cc: LayerTreeHostImpl uses element id to tick animations

cc/layers/scrollbar_layer_impl_base.cc

 // Copyright 2013 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/layers/scrollbar_layer_impl_base.h"
 
 #include <algorithm>
+#include "cc/trees/effect_node.h"
 #include "cc/trees/layer_tree_impl.h"
 #include "ui/gfx/geometry/rect_conversions.h"
 
 namespace cc {
 
 ScrollbarLayerImplBase::ScrollbarLayerImplBase(
     LayerTreeImpl* tree_impl,
     int id,
     ScrollbarOrientation orientation,
     bool is_left_side_vertical_scrollbar,
@@ skipping 175 matching lines @@
             ? bounds().width() - thumb_thickness
             : thumb_thickness_adjustment,
         thumb_offset,
         thumb_thickness - thumb_thickness_adjustment,
         thumb_length);
   }
 
   return gfx::ToEnclosingRect(thumb_rect);
 }
 
+void ScrollbarLayerImplBase::SetOverlayScrollbarLayerOpacityAnimated(
+    float opacity) {
+  DCHECK(is_overlay_scrollbar());
+  if (!layer_tree_impl())
+    return;
+
+  PropertyTrees* property_trees = layer_tree_impl()->property_trees();
+  int effect_node_index =
+      property_trees->effect_tree.FindNodeIndexFromOwningLayerId(id());
+  // If this method is called during LayerImpl::PushPropertiesTo, we may not yet
+  // have valid owning_layer_id_to_node_index entries in effect tree as property
+  // trees are pushed after layers during activation. We can skip updating
+  // opacity in that case as we are only registering a scrollbar and because
+  // opacity will be overwritten anyway when property trees are pushed.
+  if (effect_node_index == EffectTree::kInvalidNodeId)
+    return;
+
+  DCHECK_EQ(effect_tree_index(), effect_node_index);
+  if (EffectNode* node =
+          property_trees->effect_tree.Node(effect_tree_index())) {

[ajuma, 2017/03/21 22:00:30]

Since we're already checking for an invalid id above do we still need this null
check?

[weiliangc, 2017/03/22 18:44:15]

I early out for the DCHECK now because when the layer list and the property tree
are not synced up we could be accessing a non existence node here.

+    if (node->opacity == opacity)
+      return;
+
+    layer_tree_impl()->AddToOpacityAnimationsMap(id(), opacity);
+
+    node->opacity = opacity;
+    node->effect_changed = true;
+    property_trees->changed = true;
+    property_trees->effect_tree.set_needs_update(true);
+    layer_tree_impl()->set_needs_update_draw_properties();
+  }
+}
+
 }  // namespace cc