mus drag and drop: Add API to let the client cancel the drag.

services/ui/ws/window_tree.cc

 // Copyright 2014 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 "services/ui/ws/window_tree.h"
 
 #include <stddef.h>
 
 #include <utility>
 
@@ skipping 1528 matching lines @@
 
   // Here, we need to dramatically change how the mouse pointer works. Once
   // we've started a drag drop operation, cursor events don't go to windows as
   // normal.
   WindowManagerState* wms = display_root->window_manager_state();
   window_server_->StartDragLoop(change_id, window, this);
   wms->SetDragDropSourceWindow(this, window, this, drag_pointer,
                                std::move(drag_data), drag_operation);
 }
 
+void WindowTree::CancelDragDrop(Id window_id) {
+  ServerWindow* window = GetWindowByClientId(ClientWindowId(window_id));
+  bool success = window && access_policy_->CanInitiateDragLoop(window);

[sky, 2016/09/21 20:40:56]

I wouldn't bother with the success member. That is mostly used as a way to
inform the client of the result, but there is no callback for cancel, so no
point in the member.

Also, DVLOG the error cases please.

+  if (!success)
+    return;
+
+  if (window != window_server_->GetCurrentDragLoopWindow())
+    return;
+
+  if (window_server_->GetCurrentDragLoopInitiator() != this)
+    return;
+
+  WindowManagerDisplayRoot* display_root = GetWindowManagerDisplayRoot(window);
+  if (!display_root)
+    return;
+
+  WindowManagerState* wms = display_root->window_manager_state();
+  wms->CancelDragDrop();
+}
+
 void WindowTree::PerformWindowMove(uint32_t change_id,
                                    Id window_id,
                                    ui::mojom::MoveLoopSource source,
                                    const gfx::Point& cursor) {
   ServerWindow* window = GetWindowByClientId(ClientWindowId(window_id));
   bool success = window && access_policy_->CanInitiateMoveLoop(window);
   if (!success || !ShouldRouteToWindowManager(window)) {
     // We need to fail this move loop change, otherwise the client will just be
     // waiting for |change_id|.
     DVLOG(1) << "PerformWindowMove failed (access denied).";
@@ skipping 313 matching lines @@
   client()->OnCompleteDrop(client_window_id.id, event_flags, cursor_offset,
                            effect_bitmask, callback);
 }
 
 void WindowTree::PerformOnDragDropDone() {
   client()->OnDragDropDone();
 }
 
 }  // namespace ws
 }  // namespace ui