Refactor compositor event handling path to be callback-based

content/renderer/input/input_event_filter.cc

Index: content/renderer/input/input_event_filter.cc
diff --git a/content/renderer/input/input_event_filter.cc b/content/renderer/input/input_event_filter.cc
index 076eec831668228b1cadaa95a7c005f4f70700cc..96f799c2caf9126bab285473931a7c530505273e 100644
--- a/content/renderer/input/input_event_filter.cc
+++ b/content/renderer/input/input_event_filter.cc
@@ -53,7 +53,6 @@ InputEventFilter::InputEventFilter(
       main_listener_(main_listener),
       sender_(NULL),
       target_task_runner_(target_task_runner),
-      current_overscroll_params_(NULL),
       renderer_scheduler_(NULL) {
   DCHECK(target_task_runner_.get());
   DCHECK(main_task_runner_->BelongsToCurrentThread());
@@ -90,11 +89,12 @@ void InputEventFilter::UnregisterRoutingID(int routing_id) {
 }
 
 void InputEventFilter::DidOverscroll(int routing_id,
-                                     const DidOverscrollParams& params) {
-  if (current_overscroll_params_) {
-    current_overscroll_params_->reset(new DidOverscrollParams(params));
+                                     const DidOverscrollParams& params,
+                                     bool bundle_ack_with_triggering_event) {
+  // Overscroll message will be bundled with triggering event ACK, saving an
+  // IPC.
+  if (bundle_ack_with_triggering_event)
     return;
-  }
 
   SendMessage(std::unique_ptr<IPC::Message>(
       new InputHostMsg_DidOverscroll(routing_id, params)));
@@ -165,9 +165,7 @@ bool InputEventFilter::OnMessageReceived(const IPC::Message& message) {
   return true;
 }
 
-InputEventFilter::~InputEventFilter() {
-  DCHECK(!current_overscroll_params_);
-}
+InputEventFilter::~InputEventFilter() {}
 
 void InputEventFilter::ForwardToHandler(const IPC::Message& message) {
   DCHECK(!handler_.is_null());
@@ -192,22 +190,24 @@ void InputEventFilter::ForwardToHandler(const IPC::Message& message) {
       ui::WebInputEventTraits::Clone(*std::get<0>(params));
   ui::LatencyInfo latency_info = std::get<1>(params);
   InputEventDispatchType dispatch_type = std::get<2>(params);
+
   DCHECK(event);
   DCHECK(dispatch_type == DISPATCH_TYPE_BLOCKING ||
          dispatch_type == DISPATCH_TYPE_NON_BLOCKING);
 
-  bool send_ack = dispatch_type == DISPATCH_TYPE_BLOCKING;
-
-  // Intercept |DidOverscroll| notifications, bundling any triggered overscroll
-  // response with the input event ack.
-  std::unique_ptr<DidOverscrollParams> overscroll_params;
-  base::AutoReset<std::unique_ptr<DidOverscrollParams>*>
-      auto_reset_current_overscroll_params(
-          &current_overscroll_params_, send_ack ? &overscroll_params : NULL);
-
-  InputEventAckState ack_state =
-      handler_.Run(routing_id, event.get(), &latency_info);
+  handler_.Run(routing_id, std::move(event), latency_info,
+               base::Bind(&InputEventFilter::DidForwardToHandler, this,
+                          routing_id, dispatch_type));
+};
 
+void InputEventFilter::DidForwardToHandler(
+    int routing_id,
+    InputEventDispatchType dispatch_type,
+    InputEventAckState ack_state,
+    ui::ScopedWebInputEvent event,
+    const ui::LatencyInfo& latency_info,
+    std::unique_ptr<DidOverscrollParams> overscroll_params) {
+  bool send_ack = dispatch_type == DISPATCH_TYPE_BLOCKING;
   uint32_t unique_touch_event_id =
       ui::WebInputEventTraits::GetUniqueTouchEventId(*event);
   WebInputEvent::Type type = event->type;