cc: Make BackToBackBeginFrameSource a SyntheticBeginFrameSource.

cc/scheduler/begin_frame_source.cc

Index: cc/scheduler/begin_frame_source.cc
diff --git a/cc/scheduler/begin_frame_source.cc b/cc/scheduler/begin_frame_source.cc
index 5c20a86caf2c4b11fd1fd54d48b74ca78a893cc7..2f56c890eb6aa7db8a29ee2a60ff9c95bd61ae4b 100644
--- a/cc/scheduler/begin_frame_source.cc
+++ b/cc/scheduler/begin_frame_source.cc
@@ -9,6 +9,7 @@
 #include "base/auto_reset.h"
 #include "base/location.h"
 #include "base/logging.h"
+#include "base/memory/ptr_util.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/stringprintf.h"
 #include "base/trace_event/trace_event.h"
@@ -97,92 +98,37 @@ void BeginFrameSourceBase::SetBeginFrameSourcePaused(bool paused) {
 // BackToBackBeginFrameSource --------------------------------------------
 BackToBackBeginFrameSource::BackToBackBeginFrameSource(
     base::SingleThreadTaskRunner* task_runner)
-    : BeginFrameSourceBase(), task_runner_(task_runner), weak_factory_(this) {
-  DCHECK(task_runner);
+    : SyntheticBeginFrameSource(
+          base::MakeUnique<DelayBasedTimeSource>(task_runner)) {
+  SetUnthrottled(true);
 }
 
-BackToBackBeginFrameSource::~BackToBackBeginFrameSource() {
-}
-
-base::TimeTicks BackToBackBeginFrameSource::Now() {
-  return base::TimeTicks::Now();
-}
-
-// BeginFrameSourceBase support
-void BackToBackBeginFrameSource::AddObserver(BeginFrameObserver* obs) {
-  BeginFrameSourceBase::AddObserver(obs);
-  pending_begin_frame_observers_.insert(obs);
-  PostPendingBeginFramesTask();
-}
-
-void BackToBackBeginFrameSource::RemoveObserver(BeginFrameObserver* obs) {
-  BeginFrameSourceBase::RemoveObserver(obs);
-  pending_begin_frame_observers_.erase(obs);
-  if (pending_begin_frame_observers_.empty())
-    begin_frame_task_.Cancel();
-}
-
-void BackToBackBeginFrameSource::DidFinishFrame(BeginFrameObserver* obs,
-                                                size_t remaining_frames) {
-  BeginFrameSourceBase::DidFinishFrame(obs, remaining_frames);
-  if (remaining_frames == 0 && observers_.find(obs) != observers_.end()) {
-    pending_begin_frame_observers_.insert(obs);
-    PostPendingBeginFramesTask();
-  }
-}
-
-void BackToBackBeginFrameSource::PostPendingBeginFramesTask() {
-  DCHECK(needs_begin_frames());
-  DCHECK(!pending_begin_frame_observers_.empty());
-  if (begin_frame_task_.IsCancelled()) {
-    begin_frame_task_.Reset(
-        base::Bind(&BackToBackBeginFrameSource::SendPendingBeginFrames,
-                   weak_factory_.GetWeakPtr()));
-    task_runner_->PostTask(FROM_HERE, begin_frame_task_.callback());
-  }
-}
-
-void BackToBackBeginFrameSource::SendPendingBeginFrames() {
-  DCHECK(needs_begin_frames());
-  DCHECK(!begin_frame_task_.IsCancelled());
-  begin_frame_task_.Cancel();
-
-  base::TimeTicks now = Now();
-  BeginFrameArgs args = BeginFrameArgs::Create(
-      BEGINFRAME_FROM_HERE, now, now + BeginFrameArgs::DefaultInterval(),
-      BeginFrameArgs::DefaultInterval(), BeginFrameArgs::NORMAL);
-
-  std::set<BeginFrameObserver*> pending_observers;
-  pending_observers.swap(pending_begin_frame_observers_);
-  for (BeginFrameObserver* obs : pending_observers)
-    obs->OnBeginFrame(args);
-}
+BackToBackBeginFrameSource::~BackToBackBeginFrameSource() = default;
 
 // SyntheticBeginFrameSource ---------------------------------------------
 SyntheticBeginFrameSource::SyntheticBeginFrameSource(
-    base::SingleThreadTaskRunner* task_runner,
-    base::TimeDelta initial_vsync_interval)
-    : time_source_(
-          DelayBasedTimeSource::Create(initial_vsync_interval, task_runner)) {
-  time_source_->SetClient(this);
-}
-
-SyntheticBeginFrameSource::SyntheticBeginFrameSource(
     std::unique_ptr<DelayBasedTimeSource> time_source)
-    : time_source_(std::move(time_source)) {
+    : time_source_(std::move(time_source)),
+      last_interval_(time_source_->Interval()) {
   time_source_->SetClient(this);
 }
 
