Refactor delegated scrolling in LayerImpl

cc/layers/layer_impl.h

 // 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.
 
 #ifndef CC_LAYERS_LAYER_IMPL_H_
 #define CC_LAYERS_LAYER_IMPL_H_
 
 #include <map>
 #include <set>
 #include <string>
@@ skipping 62 matching lines @@
   DRAW_MODE_NONE,
   DRAW_MODE_HARDWARE,
   DRAW_MODE_SOFTWARE,
   DRAW_MODE_RESOURCELESS_SOFTWARE
 };
 
 class CC_EXPORT LayerImpl : public LayerAnimationValueObserver,
                             public LayerAnimationValueProvider,
                             public AnimationDelegate {
  public:
-  // Allows for the ownership of the total scroll offset to be delegated outside
-  // of the layer.
-  class ScrollOffsetDelegate {
+  // An embedder will use ExternalScrollOffsetListener to know when the
+  // LayerImpl's scroll offset has changed. ExternalScrollOffsetListener
+  // carries information of whether external fling is active.
+  class ExternalScrollOffsetListener {

[aelias_OOO_until_Jul13, 2015/04/06 19:47:49]

What do you think about going further and get rid of the listener class
entirely?  LayerImpl can just call layer_tree_impl()->IsExternalFlingActive()
and layer_tree_impl()->DidUpdateScrollOffset(id()).  DidUpdateScrollOffset would
take an int layer_id as argument and LayerTreeImpl would compare it to
InnerViewportScrollLayer()->id() and OuterViewportScrollLayer()->id() to know if
it's the root scroll, and ignore it otherwise.

[hush (inactive), 2015/04/06 23:08:03]

yes. I added DidUpdateScrollOffset and IsExternalFlingActive in LTI.
I also added an extra state in LTI to indicate the root layer scroll offset is
being distributed to the inner/ outer viewport scroll layers and LTI will ignore
DidUpdateScrollOffset in this case.

    public:
-    virtual void SetCurrentScrollOffset(const gfx::ScrollOffset& new_value) = 0;
-    virtual gfx::ScrollOffset GetCurrentScrollOffset() = 0;
     virtual bool IsExternalFlingActive() const = 0;
-    virtual void Update() const = 0;
+    // Tells the embedder to update because the LayerImpl has changed its scroll
+    // offset.
+    virtual void DidUpdateScrollOffset() const = 0;
   };
 
   typedef SyncedProperty<AdditionGroup<gfx::ScrollOffset>> SyncedScrollOffset;
   typedef LayerImplList RenderSurfaceListType;
   typedef LayerImplList LayerListType;
   typedef RenderSurfaceImpl RenderSurfaceType;
 
   enum RenderingContextConstants { NO_RENDERING_CONTEXT = 0 };
 
   static scoped_ptr<LayerImpl> Create(
@@ skipping 282 matching lines @@
   void SetBoundsDelta(const gfx::Vector2dF& bounds_delta);
   gfx::Vector2dF bounds_delta() const { return bounds_delta_; }
 
   void SetContentBounds(const gfx::Size& content_bounds);
   gfx::Size content_bounds() const { return draw_properties_.content_bounds; }
 
   float contents_scale_x() const { return draw_properties_.contents_scale_x; }
   float contents_scale_y() const { return draw_properties_.contents_scale_y; }
   void SetContentsScale(float contents_scale_x, float contents_scale_y);
 
-  void SetScrollOffsetDelegate(ScrollOffsetDelegate* scroll_offset_delegate);
-  void RefreshFromScrollDelegate();
+  void SetExternalScrollOffsetListener(ExternalScrollOffsetListener* listener);
   bool IsExternalFlingActive() const;
 
+  // Visible for testing.

[aelias_OOO_until_Jul13, 2015/04/06 19:47:49]

It's not clear what methods below this comment refers to, and I think
non-test-code could still legitimately call SetCurrentScrollOffset in the
future, so I'd suggest removing the comment.

[hush (inactive), 2015/04/06 23:08:03]

Removed the comment.

   void SetCurrentScrollOffset(const gfx::ScrollOffset& scroll_offset);
   void PushScrollOffsetFromMainThread(const gfx::ScrollOffset& scroll_offset);
   // This method is similar to PushScrollOffsetFromMainThread but will cause the
   // scroll offset given to clobber any scroll changes on the active tree in the
   // time until this value is pushed to the active tree.
   void PushScrollOffsetFromMainThreadAndClobberActiveValue(
       const gfx::ScrollOffset& scroll_offset);
   gfx::ScrollOffset PullDeltaForMainThread();
   gfx::ScrollOffset CurrentScrollOffset() const;
   gfx::ScrollOffset BaseScrollOffset() const;
@@ skipping 258 matching lines @@
   int layer_id_;
   LayerTreeImpl* layer_tree_impl_;
 
   // Properties dynamically changeable on active tree.
   scoped_refptr<SyncedScrollOffset> scroll_offset_;
   gfx::Vector2dF bounds_delta_;
 
   // Properties synchronized from the associated Layer.
   gfx::Point3F transform_origin_;
   gfx::Size bounds_;
-  ScrollOffsetDelegate* scroll_offset_delegate_;
+  ExternalScrollOffsetListener* scroll_offset_listener_;
   LayerImpl* scroll_clip_layer_;
   bool scrollable_ : 1;
   bool should_scroll_on_main_thread_ : 1;
   bool have_wheel_event_handlers_ : 1;
   bool have_scroll_event_handlers_ : 1;
 
   static_assert(SCROLL_BLOCKS_ON_MAX < (1 << 3), "ScrollBlocksOn too big");
   ScrollBlocksOn scroll_blocks_on_ : 3;
 
   bool user_scrollable_horizontal_ : 1;
@@ skipping 88 matching lines @@
 
   std::vector<FrameTimingRequest> frame_timing_requests_;
   bool frame_timing_requests_dirty_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_LAYER_IMPL_H_