    state->SetInteger("interval_us", interval_.InMicroseconds());
+    return state.PassAs<base::Value>();
+  }
+
+ protected:
+  virtual std::string GetType() const = 0;
+
+  base::TimeDelta interval_;
+  FrameSink* sink_;
+};
+
+/**
+ * A frame source which proxies BeginFrame from an external OutputSurface such
+ * as on Android.
+ */
+class ExternalFrameSource : public FrameSource {
+ public:
+  ExternalFrameSource(FrameSink* sink, SchedulerClient* client)
+      : client_(client), FrameSource(sink) {
+    DCHECK(client_);
+  }
+  virtual ~ExternalFrameSource() {}
+
+  virtual void BeginFrame(BeginFrameArgs* args) { sink_->BeginFrame(args); }
+
+  virtual void SetNeedsBeginFrame(bool needs_begin_frame) OVERRIDE {
+    DCHECK(client_);
+    client_->SetNeedsBeginFrame(needs_begin_frame);
+  }
+
+  virtual scoped_ptr<base::Value> AsValue() const OVERRIDE {
+    scoped_ptr<base::DictionaryValue> state = FrameSource::AsValue();
+    state->SetInteger("client", client_);
+    return state.PassAs<base::Value>();
+  }
+
+ protected:
+  virtual std::string GetType() const OVERRIDE { return "ExternalFrameSource"; }
+
+ private:
+  SchedulerClient* client_;
+};
+
+/**
+ * A frame source which throttles down another begin frame source.
+ */
+class ThrottledFrameSource : public FrameSource, FrameSink {

[brianderson, 2014/05/05 16:50:05]

In what cases would this class be used? Is this to throttle down to 30 fps or 20
fps for example?

[mithro-old, 2014/05/05 17:10:53]

Yes, examples I can think of are;
 * Throttle an ExternalFrameSource (or the synthetic version) down to
30fps/15fps etc
 * Throttling the BackToBackFrameSource to a maximum fps rate.

[brianderson, 2014/05/05 17:37:49]

Cool. What would the interface look like to change the rate of throttling? Would
we set a decimation factor or some kind of minimum interval?

+ public:
+  ThrottledFrameSource(FrameSink* sink,
+                       const scoped_refptr<FrameSource>& source,
+                       base::TimeDelta interval)
+      : source_(source),
+        interval_(interval),
+        last_frame_args_(),
+        FrameSource(sink) {
+    source_->SetSink(this);
+  }
+  virtual ~ThrottledFrameSource() {}
+
+  virtual void BeginFrame(BeginFrameArgs* args) OVERRIDE {
+    if (last_frame_args_.frame_time + interval_ > args.frame_time)
+      return;
+    source_->SendBeginFrame(sink, args);
+  }
+
+  virtual void SetNeedsBeginFrame(bool needs_begin_frame) OVERRIDE {
+    source_->SetNeedsBeginFrame(needs_begin_frame);
+  }
+
+  virtual scoped_ptr<base::Value> AsValue() const OVERRIDE {
+    scoped_ptr<base::DictionaryValue> state = FrameSource::AsValue();
+    state->SetValue("source", source_->AsValue().release());
+    return state.PassAs<base::Value>();
+  }
+
+ protected:
+  virtual std::string GetType() const OVERRIDE {
+    return "ThrottledFrameSource";
+  }
+
+ private:
+  BeginFrameArgs last_frame_args_;
+  scoped_refptr<base::FrameSource> source_;
+};
+
+/**
+ * Frame source which needs a task runner object.
+ */
+class TaskRunnerFrameSource : public FrameSource {
+ public:
+  TaskRunnerFrameSource(FrameSink* sink,
+                        base::SingleThreadTaskRunner* task_runner)
+      : task_runner_(task_runner), FrameSource(sink) {
+    DCHECK(task_runner);
+  }
+  virtual ~TaskRunnerFrameSource() {}
+
+ protected:
+  base::SingleThreadTaskRunner task_runner_;
+};
+
+/**
+ * A frame source which sends a BeginFrame as soon as SetNeedsBeginFrame is
+ * requested.
+ */
+class BackToBackFrameSource : public TaskRunnerFrameSource {

[brianderson, 2014/05/05 16:50:05]

Would this be used as the "vsync disabled" BeginFrame source? 

