Adds a class that makes it easy to subclass a HWND and filter messages sent to it.

ui/base/win/hwnd_subclass.h

+// Copyright (c) 2012 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 UI_BASE_WIN_HWND_SUBCLASS_H_
+#define UI_BASE_WIN_HWND_SUBCLASS_H_
+#pragma once
+
+#include <windows.h>
+
+#include "base/gtest_prod_util.h"
+#include "base/memory/scoped_ptr.h"
+#include "ui/base/ui_export.h"
+#include "ui/base/view_prop.h"
+#include "ui/gfx/native_widget_types.h"

[tfarina, 2012/04/27 21:06:02]

nit: do you really need this include?

+
+namespace ui {
+
+// Classes implementing this interface get the opportunity to handle and consume
+// messages before they are sent to their target HWND.
+class UI_EXPORT HWNDMessageFilter {
+ public:
+  virtual ~HWNDMessageFilter() {}
+
+  // A derived class overrides this method to perform filtering of the messages
+  // before the |original_wnd_proc_| sees them. Return true to consume the
+  // message and prevent |original_wnd_proc_| from seeing them at all, false to
+  // allow it to process them.
+  virtual bool FilterMessage(HWND hwnd,
+                             UINT message,
+                             WPARAM w_param,
+                             LPARAM l_param,
+                             LRESULT* l_result) = 0;
+};
+
+// An object that instance-subclasses a window. If the window has already been
+// instance-subclassed, that subclassing is lost.
+class UI_EXPORT HWNDSubclass {
+ public:
+  explicit HWNDSubclass(HWND target);
+  ~HWNDSubclass();
+
+  // HWNDSubclass takes ownership of the filter.
+  void SetFilter(HWNDMessageFilter* filter);
+
+  LRESULT OnWndProc(HWND hwnd, UINT message, WPARAM w_param, LPARAM l_param);
+
+ private:
+  FRIEND_TEST_ALL_PREFIXES(HWNDSubclassTest, Filtering);
+
+  static WNDPROC GetCurrentWndProc(HWND target);
+
+  HWND target_;
+  scoped_ptr<HWNDMessageFilter> filter_;
+  WNDPROC original_wnd_proc_;
+  ui::ViewProp prop_;
+
+  DISALLOW_COPY_AND_ASSIGN(HWNDSubclass);
+};
+
+}  // namespace ui
+
+#endif  // UI_BASE_WIN_HWND_SUBCLASS_H_