Make first TouchStart and first TouchMove events on a flinging layer non-blocking

content/renderer/input/main_thread_event_queue.cc

 // Copyright 2016 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/renderer/input/main_thread_event_queue.h"
 #include "content/common/input/event_with_latency_info.h"
 #include "content/common/input_messages.h"
 
 namespace content {
 
@@ skipping 23 matching lines @@
   ScopedWebInputEventWithLatencyInfo::CoalesceWith(other);
 }
 
 MainThreadEventQueue::MainThreadEventQueue(
     int routing_id,
     MainThreadEventQueueClient* client,
     const scoped_refptr<base::SingleThreadTaskRunner>& main_task_runner)
     : routing_id_(routing_id),
       client_(client),
       is_flinging_(false),
+      last_touch_start_forced_nonblocking_(false),
+      enable_fling_passive_listener_flag_(base::FeatureList::IsEnabled(
+          features::kPassiveEventListenersDueToFling)),
       main_task_runner_(main_task_runner) {}
 
 MainThreadEventQueue::~MainThreadEventQueue() {}
 
 bool MainThreadEventQueue::HandleEvent(
     ui::ScopedWebInputEvent event,
     const ui::LatencyInfo& latency,
     InputEventDispatchType original_dispatch_type,
     InputEventAckState ack_result) {
   DCHECK(original_dispatch_type == DISPATCH_TYPE_BLOCKING ||
          original_dispatch_type == DISPATCH_TYPE_NON_BLOCKING);
   DCHECK(ack_result == INPUT_EVENT_ACK_STATE_SET_NON_BLOCKING ||
          ack_result == INPUT_EVENT_ACK_STATE_NOT_CONSUMED);
 
   bool non_blocking = original_dispatch_type == DISPATCH_TYPE_NON_BLOCKING ||
                       ack_result == INPUT_EVENT_ACK_STATE_SET_NON_BLOCKING;
-
-  InputEventDispatchType dispatch_type =
-      non_blocking ? DISPATCH_TYPE_NON_BLOCKING : DISPATCH_TYPE_BLOCKING;
-
   bool is_wheel = event->type == blink::WebInputEvent::MouseWheel;
   bool is_touch = blink::WebInputEvent::isTouchEventType(event->type);
 
   if (is_touch) {
     blink::WebTouchEvent* touch_event =
         static_cast<blink::WebTouchEvent*>(event.get());
     touch_event->dispatchedDuringFling = is_flinging_;
     // Adjust the |dispatchType| on the event since the compositor
     // determined all event listeners are passive.
     if (non_blocking) {
       touch_event->dispatchType =
           blink::WebInputEvent::ListenersNonBlockingPassive;
     }
+    if (touch_event->type == blink::WebInputEvent::TouchStart)
+      last_touch_start_forced_nonblocking_ = false;
+
+    if (enable_fling_passive_listener_flag_ &&
+        touch_event->touchStartOrFirstTouchMove &&
+        touch_event->dispatchType == blink::WebInputEvent::Blocking) {
+      // If the touch start is forced to be passive due to fling, its following
+      // touch move should also be passive.
+      if (is_flinging_ || last_touch_start_forced_nonblocking_) {
+        touch_event->dispatchType =
+            blink::WebInputEvent::ListenersForcedNonBlockingDueToFling;
+        non_blocking = true;
+        last_touch_start_forced_nonblocking_ = true;

[tdresser, 2016/08/18 12:43:53]

Sorry, let's call this last_touch_start_forced_nonblocking_due_to_fling_.

I'm going to plumb the main thread responsiveness intervention into the main
thread event queue too, so we should keep this unambiguous.

+      }
+    }
   }
   if (is_wheel && non_blocking) {
     // Adjust the |dispatchType| on the event since the compositor
     // determined all event listeners are passive.
     static_cast<blink::WebMouseWheelEvent*>(event.get())
         ->dispatchType = blink::WebInputEvent::ListenersNonBlockingPassive;
   }
 
+  InputEventDispatchType dispatch_type =
+      non_blocking ? DISPATCH_TYPE_NON_BLOCKING : DISPATCH_TYPE_BLOCKING;
   std::unique_ptr<EventWithDispatchType> event_with_dispatch_type(
       new EventWithDispatchType(std::move(event), latency, dispatch_type));
 
   QueueEvent(std::move(event_with_dispatch_type));
 
   // send an ack when we are non-blocking.
   return non_blocking;
 }
 
 void MainThreadEventQueue::PopEventOnMainThread() {
@@ skipping 40 matching lines @@
     base::AutoLock lock(event_queue_lock_);
     size_t size_before = events_.size();
     events_.Queue(std::move(event));
     send_notification = events_.size() != size_before;
   }
   if (send_notification)
     SendEventNotificationToMainThread();
 }
 
 }  // namespace content