Refactor delegated scrolling in LayerImpl

cc/trees/layer_tree_impl.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/layer_tree_impl.h"
 
 #include <algorithm>
 #include <limits>
 #include <set>
 
@@ skipping 46 matching lines @@
 
   gfx::ScrollOffset GetCurrentScrollOffset() override {
     return layer_tree_impl_->GetDelegatedScrollOffset(layer_);
   }
 
   bool IsExternalFlingActive() const override {
     return delegate_->IsExternalFlingActive();
   }
 
   void Update() const override {
-    layer_tree_impl_->UpdateScrollOffsetDelegate();
+    layer_tree_impl_->UpdateScrollOffsetDelegateIfNeeded();
   }
 
  private:
   LayerImpl* layer_;
   LayerScrollOffsetDelegate* delegate_;
   LayerTreeImpl* layer_tree_impl_;
   gfx::ScrollOffset last_set_scroll_offset_;
 };
 
 LayerTreeImpl::LayerTreeImpl(
@@ skipping 985 matching lines @@
       outer_viewport_scroll_layer_->SetScrollOffsetDelegate(
           outer_viewport_scroll_delegate_proxy_.get());
     }
 
     if (inner_viewport_scroll_layer_)
       inner_viewport_scroll_layer_->RefreshFromScrollDelegate();
     if (outer_viewport_scroll_layer_)
       outer_viewport_scroll_layer_->RefreshFromScrollDelegate();
 
     if (inner_viewport_scroll_layer_)
-      UpdateScrollOffsetDelegate();
+      UpdateScrollOffsetDelegateIfNeeded();
   }
 }
 
 void LayerTreeImpl::OnRootLayerDelegatedScrollOffsetChanged() {
   DCHECK(root_layer_scroll_offset_delegate_);
-  if (inner_viewport_scroll_layer_) {
+  root_layer_scroll_offset_delegate_->SetBlockUpdates(true);
+  if (inner_viewport_scroll_layer_)
     inner_viewport_scroll_layer_->RefreshFromScrollDelegate();
-  }
-  if (outer_viewport_scroll_layer_) {
+
+  if (outer_viewport_scroll_layer_)
     outer_viewport_scroll_layer_->RefreshFromScrollDelegate();
-  }
+
+  root_layer_scroll_offset_delegate_->SetBlockUpdates(false);
+  UpdateScrollOffsetDelegateIfNeeded();
 }
 
-void LayerTreeImpl::UpdateScrollOffsetDelegate() {
-  DCHECK(InnerViewportScrollLayer());
-  DCHECK(!OuterViewportScrollLayer() || outer_viewport_scroll_delegate_proxy_);
-  DCHECK(root_layer_scroll_offset_delegate_);
-
+void LayerTreeImpl::UpdateScrollOffsetDelegateIfNeeded() {
+  if (!root_layer_scroll_offset_delegate_)
+    return;
+  if (!InnerViewportScrollLayer() || !inner_viewport_scroll_delegate_proxy_)
+    return;
+  if (OuterViewportScrollLayer() && !outer_viewport_scroll_delegate_proxy_)
+    return;
   gfx::ScrollOffset offset =
       inner_viewport_scroll_delegate_proxy_->last_set_scroll_offset();
 
   if (OuterViewportScrollLayer())
     offset += outer_viewport_scroll_delegate_proxy_->last_set_scroll_offset();
 
   root_layer_scroll_offset_delegate_->UpdateRootLayerState(
       offset, TotalMaxScrollOffset(), ScrollableSize(),
       current_page_scale_factor(), min_page_scale_factor(),
       max_page_scale_factor());
@@ skipping 536 matching lines @@
     scoped_ptr<PendingPageScaleAnimation> pending_animation) {
   pending_page_scale_animation_ = pending_animation.Pass();
 }
 
 scoped_ptr<PendingPageScaleAnimation>
     LayerTreeImpl::TakePendingPageScaleAnimation() {
   return pending_page_scale_animation_.Pass();
 }
 
 }  // namespace cc