compositor-worker: cc->blink mutation plumbing

cc/trees/proxy_main.cc

 // Copyright 2015 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/trees/proxy_main.h"
 
 #include <algorithm>
 #include <string>
 
 #include "base/trace_event/trace_event.h"
@@ skipping 166 matching lines @@
     TRACE_EVENT_INSTANT0("cc", "EarlyOut_OutputSurfaceLost",
                          TRACE_EVENT_SCOPE_THREAD);
     channel_main_->BeginMainFrameAbortedOnImpl(
         CommitEarlyOutReason::ABORTED_OUTPUT_SURFACE_LOST,
         begin_main_frame_start_time);
     return;
   }
 
   current_pipeline_stage_ = ANIMATE_PIPELINE_STAGE;
 
+  if (begin_main_frame_state->begin_frame_callbacks) {
+    for (auto& callback : *begin_main_frame_state->begin_frame_callbacks)

[esprehn, 2016/03/31 05:03:11]

Why here? Why is this before 

ApplyScrollAndScale, WillBeginMainFrame, BeginMainFrame and AnimateLayers?

I'm not a cc owner, but if I was designing this I think I'd assume there was a
callback, and the consumer of cc should do the queuing and ordering of the
callbacks. This system doesn't give the mutator any control over what's going on
here.

[majidvp, 2016/03/31 19:43:03]

We want the mutations to get applied before beginning the main frame so the
new values are available in main frame before scripted animations are
serviced. It could be placed after ApplyScrollAndScale but before
BeginMainFrame.

The original design idea from piman@ was that eventually ApplyScrollAndScale
will become one of these callbacks itself and as you are suggesting proxy main
will not care about the ordering.

[danakj, 2016/04/05 17:45:21]

After ApplyScrollAndScale may make sense for now, then whatever callbacks get
registered can use what they do. Later, you'd just have to register callbacks in
the right order if you had dependencies.

But if it's meant to do things that will influence the main frame then before
BeginMainFrame makes sense. I don't see anything about ordering wrt
WillBeginMainFrame mattering right now.

+      callback.Run();
+  }
+
   layer_tree_host_->ApplyScrollAndScale(
       begin_main_frame_state->scroll_info.get());
 
   layer_tree_host_->WillBeginMainFrame();
 
   layer_tree_host_->BeginMainFrame(begin_main_frame_state->begin_frame_args);
   layer_tree_host_->AnimateLayers(
       begin_main_frame_state->begin_frame_args.frame_time);
 
   // Recreate all UI resources if there were evicted UI resources when the impl
@@ skipping 267 matching lines @@
     return false;
   channel_main_->SetNeedsCommitOnImpl();
   return true;
 }
 
 bool ProxyMain::IsMainThread() const {
   return task_runner_provider_->IsMainThread();
 }
 
 }  // namespace cc