mus drag and drop: return the completed effect to the caller.

services/ui/public/interfaces/window_tree.mojom

Index: services/ui/public/interfaces/window_tree.mojom
diff --git a/services/ui/public/interfaces/window_tree.mojom b/services/ui/public/interfaces/window_tree.mojom
index 6d4782528b725a5ea7beb1088809b7f37f3846d0..1c87bba3fe09c26fd84812f89fba601985e011ca 100644
--- a/services/ui/public/interfaces/window_tree.mojom
+++ b/services/ui/public/interfaces/window_tree.mojom
@@ -452,8 +452,12 @@ interface WindowTreeClient {
                  gfx.mojom.Point position,
                  uint32 effect_bitmask) => (uint32 effect_taken);
 
-  // Called after OnDragDrop completes for every connection which received an
-  // OnDragDropStart() message. This signals that a client can forget the
+  // Called on the client that requested PerformDragDrop() to return which drag
+  // action was completed. This is called instead of OnChangeCompleted().
+  OnPerformDragDropCompleted(uint32 window, bool success, uint32 effect_taken);

[Tom Sepez, 2016/09/20 23:08:40]

Do you want an enum for effect_taken?  That way mojo could validate it for you
-- or am I misunderstanding?

[Tom Sepez, 2016/09/20 23:16:52]

NM. I see its a bit field?  Might more than one of these be set at a given time
in this method?  Should it be called actions_taken in that case?

[Elliot Glaysher, 2016/09/20 23:44:15]

Here, it is a single value.

OnDragEnter() and OnDragOver() take a bitmask and return a bitmask.
OnCompleteDrop() takes a bitmask and returns an individual value.
OnPerformDragDropCompleted() returns this one individual value back to the
caller.

(Even if I were to make an enum for the contexts where it's an individual value,
I'd be casting back to their bitmask values immediately since those values are
fairly well entrenched in the views codebase. I added a bunch of static_asserts
in the views client library to assert that the mus and the views/aura versions
of these constants are the same.)

+
+  // Called after OnCompleteDrop completes for every connection which received
+  // an OnDragDropStart() message. This signals that a client can forget the
   // |drag_data| passed in via the first message.
   OnDragDropDone();