mus: Implement interwindow drag and drop

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 5adcca21706011d8fa3405bc58ed6ff48fa7a4b2..9060a1328d082019995d32ab1b7beb05999c9113 100644
--- a/services/ui/public/interfaces/window_tree.mojom
+++ b/services/ui/public/interfaces/window_tree.mojom
@@ -113,6 +113,11 @@ interface WindowTree {
   // TODO(jamescook): Convert |mask| to a path. http://crbug.com/613210
   SetHitTestMask(uint32 window_id, gfx.mojom.Rect? mask);
 
+  // Called by clients that want to accept drag and drops. Windows default to
+  // this being disabled; a window must actively opt-in to receiving OnDrag*()
+  // calls.
+  SetCanAcceptDrops(uint32 window_id, bool accepts_drops);
+
   // Sets the visibility of the specified window to |visible|. Connections are
   // allowed to change the visibility of any window they have created, as well
   // as any of their roots.
@@ -270,6 +275,16 @@ interface WindowTree {
   // Tells the window manager to cancel any in progress window move started with
   // StartWindowMove() and to revert the window bounds to how they were.
   CancelWindowMove(uint32 window_id);
+
+  // Called by the client to start a drag operation. |source_window_id| is the
+  // source window, |drag_data| is the entire set of mime to raw data
+  // mapping. We send this during the start of the drag because most views
+  // clients will try to read all this data on first entry.
+  PerformDragDrop(uint32 change_id,
+                  uint32 source_window_id,
+                  int32 drag_pointer,
+                  map<string, array<uint8>> drag_data,
+                  uint32 drag_operation);
 };
 
 // Changes to windows are not sent to the connection that originated the
@@ -400,6 +415,46 @@ interface WindowTreeClient {
 
   OnWindowPredefinedCursorChanged(uint32 window_id, Cursor cursor_id);
 
+  // Called when the mouse cursor enters a window that has opted into accepting
+  // drags for the first time, providing a list of available mime types. We
+  // want to send this set of data only one time, so this isn't part of
+  // OnDragEnter(), which occurs every time the mouse enters a window.
+  OnDragStart(uint32 window,

[sky, 2016/09/08 23:37:20]

Based on the name I thought this was a callback from initiating a drag. But that
isn't the case. This function is about providing the mime types. Maybe
OnDragMimeTypes(...)?

Is there a reason the window is needed here? It seems like it would be better to
call this function once per connection, rather than per once per window in a
connection.

Also, did you consider grouping these functions in an associated interface?
Placing them in a separate interface means you have better lifetime (you
wouldn't need OnDragFinish() for one). I'm thinking of something like
GetDropTarget(mime_types, associated DropTarget& drop_target).

[Elliot Glaysher, 2016/09/13 00:14:19]

I renamed this and also changed it to transfer once per connection instead of
once per window.
 
I experimented with this, but this turns a bunch of explicit
OnDragMimeTypes/OnDragFinish() calls into some implicit calls around the
lifetime of a wrapper around DropTarget. (Ownership becomes an issue to worry
about since WindowTree would have to offer the same DropTarget for multiple
windows; DragController asks DragSource for the DropTargetConnection (or
DropTarget in this world) per window, and WindowTree can pass a different
WindowTree back.) 

+              map<string, array<uint8>> drag_data);
+
+  // Called when the mouse cursor enters a window that has opted into
+  // accepting drags through SetAcceptsDrags(), providing a list
+  // of available mime types.  Returns a bitmask of the supported
+  // operations.
+  OnDragEnter(uint32 window,
+              uint32 key_state,
+              gfx.mojom.Point position,
+              uint32 effect_bitmask) => (uint32 supported_op_bitmask);
+
+  // Called when the pointer moves over the window after the initial
+  // DragEnter. Returns a bitmask of the supported operations at this
+  // location.
+  OnDragOver(uint32 window,
+             uint32 key_state,
+             gfx.mojom.Point position,
+             uint32 effect_bitmask) => (uint32 supported_op_bitmask);
+
+  // Called when the pointer leaves a window or if the drop is
+  // canceled.
+  OnDragLeave(uint32 window);
+
+  // Called when the drop occurs on a window. Returns the action
+  // taken.
+  OnDragDrop(uint32 window,

[sky, 2016/09/08 23:37:20]

The 'Drag' in the name here is mildly confusing. How about OnDrop or
OnPerformDrop to better indicate this should process the actual drop.

[Elliot Glaysher, 2016/09/13 00:14:19]

Renamed to OnCompleteDrop (I'm already using the word perform in another
context).

+             uint32 key_state,
+             gfx.mojom.Point position,
+             uint32 effect_bitmask) => (uint32 effect_taken);
+
+  // Called after OnDragDrop completes for every |window| which received an
+  // OnDragStart() message. This signals that a client can forget the
+  // |drag_data| passed in via the first message.
+  OnDragFinish(uint32 window);
+
   // A change initiated from the client has completed. See description of
   // change ids for details.
   OnChangeCompleted(uint32 change_id, bool success);