Add aura::WindowIdRegistry.

ui/aura/window_id_registry.cc

Index: ui/aura/window_id_registry.cc
diff --git a/ui/aura/window_id_registry.cc b/ui/aura/window_id_registry.cc
new file mode 100644
index 0000000000000000000000000000000000000000..a666164ce02625521e17f948f46b8b8bd5271fda
--- /dev/null
+++ b/ui/aura/window_id_registry.cc
@@ -0,0 +1,49 @@
+// Copyright 2013 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/aura/window_id_registry.h"
+
+#include "ui/aura/window.h"
+
+namespace aura {
+
+// static
+WindowIdRegistry* WindowIdRegistry::GetInstance() {
+  return Singleton<WindowIdRegistry>::get();
+}
+
+int WindowIdRegistry::GetIdForWindow(Window* window) {
+  // First check if an Id is already assigned to the |window|.
+  std::map<Window*, int>::iterator it = window_to_id_map_.find(window);
+  if (it != window_to_id_map_.end()) {
+    return it->second;
+  }
+
+  // If the windows doesn't have an Id yet assign it.
+  int id = next_id_++;
+  window_to_id_map_[window] = id;
+  id_to_window_map_[id] = window;
+  window->AddObserver(this);
+  return id;
+}
+
+Window* WindowIdRegistry::GetWindowById(int id) {
+  std::map<int, Window*>::iterator it = id_to_window_map_.find(id);
+  return (it != id_to_window_map_.end()) ? it->second : NULL;
+}
+
+WindowIdRegistry::WindowIdRegistry()
+    : next_id_(0) {
+}
+
+WindowIdRegistry::~WindowIdRegistry() {}
+
+void WindowIdRegistry::OnWindowDestroying(Window* window) {
+  std::map<Window*, int>::iterator it = window_to_id_map_.find(window);
+  DCHECK(it != window_to_id_map_.end());
+  id_to_window_map_.erase(it->second);
+  window_to_id_map_.erase(it);
+}
+
+}  // namespace aura