Refactor LayerTreeImpl's scrollbar map to be keyed on element ids

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 77 matching lines @@
   bool IsSyncTree() const;
   LayerImpl* FindActiveTreeLayerById(int id);
   LayerImpl* FindPendingTreeLayerById(int id);
   bool PinchGestureActive() const;
   BeginFrameArgs CurrentBeginFrameArgs() const;
   base::TimeDelta CurrentBeginFrameInterval() const;
   gfx::Rect DeviceViewport() const;
   gfx::Size DrawViewportSize() const;
   const gfx::Rect ViewportRectForTilePriority() const;
   std::unique_ptr<ScrollbarAnimationController>
-  CreateScrollbarAnimationController(int scroll_layer_id);
+  CreateScrollbarAnimationController(ElementId scroll_element_id);
   void DidAnimateScrollOffset();
   bool use_gpu_rasterization() const;
   GpuRasterizationStatus GetGpuRasterizationStatus() const;
   bool create_low_res_tiling() const;
   bool RequiresHighResToDraw() const;
   bool SmoothnessTakesPriority() const;
   VideoFrameControllerClient* GetVideoFrameControllerClient() const;
   MutatorHost* mutator_host() const {
     return layer_tree_host_impl_->mutator_host();
   }
@@ skipping 279 matching lines @@
   bool IsUIResourceOpaque(UIResourceId uid) const;
 
   void RegisterPictureLayerImpl(PictureLayerImpl* layer);
   void UnregisterPictureLayerImpl(PictureLayerImpl* layer);
   const std::vector<PictureLayerImpl*>& picture_layers() const {
     return picture_layers_;
   }
 
   void RegisterScrollbar(ScrollbarLayerImplBase* scrollbar_layer);
   void UnregisterScrollbar(ScrollbarLayerImplBase* scrollbar_layer);
-  ScrollbarSet ScrollbarsFor(int scroll_layer_id) const;
+  ScrollbarSet ScrollbarsFor(ElementId scroll_element_id) const;
 
   void RegisterScrollLayer(LayerImpl* layer);
   void UnregisterScrollLayer(LayerImpl* layer);
 
   void AddSurfaceLayer(LayerImpl* layer);
   void RemoveSurfaceLayer(LayerImpl* layer);
   const LayerImplList& SurfaceLayers() const { return surface_layers_; }
 
   LayerImpl* FindFirstScrollingLayerOrDrawnScrollbarThatIsHitByPoint(
       const gfx::PointF& screen_space_point);
@@ skipping 116 matching lines @@
   std::unordered_map<ElementId, gfx::Transform, ElementIdHash>
       element_id_to_transform_animations_;
   std::unordered_map<ElementId, FilterOperations, ElementIdHash>
       element_id_to_filter_animations_;
 
   // Maps from clip layer ids to scroll layer ids.  Note that this only includes
   // the subset of clip layers that act as scrolling containers.  (This is
   // derived from LayerImpl::scroll_clip_layer_ and exists to avoid O(n) walks.)
   std::unordered_map<int, int> clip_scroll_map_;
 
-  // Maps scroll layer ids to scrollbar layer ids.  For each scroll layer, there
-  // may be 1 or 2 scrollbar layers (for vertical and horizontal).  (This is
-  // derived from ScrollbarLayerImplBase::scroll_layer_id_ and exists to avoid
-  // O(n) walks.)
-  std::multimap<int, int> scrollbar_map_;
+  // Maps scroll element ids to scrollbar layer ids. For each scroll layer,
+  // there may be 1 or 2 scrollbar layers (for vertical and horizontal). (This
+  // is derived from ScrollbarLayerImplBase::scroll_element_id_ and exists to
+  // avoid O(n) walks.)
+  std::multimap<ElementId, int> scrollbar_map_;

[wkorman, 2017/04/17 17:44:35]

Suggestion, consider rename to something like element_id_to_scroll_layer_ids,
which makes code referencing more self documenting. We've been using this for
newer named maps in cc.

[wkorman, 2017/04/17 17:44:35]

Future optimization potential -- similar to recent brettw@ change, maybe we
should look at internal impl of multimap, since we know there will never be more
than two, I expect this has inefficient memory overhead.

Could switch to analogue of flat_map for multimap, or a map whose value is a
tuple/vector/array-of-two.

See https://codereview.chromium.org/2801603010

[pdr., 2017/04/19 18:02:18]

Switched name from scrollbar_map_ to element_id_to_scrollbar_layer_ids_.

Yeah I think a flat_map and/or map whose value is a pair would be ideal here.
Mind if I pursue this in a followup? I'd like to keep this patch as just the
layer id -> element id change. I've added a TODO above
element_id_to_scrollbar_layer_ids_ to update to a better datastructure.

[wkorman, 2017/04/19 18:07:09]

Yeah, definitely better as separate change.

 
   std::vector<PictureLayerImpl*> picture_layers_;
   LayerImplList surface_layers_;
 
   // List of visible layers for the most recently prepared frame.
   LayerImplList render_surface_layer_list_;
   // After drawing the |render_surface_layer_list_| the areas in this region
   // would not be fully covered by opaque content.
   Region unoccluded_screen_space_region_;
 
@@ skipping 31 matching lines @@
 
   std::unique_ptr<PendingPageScaleAnimation> pending_page_scale_animation_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(LayerTreeImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_IMPL_H_