Drag and drop cleans up touch sequences from the source window.

ui/events/gestures/gesture_recognizer_impl.cc

 // Copyright (c) 2012 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/gestures/gesture_recognizer_impl.h"
 
 #include <stddef.h>
 
 #include <limits>
 #include <memory>
@@ skipping 111 matching lines @@
 
 void GestureRecognizerImpl::CancelActiveTouchesExcept(
     GestureConsumer* not_cancelled) {
   for (const auto& consumer_provider : consumer_gesture_provider_) {
     if (consumer_provider.first == not_cancelled)
       continue;
     CancelActiveTouches(consumer_provider.first);
   }
 }
 
-void GestureRecognizerImpl::TransferEventsTo(GestureConsumer* current_consumer,
+void GestureRecognizerImpl::BeginDragAndDrop(GestureConsumer* current_consumer,
                                              GestureConsumer* new_consumer) {
+  // This method transfers the gesture stream from |current_consumer| to
+  // |new_consumer|, while ensuring that both retain a touch event stream which
+  // is reasonably valid. In order to do this we
+  // - record what pointers are currently down on |current_consumer|
+  // - cancel touches on consumers other than |current_consumer|
+  // - move the gesture provider from |current_consumer| to |new_consumer|
+  // - synthesize touch presses on |current_consumer|, to get the state of its
+  //   new, empty gesture provider in line with the touch state of the consumer
+  //   itself
+  // - synthesize touch cancels on |current_consumer|
+  // - retarget the pointers that were previously targeted to
+  //   |current_consumer| to |new_consumer|.
+  // NOTE: This currently doesn't synthesize touch press events on
+  // |new_consumer|, so the event stream it sees is still invalid.
+  DCHECK(current_consumer);
+  DCHECK(new_consumer);
+  GestureEventHelper* helper = FindDispatchHelperForConsumer(current_consumer);
+
+  std::vector<int> touchids_targeted_at_current;
+
+  for (const auto& touch_id_target: touch_id_target_) {
+    if (touch_id_target.second == current_consumer)
+      touchids_targeted_at_current.push_back(touch_id_target.first);
+  }
+
+  CancelActiveTouchesExcept(current_consumer);
+
+  std::vector<std::unique_ptr<TouchEvent>> pressing_touches =
+      GetEventPerPointForConsumer(current_consumer, ET_TOUCH_PRESSED);
+  std::vector<std::unique_ptr<TouchEvent>> cancelling_touches =
+      GetEventPerPointForConsumer(current_consumer, ET_TOUCH_CANCELLED);
+
+  TransferConsumer(current_consumer, new_consumer, &consumer_gesture_provider_);
+
+  if (!helper)
+    return;
+
+  // We're now in a situation where current_consumer has no gesture recognizer,
+  // but has some pointers down which need cancelling. In order to ensure that
+  // the GR sees a valid event stream, synthesize a touch press per pointer, and
+  // then synthesize a touch cancel per pointer. These extra touch presses will
+  // be filtered out in the RenderWidgetHostViewAura.
+
+  for (std::unique_ptr<TouchEvent>& event : pressing_touches)
+    helper->DispatchSyntheticTouchEvent(event.get());

[sadrul, 2016/05/03 18:38:16]

We really shouldn't dispatch a second set of touch-presses when transferring the
gesture-state/sequence. RWHVA may know to deal with it, but there can be other
consumers, and we can't expect all of them to expect this. The GR may see an
invalid event-stream, as in it sees cancels without corresponding press, but
that should be OK?

[tdresser, 2016/05/03 18:56:42]

That would mean we'd need to relax this:
https://code.google.com/p/chromium/codesearch#chromium/src/ui/events/gestures...

It's pretty nice to be able to ignore all touches that don't have a
corresponding press.

I could perhaps add a back-door to update the GR state, instead of dispatching
presses?

[sadrul, 2016/05/04 13:28:18]

If not overly complicated, then yeah, I would go with that.

