[Android] Short-circuit touch forwarding when no touch handlers exist

content/browser/android/content_view_core_impl.cc

Index: content/browser/android/content_view_core_impl.cc
diff --git a/content/browser/android/content_view_core_impl.cc b/content/browser/android/content_view_core_impl.cc
index 8c322d27848ba8ccc29718ba75e1ffce88089c2e..c1b7b3ee14fc3f672076b0a4bba213487b4872f7 100644
--- a/content/browser/android/content_view_core_impl.cc
+++ b/content/browser/android/content_view_core_impl.cc
@@ -1028,11 +1028,7 @@ void ContentViewCoreImpl::CancelActiveTouchSequenceIfNecessary() {
 
   scoped_ptr<ui::MotionEvent> cancel_event = current_down_event->Cancel();
   DCHECK(cancel_event);
-  if (!gesture_provider_.OnTouchEvent(*cancel_event))
-    return;
-
-  rwhv->SendTouchEvent(
-      CreateWebTouchEventFromMotionEvent(*cancel_event, 1.f / dpi_scale()));
+  OnMotionEvent(*cancel_event);
 }
 
 jboolean ContentViewCoreImpl::OnTouchEvent(JNIEnv* env,
@@ -1052,6 +1048,7 @@ jboolean ContentViewCoreImpl::OnTouchEvent(JNIEnv* env,
                                            jfloat touch_major_0,
                                            jfloat touch_major_1) {
   RenderWidgetHostViewAndroid* rwhv = GetRenderWidgetHostViewAndroid();
+  // Avoid synthesizing a touch event if it cannot be forwarded.
   if (!rwhv)
     return false;
 
@@ -1071,11 +1068,7 @@ jboolean ContentViewCoreImpl::OnTouchEvent(JNIEnv* env,
                            touch_major_0,
                            touch_major_1);
 
-  if (!gesture_provider_.OnTouchEvent(event))
-    return false;
-
-  rwhv->SendTouchEvent(WebTouchEventBuilder::Build(event, 1.f / dpi_scale()));
-  return true;
+  return OnMotionEvent(event);
 }
 
 float ContentViewCoreImpl::GetDpiScale() const {
@@ -1131,6 +1124,28 @@ WebGestureEvent ContentViewCoreImpl::MakeGestureEvent(
       type, time_ms / 1000.0, x / dpi_scale(), y / dpi_scale());
 }
 
+bool ContentViewCoreImpl::OnMotionEvent(const ui::MotionEvent& event) {
+  RenderWidgetHostViewAndroid* rwhv = GetRenderWidgetHostViewAndroid();
+  if (!rwhv)
+    return false;
+
+  if (!gesture_provider_.OnTouchEvent(event))
+    return false;
+
+  // If there are no touch handlers and the touch queue is empty, touch
+  // forwarding can be short-circuited completely.

[tdresser, 2014/03/28 17:55:27]

It feels like this comment is breaking encapsulation by referring to
implementation details of the input router.

Would it be reasonable to reword this more along the lines of "If we don't need
to forward the event to the RWHI, touch forwarding can be short-circuited
completely."

[jdduke (slow), 2014/03/28 18:33:55]

Yes, though in my mind those details are vital to understanding why we can allow
this.  However, I suppose if a dev is interested enough in the way they can
refer to the implementation :) In that case, I'll just remove the comment.

[tdresser, 2014/03/28 18:40:05]

SGTM.

+  RenderWidgetHostImpl* host = RenderWidgetHostImpl::From(
+      rwhv->GetRenderWidgetHost());
+  if (!host->ShouldForwardTouchEvent()) {
+    ConfirmTouchEvent(INPUT_EVENT_ACK_STATE_NO_CONSUMER_EXISTS);
+    return true;
+  }
+
+  rwhv->SendTouchEvent(
+      CreateWebTouchEventFromMotionEvent(event, 1.f / dpi_scale()));
+  return true;
+}
+
 void ContentViewCoreImpl::SendGestureEvent(
     const blink::WebGestureEvent& event) {
   RenderWidgetHostViewAndroid* rwhv = GetRenderWidgetHostViewAndroid();