In this case, one issue we'd need to solve with the interface between the
Scheduler and a FrameSource is that the Scheduler expects a stream of
BeginFrames after a single call to SetNeedsBeginFrame(true). As in,
SetNeedsBeginFrame is level triggered and not edge triggered.

The problem with a "vsync disabled" source is that, for it to behave like other
sources, we should receive an infinite stream of BeginFrames. Maybe we should
add a method to the FrameSource, in addition to the SetNeedsBeginFrame, called
ReadyToReceiveNextBeginFrame.

[mithro-old, 2014/05/05 17:10:53]

Is there any reason we want to keep it level triggered rather then edge
triggered? (Maybe because of some sort of pipelining?)

From looking at the various code, it seems we calculate if we actually want to
keep rendering frames every frame (as an animation could finish).

[brianderson, 2014/05/05 17:37:49]

It is level triggered at the interface between the Renderer and Browser for
pipelining / latency hiding reasons.

Keeping it level triggered within the Renderer allows us to use the
BeginRetroFrame logic since we can receive new BeginFrames while still handling
the previous BeginFrame. Using an edge triggered interface with the retroactive
logic gets messy.

We don't use the BeginRetro frame logic for the unthrottled case though, since
there would be an infinite number of retroactive frames and we only care about
the most recent one.
Yeah. We probably calculate this many times per frame even, but we need some way
to figure out when to toggle the level.

+ public:
+  virtual void SetNeedsBeginFrame(bool needs_begin_frame) OVERRIDE {
+    if (needs_begin_frame) {
+      task_runner_->PostDelayedTask(
+          FROM_HERE,
+          base::Bind(&BackToBackFrameSource::PostBeginFrame,
+                     weak_factory_.GetWeakPtr()));
+    }
+  }
+
+  void PostBeginFrame() {
+    base::TimeTicks now = gfx::FrameTime::Now();
+    BeginFrameArgs begin_frame_args =
+        BeginFrameArgs::Create(now, base::TimeTicks(), base::TimeDelta());
+    FrameSource::SendBeginFrame(args);
+  }
+
+ protected:
+  virtual std::string GetType() const OVERRIDE {
+    return "BackToBackFrameSource";
+  }
+};
+
+/**
+ * A frame source which is locked an external vsync source and generates
+ * BeginFrameArgs for it.
+ */
+class SyntheticFrameSource : public TaskRunnerFrameSource,
+                             public TimeSourceClient {
+ public:
+  SyntheticFrameSource(FrameSink* sink,
+                       base::SingleThreadTaskRunner* task_runner)
+      : TaskRunnerFrameSource(sink, task_runner) {
+    if (gfx::FrameTime::TimestampsAreHighRes()) {
+      time_source_ = DelayBasedTimeSourceHighRes::Create(sink_->VSyncInterval(),
+                                                         task_runner);
+    } else {
+      time_source_ =
+          DelayBasedTimeSource::Create(sink_->VSyncInterval(), task_runner);
+    }
+    time_source_->SetClient(this);
+  }
+  virtual ~SyntheticFrameSource() {}
+
+  // Activates future BeginFrames and, if activating, pushes the most
+  // recently missed BeginFrame to the back of a retroactive queue.
+  void SetNeedsBeginFrame(bool needs_begin_frame) OVERRIDE {
+    base::TimeTicks missed_tick_time =
+        time_source_->SetActive(needs_begin_frame);
+    if (!missed_tick_time.is_null()) {
+      BaseFrameSource::SendBeginFrame(
+          CreateSyntheticBeginFrameArgs(missed_tick_time));
+    }
+  }
+
+  // Updates the phase and frequency of the timer.
+  virtual void UpdateFrameSource(base::TimeTicks timebase,
+                                 base::TimeDelta interval) OVERRIDE {
+    TaskRunnerFrameSource::UpdateFrameSource(timebase, interval);
+    time_source_->SetTimebaseAndInterval(timebase, interval);
+  }
+
+  virtual scoped_ptr<base::Value> AsValue() const OVERRIDE {
+    scoped_ptr<base::DictionaryValue> state = FrameSource::AsValue();
+    state->SetValue("source", source_->AsValue().release());
+    return state.PassAs<base::Value>();
+  }
+
+  // TimeSourceClient implementation of OnTimerTick triggers a BeginFrame.
+  virtual void OnTimerTick() OVERRIDE {
+    BeginFrameArgs begin_frame_args(
+        CreateSyntheticBeginFrameArgs(time_source_->LastTickTime()));
+    SendBeginFrame(begin_frame_args);
+  }
+
+ protected:
+  virtual std::string GetType() const OVERRIDE {
+    return "SyntheticFrameSource";
+  }
+
+ private:
+  FrameSink* sink_;
+  scoped_refptr<TimeSource> time_source_;
+
+  BeginFrameArgs CreateSyntheticBeginFrameArgs(base::TimeTicks frame_time) {
+    base::TimeTicks deadline =
+        time_source_->NextTickTime() - sink_->EstimatedParentDrawTime();
+    return BeginFrameArgs::Create(frame_time, deadline, interval_);
+  }
+};
+
+/**
+ * A virtual frame source which lets you switch between two other frame sources
+ * (making sure the BeginFrameArgs stays monotonic).
+ */
+class DualFrameSource : public FrameSource, public FrameSink {

