mus: Implement interwindow drag and drop

services/ui/ws/current_drag_operation_source.h

+// Copyright 2016 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.
+
+#ifndef SERVICES_UI_WS_CURRENT_DRAG_OPERATION_SOURCE_H_
+#define SERVICES_UI_WS_CURRENT_DRAG_OPERATION_SOURCE_H_
+
+namespace ui {
+namespace ws {
+
+class DragTargetConnection;
+class ServerWindow;
+struct WindowId;
+
+// An interface implemented by the object that caused a CurrentDragOperation to
+// exist, and which acts as a delegate to it.
+class CurrentDragOperationSource {
+ public:
+  virtual ~CurrentDragOperationSource() {}
+
+  // Called when a drag operation is completed.
+  virtual void OnDragOver(bool success) = 0;

[sky, 2016/09/06 21:11:27]

On initial look I read this as 'drag moved ontop of a window'. But that isn't
the case. How about DragCompleted or DragFinished? And document what success
means.

+
+  virtual ServerWindow* GetWindowById(const WindowId& id) = 0;
+
+  virtual DragTargetConnection* GetDragTargetWithRoot(

[sky, 2016/09/06 21:11:27]

Document what this should do. I assume find deepest, it's good to document that.

+      const ServerWindow* window) = 0;
+};
+
+}  // namespace ws
+}  // namespace ui
+
+#endif  // SERVICES_UI_WS_CURRENT_DRAG_OPERATION_SOURCE_H_