-SyntheticBeginFrameSource::~SyntheticBeginFrameSource() {}
+SyntheticBeginFrameSource::~SyntheticBeginFrameSource() = default;
 
 void SyntheticBeginFrameSource::OnUpdateVSyncParameters(
     base::TimeTicks timebase,
     base::TimeDelta interval) {
-  if (!authoritative_interval_.is_zero())
+  if (!authoritative_interval_.is_zero()) {
     interval = authoritative_interval_;
+  } else if (interval.is_zero()) {
+    // TODO(brianderson): We should not be receiving 0 intervals.
+    interval = BeginFrameArgs::DefaultInterval();
+  }
 
   last_timebase_ = timebase;
-  time_source_->SetTimebaseAndInterval(timebase, interval);
+  last_interval_ = interval;
+  if (!unthrottled_)
+    time_source_->SetTimebaseAndInterval(timebase, interval);
 }
 
 void SyntheticBeginFrameSource::SetAuthoritativeVSyncInterval(
@@ -191,20 +137,44 @@ void SyntheticBeginFrameSource::SetAuthoritativeVSyncInterval(
   OnUpdateVSyncParameters(last_timebase_, interval);
 }
 
+void SyntheticBeginFrameSource::SetUnthrottled(bool unthrottled) {
+  if (unthrottled)
+    time_source_->SetTimebaseAndInterval(base::TimeTicks(), base::TimeDelta());
+  else
+    time_source_->SetTimebaseAndInterval(last_timebase_, last_interval_);
+  unthrottled_ = unthrottled;
+}
+
 BeginFrameArgs SyntheticBeginFrameSource::CreateBeginFrameArgs(
     base::TimeTicks frame_time,
     BeginFrameArgs::BeginFrameArgsType type) {
+  if (unthrottled_) {
+    // When unthrottled, the NextTickTime is not meaningful, so just use the
+    // current interval as the deadline.
+    return BeginFrameArgs::Create(BEGINFRAME_FROM_HERE, frame_time,
+                                  frame_time + last_interval_, last_interval_,
+                                  type);
+  }
   return BeginFrameArgs::Create(BEGINFRAME_FROM_HERE, frame_time,
-                                time_source_->NextTickTime(),
-                                time_source_->Interval(), type);
+                                time_source_->NextTickTime(), last_interval_,
+                                type);
 }
 
 // BeginFrameSource support
 void SyntheticBeginFrameSource::AddObserver(BeginFrameObserver* obs) {
   BeginFrameSourceBase::AddObserver(obs);
+
+  if (unthrottled_) {
+    // When unthrottled, ensure the time source is ticking for the first frame
+    // this observer sees (since it only ticks when an observer is ready), and
+    // don't post a MISSED frame since unthrottled just goes as fast as it can.
+    time_source_->SetActive(true);
+    frame_finished_observers_.insert(obs);
+    return;
+  }
+
   BeginFrameArgs args = CreateBeginFrameArgs(
-      time_source_->NextTickTime() - time_source_->Interval(),
-      BeginFrameArgs::MISSED);
+      time_source_->NextTickTime() - last_interval_, BeginFrameArgs::MISSED);
   BeginFrameArgs last_args = obs->LastUsedBeginFrameArgs();
   if (!last_args.IsValid() ||
       (args.frame_time >
@@ -213,23 +183,55 @@ void SyntheticBeginFrameSource::AddObserver(BeginFrameObserver* obs) {
   }
 }
 
+void SyntheticBeginFrameSource::RemoveObserver(BeginFrameObserver* obs) {
+  BeginFrameSourceBase::RemoveObserver(obs);
+  frame_finished_observers_.erase(obs);
+}
+
 void SyntheticBeginFrameSource::OnNeedsBeginFramesChanged(
     bool needs_begin_frames) {
   time_source_->SetActive(needs_begin_frames);
 }
 
+void SyntheticBeginFrameSource::DidFinishFrame(BeginFrameObserver* obs,
+                                               size_t remaining_frames) {
+  if (remaining_frames == 0 && observers_.find(obs) != observers_.end()) {
+    frame_finished_observers_.insert(obs);
+    time_source_->SetActive(true);
+  }
+}
+
 // DelayBasedTimeSourceClient support
 void SyntheticBeginFrameSource::OnTimerTick() {
-  BeginFrameArgs args = CreateBeginFrameArgs(time_source_->LastTickTime(),
-                                             BeginFrameArgs::NORMAL);
+  // This must be captured before calling SetActive(false) on the time_source_.
+  base::TimeTicks now = time_source_->LastTickTime();
+
+  if (unthrottled_) {
+    // When unthrottled, we only want to tick when an observer finishes a frame,
+    // so stop and wait for that.
+    time_source_->SetActive(false);
+  }
+
+  BeginFrameArgs args = CreateBeginFrameArgs(now, BeginFrameArgs::NORMAL);
   std::set<BeginFrameObserver*> observers(observers_);
-  for (auto& it : observers) {
-    BeginFrameArgs last_args = it->LastUsedBeginFrameArgs();
-    if (!last_args.IsValid() ||
-        (args.frame_time >
-         last_args.frame_time + args.interval / kDoubleTickDivisor)) {
-      it->OnBeginFrame(args);
+  for (auto& obs : observers) {
+    bool on_begin_frame = false;
+    if (unthrottled_) {
+      // When unthrottled, we only want to generate a new begin frame for each
+      // observer when its previous one finished.
+      on_begin_frame = frame_finished_observers_.count(obs) > 0;
+    } else {
+      // When ticking at a throttled rate we want to generate a new begin frame
+      // regularly, but not too often.
+      BeginFrameArgs last_args = obs->LastUsedBeginFrameArgs();
+      on_begin_frame =
+          !last_args.IsValid() ||
+          (args.frame_time >
+           last_args.frame_time + args.interval / kDoubleTickDivisor);
     }
+    frame_finished_observers_.erase(obs);
+    if (on_begin_frame)
+      obs->OnBeginFrame(args);
   }
 }