Articulate the cancel behavior in the WebTouchEvent.

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"
@@ skipping 93 matching lines @@
   point.radiusX,
   point.radiusY,
   point.rotationAngle,
   point.force,
   point.tiltX,
   point.tiltY);
 }
 
 void ApppendEventDetails(const WebTouchEvent& event, std::string* result) {
   StringAppendF(result,
-                "{\n Touches: %u, Cancelable: %d, CausesScrolling: %d,"
+                "{\n Touches: %u, DispatchType: %d, CausesScrolling: %d,"
                 " uniqueTouchEventId: %u\n[\n",
-                event.touchesLength, event.cancelable,
+                event.touchesLength, event.dispatchType,
                 event.movedBeyondSlopRegion, event.uniqueTouchEventId);
   for (unsigned i = 0; i < event.touchesLength; ++i)
     ApppendTouchPointDetails(event.touches[i], result);
   result->append(" ]\n}");
 }
 
 bool CanCoalesce(const WebKeyboardEvent& event_to_coalesce,
                  const WebKeyboardEvent& event) {
   return false;
 }
@@ skipping 347 matching lines @@
     case WebInputEvent::MouseLeave:
     case WebInputEvent::ContextMenu:
     case WebInputEvent::GestureScrollBegin:
     case WebInputEvent::GestureScrollEnd:
     case WebInputEvent::GestureShowPress:
     case WebInputEvent::GestureTapUnconfirmed:
     case WebInputEvent::GestureTapDown:
     case WebInputEvent::GestureTapCancel:
     case WebInputEvent::GesturePinchBegin:
     case WebInputEvent::GesturePinchEnd:
+      return false;
+
+    // Touch cancel should always be non-blocking.
     case WebInputEvent::TouchCancel:
+      DCHECK_NE(WebInputEvent::Blocking,
+                static_cast<const WebTouchEvent&>(event).dispatchType);
       return false;
+
+    // Touch start and touch end indicate whether they are non-blocking
+    // (aka cancelable) on the event.

[tdresser, 2016/04/18 19:40:45]

The aka here is backwards.

non-blocking -> uncancelable

[dtapuska, 2016/04/18 20:09:09]

Done.

     case WebInputEvent::TouchStart:
     case WebInputEvent::TouchEnd:
-      return static_cast<const WebTouchEvent&>(event).cancelable;
+      return static_cast<const WebTouchEvent&>(event).dispatchType ==
+             WebInputEvent::Blocking;
+
+    // Touch move events may be non-blocking but are always explicitly
+    // acknowledge by the renderer so they block the event stream.
+    case WebInputEvent::TouchMove:
     default:
       return true;
   }
 }
 
 uint32_t WebInputEventTraits::GetUniqueTouchEventId(
     const WebInputEvent& event) {
   if (WebInputEvent::isTouchEventType(event.type)) {
     return static_cast<const WebTouchEvent&>(event).uniqueTouchEventId;
   }
   return 0U;
 }
 
 }  // namespace content