cc: Avoid updating animations in the background without an active tree

cc/trees/thread_proxy.cc

Index: cc/trees/thread_proxy.cc
diff --git a/cc/trees/thread_proxy.cc b/cc/trees/thread_proxy.cc
index 4892234a42e695c8b25ea9fc223f77d6dcf54944..786b9eda39289ea65cc9d05770c5a5b46f6bb8f4 100644
--- a/cc/trees/thread_proxy.cc
+++ b/cc/trees/thread_proxy.cc
@@ -238,11 +238,16 @@ void ThreadProxy::SetVisibleOnImplThread(CompletionEvent* completion,
   TRACE_EVENT0("cc", "ThreadProxy::SetVisibleOnImplThread");
   layer_tree_host_impl_->SetVisible(visible);
   scheduler_on_impl_thread_->SetVisible(visible);
-  layer_tree_host_impl_->UpdateBackgroundAnimateTicking(
-      !scheduler_on_impl_thread_->WillDrawIfNeeded());
+  UpdateBackgroundAnimateTicking();
   completion->Signal();
 }
 
+void ThreadProxy::UpdateBackgroundAnimateTicking() {
+  layer_tree_host_impl_->UpdateBackgroundAnimateTicking(
+      !scheduler_on_impl_thread_->WillDrawIfNeeded() &&
+      layer_tree_host_impl_->active_tree()->root_layer());

[enne (OOO), 2013/08/29 00:10:10]

This seems a little redundant.  Scheduler::WillDrawIfNeeded returns
!SchedulerStateMachine::PendingDrawsShouldBeAborted which returns true if
!can_draw.  CanDraw() itself returns false if there's no active root layer. 
Does that just mean that the scheduler state machine hasn't been updated?

[brianderson, 2013/08/29 00:16:04]

It's confusing with all the negations going on. The issue was we were enabling
background ticking when !can_draw, but we shouldn't be if the reason we
!can_draw is that the active_tree's root layer is null.

+}
+
 void ThreadProxy::DoCreateAndInitializeOutputSurface() {
   TRACE_EVENT0("cc", "ThreadProxy::DoCreateAndInitializeOutputSurface");
   DCHECK(IsMainThread());
@@ -406,8 +411,7 @@ void ThreadProxy::OnCanDrawStateChanged(bool can_draw) {
   TRACE_EVENT1(
       "cc", "ThreadProxy::OnCanDrawStateChanged", "can_draw", can_draw);
   scheduler_on_impl_thread_->SetCanDraw(can_draw);
-  layer_tree_host_impl_->UpdateBackgroundAnimateTicking(
-      !scheduler_on_impl_thread_->WillDrawIfNeeded());
+  UpdateBackgroundAnimateTicking();
 }
 
 void ThreadProxy::NotifyReadyToActivate() {
@@ -928,8 +932,7 @@ void ThreadProxy::ScheduledActionCommit() {
 
   SetInputThrottledUntilCommitOnImplThread(false);
 
-  layer_tree_host_impl_->UpdateBackgroundAnimateTicking(
-      !scheduler_on_impl_thread_->WillDrawIfNeeded());
+  UpdateBackgroundAnimateTicking();
 
   next_frame_is_newly_committed_frame_on_impl_thread_ = true;
 
@@ -1002,7 +1005,7 @@ ThreadProxy::ScheduledActionDrawAndSwapInternal(bool forced_draw) {
   if (layer_tree_host_impl_->pending_tree())
     layer_tree_host_impl_->pending_tree()->UpdateDrawProperties();
   layer_tree_host_impl_->Animate(monotonic_time, wall_clock_time);
-  layer_tree_host_impl_->UpdateBackgroundAnimateTicking(false);
+  UpdateBackgroundAnimateTicking();
 
   base::TimeTicks start_time = base::TimeTicks::HighResNow();
   base::TimeDelta draw_duration_estimate = DrawDurationEstimate();
@@ -1049,7 +1052,6 @@ ThreadProxy::ScheduledActionDrawAndSwapInternal(bool forced_draw) {
     result.did_draw = true;
   }
   layer_tree_host_impl_->DidDrawAllLayers(frame);
-
   layer_tree_host_impl_->UpdateAnimationState(start_ready_animations);
 
   // Check for a pending CompositeAndReadback.
@@ -1500,6 +1502,8 @@ void ThreadProxy::DidActivatePendingTree() {
     completion_event_for_commit_held_on_tree_activation_ = NULL;
   }
 
+  UpdateBackgroundAnimateTicking();
+
   commit_to_activate_duration_history_.InsertSample(
       base::TimeTicks::HighResNow() - commit_complete_time_);
 }