[brianderson, 2014/05/05 16:50:05]

Is this going to be used to switch between background and foreground ticking? I
was hoping we could use a single BeginFrame source in that case and throttle the
SetNeedsBeginFrame instead to avoid the monotonic issues.

[mithro-old, 2014/05/05 17:10:53]

Yeah. The thought here was to allow switching between background / foreground.

Using throttling has the issue that we are still getting woken up every 16ms
when we only want to do something every second (which is bad for battery).

In theory we could do something smart like the background ticking frequency be a
exact multiple of the foreground ticking and have the phase match. That gets
complicated when vsync is constantly being adjusted though.

+ public:
+  DualFrameSource(FrameSink* sink,
+                  const scoped_refptr<FrameSource>& source_primary,
+                  const scoped_refptr<FrameSource>& source_secondary)
+      : source_primary_(source_primary),
+        source_secondary_(source_secondary),
+        needs_begin_frame_(false),
+        last_frame_args_() {
+    source_primary_->SetSink(this);
+    source_secondary_->SetSink(this);
+    SwitchSource(SourcePrimary());
+  }
+
+  virtual void BeginFrame(BeginFrameArgs* args) OVERRIDE {
+    // When switching clock sources you can get a BeginFrameArgs with a
+    // frame_time before the last value.
+    if (last_frame_args_.frame_time > args.frame_time)
+      args.frame_time = last_frame_args_.frame_time;
+
+    FrameSource::SendBeginFrame(args);
+    last_frame_args_ = args;
+  }
+
+  virtual void SetNeedsBeginFrame(bool needs_begin_frame) OVERRIDE {
+    DCHECK(active_source_);
+    needs_begin_frame_ = needs_begin_frame;
+    active_source_->SetNeedsBeginFrame(needs_begin_frame);
+  }
+
+  virtual scoped_ptr<base::Value> AsValue() const OVERRIDE {
+    scoped_ptr<base::DictionaryValue> state = FrameSource::AsValue();
+    if (active_source_ == SourcePrimary())
+      state->SetString("active", "primary");
+    if (active_source_ == SourceSecondary())
+      state->SetString("active", "secondary");
+    state->SetValue("primary", source_primary_->AsValue().release());
+    state->SetValue("secondary", source_secondary_->AsValue().release());
+    state->SetValue("last_frame_args", last_frame_args_->AsValue().release());
+    return state.PassAs<base::Value>();
+  }
+
+  // ---------------------------------------------------------------------
+
+  void SwitchSource(base::FrameSource* new_source) {
+    DCHECK(new_source == SourcePrimary() || new_source == SourceSecondary());
+
+    if (needs_begin_frame_)
+      active_source_.SetNeedsBeginFrame(false);
+
+    active_source_ = new_source;
+
+    if (needs_begin_frame_)
+      active_source_.SetNeedsBeginFrame(true);
+  }
+
+  base::FrameSource* SourcePrimary() { return source_primary_.get(); }
+
+  base::FrameSource* SourceSecondary() { return source_secondary_.get(); }
+
+ protected:
+  virtual std::string GetType() const OVERRIDE {
+    return std::string("DualFrameSource");
+  }
+
+ private:
+  bool needs_begin_frame_;
+  BeginFrameArgs last_frame_args_;
+  base::FrameSource* active_source_;
+  scoped_refptr<base::FrameSource> source_primary_;
+  scoped_refptr<base::FrameSource> source_secondary_;
+};
+
+}  // namespace cc