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

ui/base/win/hwnd_subclass.cc

+// 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.
+
+#include "ui/base/win/hwnd_subclass.h"
+
+#include "base/logging.h"
+#include "ui/base/win/hwnd_util.h"
+
+namespace {
+const char* kHWNDSubclassKey = "__UI_BASE_WIN_HWND_SUBCLASS_PROC__";

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

nit: const char kFoo[] = "...";

+
+LRESULT CALLBACK WndProc(HWND hwnd,
+                         UINT message,
+                         WPARAM w_param,
+                         LPARAM l_param) {
+  ui::HWNDSubclass* wrapped_wnd_proc =
+      reinterpret_cast<ui::HWNDSubclass*>(
+          ui::ViewProp::GetValue(hwnd, kHWNDSubclassKey));
+  return wrapped_wnd_proc ? wrapped_wnd_proc->OnWndProc(hwnd,

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

nit: if you move wrapped_wnd_proc->OnWndProc(...) to the next line I bet you can
make it fit in one line.

+                                                        message,
+                                                        w_param,
+                                                        l_param) : 0;

[cpu_(ooo_6.6-7.5), 2012/04/27 21:07:12]

so when the HWNDSubclass is destroyed, the messages won't go to the default
wndproc, they will be ignored. Is that what you want?

+}
+
+}

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

nit: // namespace

+
+namespace ui {
+
+HWNDSubclass::HWNDSubclass(HWND target)
+    : target_(target),
+      original_wnd_proc_(GetCurrentWndProc(target)),
+      ALLOW_THIS_IN_INITIALIZER_LIST(prop_(target, kHWNDSubclassKey, this)) {
+  ui::SetWindowProc(target_, &WndProc);
+}
+
+HWNDSubclass::~HWNDSubclass() {
+}
+
+void HWNDSubclass::SetFilter(HWNDMessageFilter* filter) {
+  filter_.reset(filter);
+}
+
+LRESULT HWNDSubclass::OnWndProc(HWND hwnd,
+                                UINT message,
+                                WPARAM w_param,
+                                LPARAM l_param) {
+  if (filter_.get()) {
+    LRESULT l_result = 0;
+    if (filter_->FilterMessage(hwnd, message, w_param, l_param, &l_result))
+      return l_result;
+  }
+
+  // In most cases, |original_wnd_proc_| will take care of calling
+  // DefWindowProc.
+  return CallWindowProc(original_wnd_proc_, hwnd, message, w_param, l_param);
+}
+
+// static
+WNDPROC HWNDSubclass::GetCurrentWndProc(HWND target) {

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

nit: Worth moving this to unnamed namespace instead of making it a static method
of HWNDSubclass?

+  return reinterpret_cast<WNDPROC>(GetWindowLong(target, GWL_WNDPROC));
+}
+
+}  // namespace ui