Overscroll and Elasticity for views::ScrollView

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 655eda22d650d9a5aa05451926f2dadbc87a606e..a2cf2bf2112b5d881800d855f49ac44da91a8665 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -283,8 +283,7 @@ LayerTreeHostImpl::~LayerTreeHostImpl() {
     input_handler_client_->WillShutdown();
     input_handler_client_ = NULL;
   }
-  if (scroll_elasticity_helper_)
-    scroll_elasticity_helper_.reset();
+  root_scroll_elasticity_helper_.reset();
 
   // The layer trees must be destroyed before the layer tree host. We've
   // made a contract with our animation controllers that the animation_host
@@ -611,13 +610,22 @@ LayerTreeHostImpl::CreateLatencyInfoSwapPromiseMonitor(
       new LatencyInfoSwapPromiseMonitor(latency, NULL, this));
 }
 
-ScrollElasticityHelper* LayerTreeHostImpl::CreateScrollElasticityHelper() {
-  DCHECK(!scroll_elasticity_helper_);
+ScrollElasticityHelper* LayerTreeHostImpl::CreateRootScrollElasticityHelper() {
+  DCHECK(!root_scroll_elasticity_helper_);
   if (settings_.enable_elastic_overscroll) {
-    scroll_elasticity_helper_.reset(
+    root_scroll_elasticity_helper_.reset(
         ScrollElasticityHelper::CreateForLayerTreeHostImpl(this));
   }
-  return scroll_elasticity_helper_.get();
+  return root_scroll_elasticity_helper_.get();
+}
+
+base::WeakPtr<ScrollElasticityHelper>
+LayerTreeHostImpl::ScrollElasticityHelperForScrollingLayer() {
+  LayerImpl* scrolling_layer_impl = CurrentlyScrollingLayer();
+  if (!scrolling_layer_impl)
+    return nullptr;
+
+  return ScrollElasticityHelperForId(scrolling_layer_impl->id())->GetWeakPtr();
 }
 
 bool LayerTreeHostImpl::GetScrollOffsetForLayer(int layer_id,
@@ -2614,8 +2622,8 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBeginImpl(
   // in input_handler_proxy instead.
   wheel_scrolling_ = IsWheelBasedScroll(type);
   scroll_state->set_is_direct_manipulation(!wheel_scrolling_);
-  // Invoke |DistributeScrollDelta| even with zero delta and velocity to ensure
-  // scroll customization callbacks are invoked.
+  // Invoke |DistributeScrollDelta| even with zero delta and velocity to process
+  // overscroll and ensure scroll customization callbacks are invoked.
   DistributeScrollDelta(scroll_state);
 
   client_->RenewTreePriority();
@@ -3523,6 +3531,38 @@ LayerTreeHostImpl::ScrollbarAnimationControllerForId(
   return i->second.get();
 }
 
+void LayerTreeHostImpl::RegisterScrollElasticityHelper(int scroll_layer_id) {
+  DCHECK(!InnerViewportScrollLayer() ||
+         scroll_layer_id != InnerViewportScrollLayer()->id());
+  DCHECK(!OuterViewportScrollLayer() ||
+         scroll_layer_id != OuterViewportScrollLayer()->id());
+  DCHECK(settings().enable_elastic_overscroll);
+
+  auto& helper = scroll_elasticity_helpers_[scroll_layer_id];
+  if (!helper)
+    helper = active_tree_->CreateScrollElasticityHelper(scroll_layer_id);
+}
+
+void LayerTreeHostImpl::UnregisterScrollElasticityHelper(int scroll_layer_id) {
+  scroll_elasticity_helpers_.erase(scroll_layer_id);
+}
+
+ScrollElasticityHelper* LayerTreeHostImpl::ScrollElasticityHelperForId(
+    int scroll_layer_id) {
+  if (!settings().enable_elastic_overscroll)
+    return nullptr;
+
+  auto i = scroll_elasticity_helpers_.find(scroll_layer_id);
+  if (i != scroll_elasticity_helpers_.end())
+    return i->second.get();
+
+  if (!root_scroll_elasticity_helper_) {
+    root_scroll_elasticity_helper_.reset(
+        ScrollElasticityHelper::CreateForLayerTreeHostImpl(this));
+  }
+  return root_scroll_elasticity_helper_.get();
+}
+
 void LayerTreeHostImpl::PostDelayedScrollbarAnimationTask(
     const base::Closure& task,
     base::TimeDelta delay) {