[VSync Queue] Plug touch ack to gesture events and flush vsync queue if necessary

ui/events/blink/input_handler_proxy.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 "ui/events/blink/input_handler_proxy.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 
@@ skipping 304 matching lines @@
   // Note: Other input can race ahead of gesture input as they don't have to go
   // through the queue, but we believe it's OK to do so.
   if (!compositor_event_queue_ ||
       !IsGestureScrollOrFlingOrPinch(event_with_callback->event().GetType())) {
     DispatchSingleInputEvent(std::move(event_with_callback),
                              tick_clock_->NowTicks());
     return;
   }
 
   if (has_ongoing_compositor_scroll_fling_pinch_) {
+    const auto& gesture_event = ToWebGestureEvent(event_with_callback->event());
+    if (gesture_event.source_device == blink::kWebGestureDeviceTouchscreen &&
+        gesture_event.source_touch_event_dispatch_type ==
+            WebInputEvent::kBlocking) {
+      // Dispatch immediately to reduce latency.
+      compositor_event_queue_->Queue(std::move(event_with_callback),
+                                     tick_clock_->NowTicks());
+      DispatchQueuedInputEvents();
+      return;
+    }
+
     bool needs_animate_input = compositor_event_queue_->empty();
     compositor_event_queue_->Queue(std::move(event_with_callback),
                                    tick_clock_->NowTicks());
     if (needs_animate_input)
       input_handler_->SetNeedsAnimateInput();
     return;
   }
 
   // We have to dispatch the event to know whether the gesture sequence will be
   // handled by the compositor or not.
@@ skipping 31 matching lines @@
     }
   }
 
   ui::LatencyInfo monitored_latency_info = event_with_callback->latency_info();
   std::unique_ptr<cc::SwapPromiseMonitor> latency_info_swap_promise_monitor =
       input_handler_->CreateLatencyInfoSwapPromiseMonitor(
           &monitored_latency_info);
 
   current_overscroll_params_.reset();
   InputHandlerProxy::EventDisposition disposition =
       HandleInputEvent(event_with_callback->event());

[chongz, 2017/05/25 20:25:28]

sadrul@ Just to recall: During our in-person discussion you suggested that we
should store |InputEventAckState| (same as |EventDisposition|) on renderer for
the Touch event and use on next Gesture event.

However events could switch between blocking and non-blocking in one scroll, and
it seems to me that the following sequence may happen:
1. TouchMove1
2. TouchMove2
3. GestureScroll1 (from TouchMove1)
4. GestureScroll2 (from TouchMove2)

Or is there anything I'm missing?

[sadrul, 2017/06/01 19:57:17]

@dtapuska@: Can you confirm if this can happen?

Actually, even if this happens, you still want to release the queue immediately,
right? So shouldn't you just do a PostTask (and from RenderWidgetInputHandler
too, maybe?) to DispatchQueuedInputEvents()?

My feedback here is that sending the relevant information from main-thread to
compositor-thread should happen directly, instead of having to route through all
of browser/aura.

 
   switch (event_with_callback->event().GetType()) {
     case blink::WebGestureEvent::kGestureScrollBegin:
     case blink::WebGestureEvent::kGestureFlingStart:
     case blink::WebGestureEvent::kGesturePinchBegin:
     case blink::WebGestureEvent::kGestureScrollUpdate:
     case blink::WebGestureEvent::kGesturePinchUpdate:
       has_ongoing_compositor_scroll_fling_pinch_ = disposition == DID_HANDLE;
       break;
 
@@ skipping 1257 matching lines @@
                  scroll_elasticity_controller_->GetWeakPtr(), gesture_event,
                  scroll_result));
 }
 
 void InputHandlerProxy::SetTickClockForTesting(
     std::unique_ptr<base::TickClock> tick_clock) {
   tick_clock_ = std::move(tick_clock);
 }
 
 }  // namespace ui