+
+  for (std::unique_ptr<TouchEvent>& event : cancelling_touches)
+    helper->DispatchSyntheticTouchEvent(event.get());
+
+  for (int touch_id : touchids_targeted_at_current)
+    touch_id_target_[touch_id] = new_consumer;
+}
+
+
+void GestureRecognizerImpl::BeginTabDrag(GestureConsumer* current_consumer,
+                                         GestureConsumer* new_consumer) {
+  // Tab Drag requires that the touches on |current_consumer| not be
+  // cancelled. We transfer the gesture provider from |current_consumer| to
+  // |new_consumer| without bothering to make the event stream valid for either
+  // consumer.
   DCHECK(current_consumer);
   DCHECK(new_consumer);
 
   CancelActiveTouchesExcept(current_consumer);
 
   TransferTouchIdToConsumerMap(current_consumer, new_consumer,
                                &touch_id_target_);
   TransferConsumer(current_consumer, new_consumer, &consumer_gesture_provider_);
 }
 
 bool GestureRecognizerImpl::GetLastTouchPointForTarget(
     GestureConsumer* consumer,
     gfx::PointF* point) {
     if (consumer_gesture_provider_.count(consumer) == 0)
       return false;
     const MotionEvent& pointer_state =
         consumer_gesture_provider_[consumer]->pointer_state();
     if (!pointer_state.GetPointerCount())
       return false;
     *point = gfx::PointF(pointer_state.GetX(), pointer_state.GetY());
     return true;
 }
 
-bool GestureRecognizerImpl::CancelActiveTouches(GestureConsumer* consumer) {
-  bool cancelled_touch = false;
+std::vector<std::unique_ptr<TouchEvent>>
+GestureRecognizerImpl::GetEventPerPointForConsumer(GestureConsumer* consumer,
+                                                   EventType type) {
+  std::vector<std::unique_ptr<TouchEvent>> cancelling_touches;
   if (consumer_gesture_provider_.count(consumer) == 0)
-    return false;
+    return cancelling_touches;
   const MotionEventAura& pointer_state =
       consumer_gesture_provider_[consumer]->pointer_state();
   if (pointer_state.GetPointerCount() == 0)
+    return cancelling_touches;
+  for (size_t i = 0; i < pointer_state.GetPointerCount(); ++i) {
+    std::unique_ptr<TouchEvent> touch_event(new TouchEvent(
+        type, gfx::Point(), EF_IS_SYNTHESIZED, pointer_state.GetPointerId(i),
+        EventTimeForNow(), 0.0f, 0.0f, 0.0f, 0.0f));
+    gfx::PointF point(pointer_state.GetX(i), pointer_state.GetY(i));
+    touch_event->set_location_f(point);
+    touch_event->set_root_location_f(point);
+    cancelling_touches.push_back(std::move(touch_event));
+  }
+  return cancelling_touches;
+}
+
+bool GestureRecognizerImpl::CancelActiveTouches(GestureConsumer* consumer) {
+  GestureEventHelper* helper =
+      FindDispatchHelperForConsumer(consumer);
+
+  if (!helper)
     return false;
-  // pointer_state is modified every time after DispatchCancelTouchEvent.
-  std::unique_ptr<MotionEvent> pointer_state_clone = pointer_state.Clone();
-  for (size_t i = 0; i < pointer_state_clone->GetPointerCount(); ++i) {
-    TouchEvent touch_event(ui::ET_TOUCH_CANCELLED, gfx::Point(),
-                           ui::EF_IS_SYNTHESIZED,
-                           pointer_state_clone->GetPointerId(i),
-                           ui::EventTimeForNow(), 0.0f, 0.0f, 0.0f, 0.0f);
-    gfx::PointF point(pointer_state_clone->GetX(i),
-                      pointer_state_clone->GetY(i));
-    touch_event.set_location_f(point);
-    touch_event.set_root_location_f(point);
-    GestureEventHelper* helper = FindDispatchHelperForConsumer(consumer);
-    if (helper)
-      helper->DispatchCancelTouchEvent(consumer, &touch_event);
-    cancelled_touch = true;
-  }
-  return cancelled_touch;
+
+  std::vector<std::unique_ptr<TouchEvent>> cancelling_touches =
+      GetEventPerPointForConsumer(consumer, ET_TOUCH_CANCELLED);
+  for (const std::unique_ptr<TouchEvent>& cancelling_touch : cancelling_touches)
+    helper->DispatchSyntheticTouchEvent(cancelling_touch.get());
+  return cancelling_touches.size() > 0U;
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 // GestureRecognizerImpl, private:
 
 GestureProviderAura* GestureRecognizerImpl::GetGestureProviderForConsumer(
     GestureConsumer* consumer) {
   GestureProviderAura* gesture_provider = consumer_gesture_provider_[consumer];
   if (!gesture_provider) {
     gesture_provider = CreateGestureProvider(consumer, this);
@@ skipping 114 matching lines @@
   std::vector<GestureEventHelper*>::iterator it;
   for (it = helpers.begin(); it != helpers.end(); ++it)
     gesture_recognizer->AddGestureEventHelper(*it);
 
   helpers.clear();
   g_gesture_recognizer_instance =
       static_cast<GestureRecognizerImpl*>(gesture_recognizer);
 }
 
 }  // namespace ui