Add an identity (id) to system drag & drop.

base/base_drop_target.h

 // Copyright (c) 2006-2008 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 BASE_BASE_DROP_TARGET_H_
 #define BASE_BASE_DROP_TARGET_H_
 
 #include <objidl.h>
 
 #include "base/ref_counted.h"
@@ skipping 67 matching lines @@
   // dnd session.
   virtual void OnDragLeave(IDataObject* data_object);
 
   // Invoked when the drop ends on the window. This should return the operation
   // that was taken.
   virtual DWORD OnDrop(IDataObject* data_object,
                        DWORD key_state,
                        POINT cursor_position,
                        DWORD effect);
 
+  // Return the drag identity.
+  static int32 GetDragIdentity() { return drag_identity_; }
+
  private:
   // Returns the cached drop helper, creating one if necessary. The returned
   // object is not addrefed. May return NULL if the object couldn't be created.
   static IDropTargetHelper* DropHelper();
 
   // The data object currently being dragged over this drop target.
   scoped_refptr<IDataObject> current_data_object_;
 
   // A helper object that is used to provide drag image support while the mouse
   // is dragging over the content area.
   //
   // DO NOT ACCESS DIRECTLY! Use DropHelper() instead, which will lazily create
   // this if it doesn't exist yet. This object can take tens of milliseconds to
   // create, and we don't want to block any window opening for this, especially
   // since often, DnD will never be used. Instead, we force this penalty to the
   // first time it is actually used.
   static IDropTargetHelper* cached_drop_target_helper_;
 
+  // The drag identity (id). An up-counter that increases when the cursor first
+  // moves over the HWND in a DnD session (OnDragEnter). 0 is reserved to mean
+  // the "no/unknown" identity, and is used for initialization. The identity is
+  // sent to the renderer in drag enter notifications. Note: the identity value
+  // is passed over the renderer NPAPI interface to gears, so use int32 instead
+  // of int here.
+  static int32 drag_identity_;
+
   // The HWND of the source. This HWND is used to determine coordinates for
   // mouse events that are sent to the renderer notifying various drag states.
   HWND hwnd_;
 
   // Whether or not we are currently processing drag notifications for drags
   // initiated in this window.
   bool suspend_;
 
   LONG ref_count_;
 
   DISALLOW_EVIL_CONSTRUCTORS(BaseDropTarget);
 };
 
 #endif  // BASE_BASE_DROP_TARGET_H_