Scroll with Layers in views::ScrollView

cc/trees/single_thread_proxy.cc

 // Copyright 2011 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/single_thread_proxy.h"
 
 #include "base/auto_reset.h"
 #include "base/memory/ptr_util.h"
 #include "base/profiler/scoped_tracker.h"
 #include "base/trace_event/trace_event.h"
@@ skipping 236 matching lines @@
     if (layer_tree_host_impl_->EvictedUIResourcesExist())
       layer_tree_host_->RecreateUIResources();
 
     // TODO(robliao): Remove ScopedTracker below once https://crbug.com/461509
     // is fixed.
     tracked_objects::ScopedTracker tracking_profile7(
         FROM_HERE_WITH_EXPLICIT_FUNCTION(
             "461509 SingleThreadProxy::DoCommit7"));
     layer_tree_host_->FinishCommitOnImplThread(layer_tree_host_impl_.get());
 
-#if DCHECK_IS_ON()
-    // In the single-threaded case, the scale and scroll deltas should never be
-    // touched on the impl layer tree.

[Ian Vollick, 2016/08/12 13:57:42]

I presume that scroll to offset can now cause scroll deltas to be collected by
the LTHI making the first DCHECK fail, but we shouldn't be pinch-zooming, so the
page scale delta should still be 1.f, right?

If that's true, can you reinstate this code for the 2nd DCHECK, and add a
comment as to why we might get scroll deltas in the single thread proxy (since
it's counterintuitive).

[tapted, 2016/08/13 09:03:53]

Done (reinstated in DoBeginMainFrame() since the ProcessScrollDeltas() is needed
for the DCHECK).

-    std::unique_ptr<ScrollAndScaleSet> scroll_info =
-        layer_tree_host_impl_->ProcessScrollDeltas();
-    DCHECK(scroll_info->scrolls.empty());
-    DCHECK_EQ(1.f, scroll_info->page_scale_delta);
-#endif
-
     if (scheduler_on_impl_thread_)
       scheduler_on_impl_thread_->DidCommit();
 
     layer_tree_host_impl_->CommitComplete();
 
     // TODO(robliao): Remove ScopedTracker below once https://crbug.com/461509
     // is fixed.
     tracked_objects::ScopedTracker tracking_profile8(
         FROM_HERE_WITH_EXPLICIT_FUNCTION(
             "461509 SingleThreadProxy::DoCommit8"));
@@ skipping 519 matching lines @@
   // Prevent new commits from being requested inside DoBeginMainFrame.
   // Note: We do not want to prevent SetNeedsAnimate from requesting
   // a commit here.
   commit_requested_ = true;
 
   DoBeginMainFrame(begin_frame_args);
 }
 
 void SingleThreadProxy::DoBeginMainFrame(
     const BeginFrameArgs& begin_frame_args) {
+  std::unique_ptr<ScrollAndScaleSet> scroll_info =
+      layer_tree_host_impl_->ProcessScrollDeltas();
+  layer_tree_host_->ApplyScrollAndScale(scroll_info.get());

[Ian Vollick, 2016/08/12 13:57:42]

Again, please add a comment as this is counterintuitive.

[tapted, 2016/08/13 09:03:53]

Done.

+
   layer_tree_host_->WillBeginMainFrame();
   layer_tree_host_->BeginMainFrame(begin_frame_args);
   layer_tree_host_->AnimateLayers(begin_frame_args.frame_time);
   layer_tree_host_->RequestMainFrameUpdate();
 
   // New commits requested inside UpdateLayers should be respected.
   commit_requested_ = false;
 
   layer_tree_host_->UpdateLayers();
 
@@ skipping 68 matching lines @@
 void SingleThreadProxy::DidFinishImplFrame() {
   layer_tree_host_impl_->DidFinishImplFrame();
 #if DCHECK_IS_ON()
   DCHECK(inside_impl_frame_)
       << "DidFinishImplFrame called while not inside an impl frame!";
   inside_impl_frame_ = false;
 #endif
 }
 
 }  // namespace cc