cc: Implement unified BeginFrame on aura

ui/compositor/compositor.cc

Index: ui/compositor/compositor.cc
diff --git a/ui/compositor/compositor.cc b/ui/compositor/compositor.cc
index 0fae1937acb265839fc48a516aa597dfa96184b0..efaa5cb81e1f6f959f9ff5ac033d0306d8ecd714 100644
--- a/ui/compositor/compositor.cc
+++ b/ui/compositor/compositor.cc
@@ -94,6 +94,8 @@ Compositor::Compositor(gfx::AcceleratedWidget widget,
   settings.main_frame_before_activation_enabled = false;
   settings.throttle_frame_production =
       !command_line->HasSwitch(switches::kDisableGpuVsync);
+  settings.forward_begin_frames_to_children =
+      command_line->HasSwitch(cc::switches::kEnableBeginFrameScheduling);
 #if !defined(OS_MACOSX)
   settings.renderer_settings.partial_swap_enabled =
       !command_line->HasSwitch(cc::switches::kUIDisablePartialSwap);
@@ -137,6 +139,9 @@ Compositor::Compositor(gfx::AcceleratedWidget widget,
 
   base::TimeTicks before_create = base::TimeTicks::Now();
   if (compositor_thread_loop_.get()) {
+    // Unified BeginFrame scheduling shouldn't be enabled with threaded
+    // compositing.
+    DCHECK(!settings.forward_begin_frames_to_children);
     host_ = cc::LayerTreeHost::CreateThreaded(
         this,
         context_factory_->GetSharedBitmapManager(),
@@ -168,6 +173,8 @@ Compositor::~Compositor() {
   CancelCompositorLock();
   DCHECK(!compositor_lock_);
 
+  DCHECK(!begin_frame_observer_list_.might_have_observers());
+
   if (root_layer_)
     root_layer_->SetCompositor(NULL);
 
@@ -302,6 +309,31 @@ bool Compositor::HasAnimationObserver(
   return animation_observer_list_.HasObserver(observer);
 }
 
+void Compositor::AddBeginFrameObserver(
+    CompositorBeginFrameObserver* observer,
+    const cc::BeginFrameArgs& last_begin_frame_args_sent_to_observer) {
+  // If |last_begin_frame_args_| is still effective, send it to the new
+  // |observer| immediately.
+  if (!last_begin_frame_args_sent_to_observer.deadline.is_null() &&
+      last_begin_frame_args_sent_to_observer != missed_begin_frame_args_) {
+    missed_begin_frame_args_.type = cc::BeginFrameArgs::MISSED;
+    observer->OnSendBeginFrame(missed_begin_frame_args_);

[mithro-old, 2014/12/16 10:50:58]

I don't think this is correct, if the observer was a proper BeginFrameObserver
the code would read something like;

if (!new_args.IsValid())
  return;

// If the last begin frame is invalid, then any new begin frame is valid.
if (observer->LastUsedBeginFrameArgs().IsValid())
  OnSendBeginFrame(new_args);

// Only allow new args have a *strictly bigger* frame_time value
if (new_args.frame_time >= observer->LastUsedBeginFrameArgs().frame_time)
  OnSendBeginFrame(new_args);

[simonhong, 2014/12/16 15:13:30]

This code can be effective when observer is removed and added again and
|missed_begin_frame_args_| is still effective.
In this case, |missed_begin_frame_args_| can be delivered immediately to
renderer.
Of course, |missed_begin_frame_args_| 's deadline can be passed.
I and brian thought that this can be handled in the Scheduler side.
Old BeginFrameArgs with MISSED type will be dropped by RetroFrame handler.
So we are sending |missed_begin_frame_args_| to renderer without much
inspection.

[brianderson, 2014/12/16 23:23:52]

@mithro's suggestion enforces monotonicity of the frame_time, which is
important, especially now that the latest patch sends a fake BeginFrame for the
first frame of dev tools.

Before the dev tools change, I think the guarantee of monotonicity from the
scheduler's begin frame source would have been good enough.

+  }
+
+  if (!begin_frame_observer_list_.might_have_observers())
+    SetChildrenNeedBeginFrames(true);
+  begin_frame_observer_list_.AddObserver(observer);
+}
+
+void Compositor::RemoveBeginFrameObserver(
+    CompositorBeginFrameObserver* observer) {
+  DCHECK(begin_frame_observer_list_.HasObserver(observer));
+  begin_frame_observer_list_.RemoveObserver(observer);
+
+  if (!begin_frame_observer_list_.might_have_observers())
+    SetChildrenNeedBeginFrames(false);
+}
+
 void Compositor::BeginMainFrame(const cc::BeginFrameArgs& args) {
   FOR_EACH_OBSERVER(CompositorAnimationObserver,
                     animation_observer_list_,
@@ -359,6 +391,13 @@ void Compositor::DidAbortSwapBuffers() {
                     OnCompositingAborted(this));
 }
 
+void Compositor::SendBeginFramesToChildren(const cc::BeginFrameArgs& args) {
+  FOR_EACH_OBSERVER(CompositorBeginFrameObserver,
+                    begin_frame_observer_list_,
+                    OnSendBeginFrame(args));
+  missed_begin_frame_args_ = args;
+}
+
 const cc::LayerTreeDebugState& Compositor::GetLayerTreeDebugState() const {
   return host_->debug_state();
 }
@@ -397,4 +436,8 @@ void Compositor::CancelCompositorLock() {
     compositor_lock_->CancelLock();
 }
 
+void Compositor::SetChildrenNeedBeginFrames(bool need_begin_frames) {
+  host_->SetChildrenNeedBeginFrames(need_begin_frames);
+}
+
 }  // namespace ui