compositor: Tick the UI animations from cc, instead of from timer callbacks.

ui/compositor/compositor.cc

Index: ui/compositor/compositor.cc
diff --git a/ui/compositor/compositor.cc b/ui/compositor/compositor.cc
index 7a1b2928a8890ac7d73b46511fa8191782788865..c62c12658474987e43abb35f9c9fa46c08549211 100644
--- a/ui/compositor/compositor.cc
+++ b/ui/compositor/compositor.cc
@@ -26,6 +26,7 @@
 #include "ui/compositor/compositor_vsync_manager.h"
 #include "ui/compositor/dip_util.h"
 #include "ui/compositor/layer.h"
+#include "ui/compositor/layer_animator_collection.h"
 #include "ui/gfx/frame_time.h"
 #include "ui/gl/gl_context.h"
 #include "ui/gl/gl_switches.h"
@@ -257,7 +258,9 @@ void Compositor::Draw() {
     // TODO(nduca): Temporary while compositor calls
     // compositeImmediately() directly.
     Layout();
-    host_->Composite(gfx::FrameTime::Now());
+    base::TimeTicks now = gfx::FrameTime::Now();
+    Animate(now);

[piman, 2014/05/23 18:20:06]

I'm not sure I understand what in the animation code triggers a compositor frame
to make sure we call Animate() the first time.

[sadrul, 2014/05/23 18:32:28]

I am not sure if there is anything to ensure this first call. Ali might know
more.

Do you suggest an explicit call to the compositor from the collection?

[ajuma, 2014/05/23 19:15:14]

There doesn't seem to be anything to ensure this. (In the existing code, adding
a LayerAnimatior to an empty AnimationContainer causes the AnimationContainer to
start its timer.) An explicit call to the compositor when adding an animator to
an empty collection seems reasonable (perhaps via a minimal
LayerAnimatorCollection<something> interface rather than exposing all of
Compositor to the collection).

+    host_->Composite(now);
   }
   if (swap_state_ == SWAP_NONE)
     NotifyEnd();
@@ -317,6 +320,12 @@ bool Compositor::HasObserver(CompositorObserver* observer) {
   return observer_list_.HasObserver(observer);
 }
 
+void Compositor::Animate(base::TimeTicks frame_begin_time) {
+  LayerAnimatorCollection::GetInstance()->Progress(frame_begin_time);

[piman, 2014/05/23 18:20:06]

We have one compositor per root window, which are on their own schedule.
Doesn't this mean that we'll evaluate all animations from all layer for each
compositor? I.e. N times if you have N root windows?

Couldn't the LayerAnimatorCollection be one per compositor, so that you only
animate each layer once per frame?

[sadrul, 2014/05/23 18:32:28]

Ah, good point. Right now, the LayerAnimator does not know about the compositor.
Should there be a LayerAnimationDelegate::GetCompositor(), so that the animator
can get to the right collection to add itself to/remove itself from?

[ajuma, 2014/05/23 19:15:14]

Maybe LayerAnimationDelegate::GetCollection(), where the implementation in Layer
calls into the compositor to get the collection?

Also, if a layer with an active animator is moved from one compositor to
another, we'll need to make sure to remove the animator from the old
compositor's collection and add it to the new one.

+  if (LayerAnimatorCollection::GetInstance()->HasActiveAnimators())
+    host_->SetNeedsAnimate();
+}
+
 void Compositor::Layout() {
   // We're sending damage that will be addressed during this composite
   // cycle, so we don't need to schedule another composite to address it.