Overscroll and Elasticity for views::ScrollView

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index 1342c662d37f22acaef8ba033e528c9913704b32..0c387796d65c754b8f834332503d3d4a397a0c22 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -24,6 +24,7 @@
 #include "cc/debug/devtools_instrumentation.h"
 #include "cc/debug/traced_value.h"
 #include "cc/input/page_scale_animation.h"
+#include "cc/input/scroll_elasticity_helper.h"
 #include "cc/input/scrollbar_animation_controller.h"
 #include "cc/input/scrollbar_animation_controller_linear_fade.h"
 #include "cc/input/scrollbar_animation_controller_thinning.h"
@@ -154,11 +155,11 @@ void LayerTreeImpl::DidUpdateScrollOffset(int layer_id) {
   // If pending tree topology changed and we still want to notify the pending
   // tree about scroll offset in the active tree, we may not find the
   // corresponding pending layer.
-  if (LayerById(layer_id)) {
+  if (LayerImpl* layer = LayerById(layer_id)) {
     // TODO(sunxd): when we have a layer_id to property_tree index map in
     // property trees, use the transform_id parameter instead of looking for
     // indices from LayerImpls.
-    transform_id = LayerById(layer_id)->transform_tree_index();
+    transform_id = layer->transform_tree_index();
   } else {
     DCHECK(!IsActiveTree());
     return;
@@ -166,8 +167,11 @@ void LayerTreeImpl::DidUpdateScrollOffset(int layer_id) {
 
   if (transform_id != -1) {
     TransformNode* node = transform_tree.Node(transform_id);
-    if (node->scroll_offset != scroll_tree.current_scroll_offset(layer_id)) {
-      node->scroll_offset = scroll_tree.current_scroll_offset(layer_id);
+    gfx::ScrollOffset scroll_offset_with_overscroll =
+        scroll_tree.current_scroll_offset(layer_id) +
+        scroll_tree.current_overscroll(layer_id);
+    if (node->scroll_offset_with_overscroll != scroll_offset_with_overscroll) {
+      node->scroll_offset_with_overscroll = scroll_offset_with_overscroll;
       node->needs_local_transform_update = true;
       transform_tree.set_needs_update(true);
     }
@@ -613,12 +617,20 @@ void LayerTreeImpl::SetCurrentlyScrollingLayer(LayerImpl* layer) {
 
   ScrollbarAnimationController* old_animation_controller =
       layer_tree_host_impl_->ScrollbarAnimationControllerForId(old_id);
+  ScrollElasticityHelper* old_elasticity_helper =
+      layer_tree_host_impl_->ScrollElasticityHelperForId(old_id);
   ScrollbarAnimationController* new_animation_controller =
       layer_tree_host_impl_->ScrollbarAnimationControllerForId(new_id);
+  ScrollElasticityHelper* new_elasticity_helper =
+      layer_tree_host_impl_->ScrollElasticityHelperForId(new_id);
+  if (old_elasticity_helper == new_elasticity_helper)
+    old_elasticity_helper = new_elasticity_helper = nullptr;
 
   if (old_animation_controller)
     old_animation_controller->DidScrollEnd();
+
   scroll_tree.set_currently_scrolling_node(new_scroll_node_id);
+
   if (new_animation_controller)
     new_animation_controller->DidScrollBegin();
 }
@@ -1375,6 +1387,13 @@ LayerTreeImpl::CreateScrollbarAnimationController(int scroll_layer_id) {
   return nullptr;
 }
 
+std::unique_ptr<ScrollElasticityHelper>
+LayerTreeImpl::CreateScrollElasticityHelper(int scroll_layer_id) {
+  DCHECK(settings().enable_elastic_overscroll);
+  return ScrollElasticityHelper::CreateForLayer(scroll_layer_id,
+                                                layer_tree_host_impl_);
+}
+
 void LayerTreeImpl::DidAnimateScrollOffset() {
   layer_tree_host_impl_->DidAnimateScrollOffset();
 }
@@ -1640,6 +1659,13 @@ void LayerTreeImpl::RegisterScrollLayer(LayerImpl* layer) {
   clip_scroll_map_.insert(
       std::pair<int, int>(layer->scroll_clip_layer_id(), layer->id()));
 
+  bool can_overscroll = layer->can_overscroll();
+  if (can_overscroll && settings().enable_elastic_overscroll &&
+      (layer->user_scrollable_horizontal() ||
+       layer->user_scrollable_vertical())) {
+    layer_tree_host_impl_->RegisterScrollElasticityHelper(layer->id());
+  }
+
   DidUpdateScrollState(layer->id());
 }
 
@@ -1647,6 +1673,7 @@ void LayerTreeImpl::UnregisterScrollLayer(LayerImpl* layer) {
   if (layer->scroll_clip_layer_id() == Layer::INVALID_ID)
     return;
 
+  layer_tree_host_impl_->UnregisterScrollElasticityHelper(layer->id());
   clip_scroll_map_.erase(layer->scroll_clip_layer_id());
 }