Observe for window's (self) deletion during activation/focus change

ui/aura/window_destruction_observer.h

Index: ui/aura/window_destruction_observer.h
diff --git a/ui/aura/window_destruction_observer.h b/ui/aura/window_destruction_observer.h
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+++ b/ui/aura/window_destruction_observer.h
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+// 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.
+
+#ifndef UI_AURA_WINDOW_DESTRUCTION_OBSERVER_H_
+#define UI_AURA_WINDOW_DESTRUCTION_OBSERVER_H_
+
+#include <set>
+
+#include "ui/aura/aura_export.h"
+#include "ui/aura/window.h"
+#include "ui/aura/window_observer.h"
+
+namespace aura {
+
+// A class for observing window (self) destruction. A side effect of a
+// window member funtion may be that the object, i.e. |this|, gets
+// deleted. This class can be used to detect such cases and to exit
+// the member function without any further access to member data. See
+// for example: aura::Window::NotifyWindowVisibilityChangedAtReceiver.
+
+class AURA_EXPORT WindowDestructionObserver : public WindowObserver {
+ public:
+  WindowDestructionObserver();
+
+  virtual ~WindowDestructionObserver();
+
+  // Returns true if |window| has been observed to be destroyed.
+  bool was_destroyed(const Window* window) const;
+
+ private:
+  // aura::WindowObserver overrides:
+  virtual void OnWindowDestroyed(Window* window) OVERRIDE;
+
+  std::set<const Window*> destroyed_windows_;
+
+  DISALLOW_COPY_AND_ASSIGN(WindowDestructionObserver);
+};
+
+} // namespace aura
+
+#endif