cc : Calculate jitter without using layer hierarchy

cc/trees/layer_tree_impl.h

 // Copyright 2012 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.
 
 #ifndef CC_TREES_LAYER_TREE_IMPL_H_
 #define CC_TREES_LAYER_TREE_IMPL_H_
 
 #include <map>
 #include <set>
 #include <string>
@@ skipping 175 matching lines @@
 
   LayerImpl* InnerViewportScrollLayer() const;
   // This function may return NULL, it is the caller's responsibility to check.
   LayerImpl* OuterViewportScrollLayer() const;
   gfx::ScrollOffset TotalScrollOffset() const;
   gfx::ScrollOffset TotalMaxScrollOffset() const;
 
   LayerImpl* InnerViewportContainerLayer() const;
   LayerImpl* OuterViewportContainerLayer() const;
   LayerImpl* CurrentlyScrollingLayer() const;
-  int LastScrolledLayerId() const;
+  int LastScrolledScrollNodeId() const;
   void SetCurrentlyScrollingLayer(LayerImpl* layer);
   void ClearCurrentlyScrollingLayer();
 
   void SetViewportLayersFromIds(int overscroll_elasticity_layer,
                                 int page_scale_layer_id,
                                 int inner_viewport_scroll_layer_id,
                                 int outer_viewport_scroll_layer_id);
   void ClearViewportLayers();
   LayerImpl* OverscrollElasticityLayer() {
     return LayerById(overscroll_elasticity_layer_id_);
@@ skipping 296 matching lines @@
   void PushTopControls(const float* top_controls_shown_ratio);
   LayerTreeHostImpl* layer_tree_host_impl_;
   int source_frame_number_;
   int is_first_frame_after_commit_tracker_;
   LayerImpl* root_layer_;
   HeadsUpDisplayLayerImpl* hud_layer_;
   PropertyTrees property_trees_;
   SkColor background_color_;
   bool has_transparent_background_;
 
-  int last_scrolled_layer_id_;
+  int last_scrolled_scroll_node_id_;

[ajuma, 2016/03/29 14:24:02]

We already have a currently_scrolling_node_id on the scroll tree, so do we still
need this id?

[jaydasika, 2016/03/29 15:57:24]

I think we need this as the one on scroll tree is reset when ScrollEnd is called

[ajuma, 2016/03/29 16:52:07]

Ah, makes sense. Does this need to get updated on each activation then? (Since,
unlike layer ids, scroll node ids aren't stable.)

[jaydasika, 2016/03/29 18:30:19]

It gets updated every activation here :
https://code.google.com/p/chromium/codesearch#chromium/src/cc/trees/layer_tre...

[ajuma, 2016/03/29 18:50:04]

When the layer passed to SetCurrentlyScrollingLayer is null,
last_scrolled_scroll_node_id_ isn't updated.

   int overscroll_elasticity_layer_id_;
   int page_scale_layer_id_;
   int inner_viewport_scroll_layer_id_;
   int outer_viewport_scroll_layer_id_;
 
   LayerSelection selection_;
 
   scoped_refptr<SyncedProperty<ScaleGroup>> page_scale_factor_;
   float min_page_scale_factor_;
   float max_page_scale_factor_;
@@ skipping 64 matching lines @@
 
   scoped_ptr<PendingPageScaleAnimation> pending_page_scale_animation_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(LayerTreeImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_IMPL_H_