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

base/base_drop_target.cc

 // 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.
 
 #include "base/base_drop_target.h"
 
 #include <shlobj.h>
 
 #include "base/logging.h"
 
 ///////////////////////////////////////////////////////////////////////////////
 
 IDropTargetHelper* BaseDropTarget::cached_drop_target_helper_ = NULL;
+int32 BaseDropTarget::drag_identity_ = 0;
 
 BaseDropTarget::BaseDropTarget(HWND hwnd)
     : hwnd_(hwnd),
       suspend_(false),
       ref_count_(0) {
   DCHECK(hwnd);
   HRESULT result = RegisterDragDrop(hwnd, this);
   DCHECK(SUCCEEDED(result));
 }
 
@@ skipping 22 matching lines @@
   if (drop_helper) {
     drop_helper->DragEnter(GetHWND(), data_object,
                            reinterpret_cast<POINT*>(&cursor_position), *effect);
   }
 
   // You can't drag and drop within the same HWND.
   if (suspend_) {
     *effect = DROPEFFECT_NONE;
     return S_OK;
   }
+
+  // Update the drag identity, skipping 0.
+  if (++drag_identity_ == 0)
+    ++drag_identity_;
+
   current_data_object_ = data_object;
   POINT screen_pt = { cursor_position.x, cursor_position.y };
   *effect = OnDragEnter(current_data_object_, key_state, screen_pt, *effect);
   return S_OK;
 }
 
 HRESULT BaseDropTarget::DragOver(DWORD key_state,
                                  POINTL cursor_position,
                                  DWORD* effect) {
   // Tell the helper that we moved over it so it can update the drag image.
@@ skipping 87 matching lines @@
 void BaseDropTarget::OnDragLeave(IDataObject* data_object) {
 }
 
 DWORD BaseDropTarget::OnDrop(IDataObject* data_object,
                              DWORD key_state,
                              POINT cursor_position,
                              DWORD effect) {
   return DROPEFFECT_NONE;
 }