Cache a screen compatible DC to avoid re-creating it excessively.

ui/gfx/screen_compatible_dc_win.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/gfx/screen_compatible_dc_win.h"
+
+#include "base/logging.h"
+#include "base/memory/singleton.h"
+#include "base/win/wrapped_window_proc.h"
+
+namespace {
+
+// Windows class name to use for the listener window.
+const wchar_t kWindowClassName[] = L"CachedCompatibleDC_Invalidator";
+
+class CachedCompatibleDC {
+ public:
+  // Singleton getter.
+  static CachedCompatibleDC* GetInstance();
+
+  // Returns the cached screen compatible DC or creates one if needed.
+  HDC GetOrCreateCompatibleDC();
+
+  // Deletes the cached DC, called on WM_DISPLAYCHANGE notifications.
+  void DeleteCachedCompatibleDC();
+
+ private:
+  // Cached device context compatible with the screen.
+  HDC compatible_dc_;
+
+  // HWND for listening to WM_DISPLAYCHANGE notifications.
+  HWND listener_window_;
+
+  CachedCompatibleDC();
+  ~CachedCompatibleDC();
+
+  friend struct DefaultSingletonTraits<CachedCompatibleDC>;
+
+  DISALLOW_COPY_AND_ASSIGN(CachedCompatibleDC);
+};
+
+// Windows callback for listening for WM_DISPLAYCHANGE messages.
+LRESULT CALLBACK ListenerWindowProc(HWND hwnd, UINT message,

[sky, 2012/02/03 23:14:42]

nit: each param on its own line.

[Alexei Svitkine (slow), 2012/02/06 14:37:38]

Done.

+                                    WPARAM wparam, LPARAM lparam) {
+  if (message == WM_DISPLAYCHANGE)
+    CachedCompatibleDC::GetInstance()->DeleteCachedCompatibleDC();
+
+  return ::DefWindowProc(hwnd, message, wparam, lparam);
+}
+
+// Creates a listener window to handle WM_DISPLAYCHANGE messages.
+HWND CreateListenerWindow() {
+  HINSTANCE hinst = 0;
+  if (!::GetModuleHandleExA(GET_MODULE_HANDLE_EX_FLAG_FROM_ADDRESS,
+                            reinterpret_cast<char*>(&ListenerWindowProc),
+                            &hinst)) {
+    NOTREACHED();
+  }
+
+  WNDCLASSEX wc = {0};
+  wc.cbSize = sizeof(wc);
+  wc.lpfnWndProc = base::win::WrappedWindowProc<ListenerWindowProc>;
+  wc.hInstance = hinst;
+  wc.lpszClassName = kWindowClassName;
+  ATOM clazz = ::RegisterClassEx(&wc);
+  DCHECK(clazz);
+
+  return ::CreateWindow(MAKEINTATOM(clazz), 0, 0, 0, 0, 0, 0, HWND_MESSAGE, 0,
+                        hinst, 0);
+}
+
+CachedCompatibleDC::CachedCompatibleDC()
+    : compatible_dc_(NULL),
+      listener_window_(NULL) {
+}
+
+CachedCompatibleDC::~CachedCompatibleDC() {
+  DeleteCachedCompatibleDC();
+  if (listener_window_) {
+    ::DestroyWindow(listener_window_);
+    ::UnregisterClass(kWindowClassName, GetModuleHandle(NULL));
+  }
+}
+
+// static
+CachedCompatibleDC* CachedCompatibleDC::GetInstance() {
+  return Singleton<CachedCompatibleDC>::get();
+}
+
+HDC CachedCompatibleDC::GetOrCreateCompatibleDC() {
+  if (!compatible_dc_) {
+    compatible_dc_ = ::CreateCompatibleDC(NULL);
+    if (!listener_window_) {
+      listener_window_ = CreateListenerWindow();
+      DCHECK(listener_window_);

[sky, 2012/02/03 23:14:42]

ui::CheckWindowCreated

[Alexei Svitkine (slow), 2012/02/06 14:37:38]

Done.

+    }
+  }
+  return compatible_dc_;
+}
+
+void CachedCompatibleDC::DeleteCachedCompatibleDC() {
+  if (compatible_dc_) {
+    ::DeleteDC(compatible_dc_);
+    compatible_dc_ = NULL;
+  }
+}
+
+}  // namespace
+
+namespace gfx {
+
+ScopedTemporaryScreenCompatibleDC::ScopedTemporaryScreenCompatibleDC()
+    : hdc_(CachedCompatibleDC::GetInstance()->GetOrCreateCompatibleDC()) {
+}
+
+ScopedTemporaryScreenCompatibleDC::~ScopedTemporaryScreenCompatibleDC() {
+}
+
+}  // namespace gfx