Make shelf item can be dragged when context menu is opened.

ash/shelf/shelf_button.cc

Index: ash/shelf/shelf_button.cc
diff --git a/ash/shelf/shelf_button.cc b/ash/shelf/shelf_button.cc
index f8c930d29ce9c4c65a5441bc013f0b3692b0ba3f..db766c2b14a70fb472de4e218f28d6aec784dfb5 100644
--- a/ash/shelf/shelf_button.cc
+++ b/ash/shelf/shelf_button.cc
@@ -26,6 +26,7 @@
 #include "ui/views/animation/ink_drop_impl.h"
 #include "ui/views/animation/square_ink_drop_ripple.h"
 #include "ui/views/controls/image_view.h"
+#include "ui/views/controls/menu/menu_controller.h"
 
 namespace {
 
@@ -461,6 +462,14 @@ void ShelfButton::OnGestureEvent(ui::GestureEvent* event) {
       shelf_view_->PointerReleasedOnButton(this, ShelfView::TOUCH, false);
       event->SetHandled();
       return;
+    case ui::ET_GESTURE_LONG_TAP: {
+      views::MenuController* controller =

[msw, 2017/05/17 23:03:30]

Can you comment on what this is doing and why?

[minch1, 2017/05/18 17:42:57]

1.LONG_PRESS the shelf items until the context menu is opened. This will  
transfer events from owner(shelf_widget) to MenuHost. 
2.Release LONG PRESS directly without dragging. The context menu is still
opened. If we touch anywhere else at this time. The context menu will close. And
MenuHost will transfer events back to the owner at this time. But Actually, we
don't want events to be transferred back in this situation.
Since there is a LONG_TAP events followed when release the press. We tried to
|set_owner_needs_gesture_events| to false here. To make sure the events will not
be transferred back in this situation.

[msw, 2017/05/18 20:42:20]

It's not clear to me (at least from that explanation) why we need to
|set_owner_needs_gesture_events(false)| instead of just handling/ignoring this
one LONG_TAP event. Also, I'm hoping that you'll add a comment in code, not just
on this CL review, readers of this code probably won't understand what's
happening or why here.

[minch1, 2017/05/18 22:02:53]

We handled the LONG_TAP here to make it will not open a context menu again.
Besides that we also want to make sure the MenuHost will not transfer events
back in this situation. That is why |set_owner_needs_gesture_events(false)|
here.

[msw, 2017/05/18 22:14:47]

Right, I don't understand *why* we don't want the MenuHost to transfer events
back in this case.

+          views::MenuController::GetActiveInstance();
+      if (controller)
+        controller->set_owner_needs_gesture_events(false);

[sky, 2017/05/18 16:33:30]

Instead of changing the menu code, could you accomplish what you need with a
post target event handler?

That said, I'm a bit hazy on why you need this. Why do you need to transfer
events?

[minch1, 2017/05/18 17:42:57]

I think the reason we need to transfer events here is because the window of the
menu capture all the events when the menu is created? But it did nothing for the
gesture events of the shelf items. So we cannot dragged the shelf items when
context menu is created. 

Transfer events from the owner to the menu. Then we can handle the events that
not happened on the menu. Make sure it will not be dropped. Then, after menu is
closed, if needed transfer the events back to make sure the owner can continue
the events.
xiyuan@, please help explain a little more if I made something wrong? Thanks.

[xiyuan, 2017/05/18 18:00:47]

Each widget has a GestureProvider that translates touch events to gesture
events. The GestureProvider starts the work when it sees a TAP_DOWN. When the
context menu is created, it captures all touch events. But since we don't yet
lift the fingure, it will not generate gestures and silently drop all touch
events. By TransferGesture, we make MenuHost continue to translate the touch
events to gesture events. We forward the unhandled gestures to Shelf widget so
that the dragging can be continued after the menu is open.

[sky, 2017/05/18 19:43:12]

Why is that a problem though? Shouldn't the user close the menu, and then
attempt the gesture to do the drag?

[minch1, 2017/05/18 22:02:53]

I guess this is a requirement from UX?

+      event->SetHandled();
+      return;
+    }
     default:
       return CustomButton::OnGestureEvent(event);
   }