Coalesce async touch move events until the ack back from render

content/common/input/web_input_event_traits.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 "content/common/input/web_input_event_traits.h"
 
 #include <bitset>
 #include <limits>
 
 #include "base/logging.h"
 #include "base/strings/stringprintf.h"
+#include "content/common/input_messages.h"
 
 using base::StringAppendF;
 using base::SStringPrintf;
 using blink::WebGestureEvent;
 using blink::WebInputEvent;
 using blink::WebKeyboardEvent;
 using blink::WebMouseEvent;
 using blink::WebMouseWheelEvent;
 using blink::WebTouchEvent;
 using blink::WebTouchPoint;
@@ skipping 458 matching lines @@
       return true;
     case WebInputEvent::TouchStart:
     case WebInputEvent::TouchMove:
     case WebInputEvent::TouchEnd:
       return !static_cast<const WebTouchEvent&>(event).cancelable;
     default:
       return false;
   }
 }
 
+scoped_ptr<IPC::Message> WebInputEventTraits::CreateAckIfNecessary(
+    const blink::WebInputEvent& event,
+    const content::InputEventAckState ack_state,
+    const ui::LatencyInfo& latency_info,
+    scoped_ptr<DidOverscrollParams> overscroll_params,
+    int routing_id) {
+  scoped_ptr<IPC::Message> response;
+  if (event.type == WebInputEvent::TouchMove) {
+    const WebTouchEvent& touch = static_cast<const WebTouchEvent&>(event);
+    if (!touch.cancelable) {
+      response = scoped_ptr<IPC::Message>(
+          new InputHostMsg_HandleUncancelableTouchMoveEvent_ACK(routing_id));
+    }
+  }
+
+  const bool send_ack = !WebInputEventTraits::IgnoresAckDisposition(event);
+  if (send_ack) {

[tdresser, 2015/03/31 14:44:07]

Could we end up overwriting response here?

We should at least:
DCHECK(!response);

[lanwei, 2015/04/01 13:14:21]

Done.

+    InputHostMsg_HandleInputEvent_ACK_Params ack;
+    ack.type = event.type;
+    ack.state = ack_state;
+    ack.latency = latency_info;
+    ack.overscroll = overscroll_params.Pass();
+    response = scoped_ptr<IPC::Message>(
+        new InputHostMsg_HandleInputEvent_ACK(routing_id, ack));
+  }
+  return response.Pass();
+}
+
 }  // namespace content