Fix Up SingletonHwnd Observer Lifetime Issues

ui/gfx/win/singleton_hwnd.h

Index: ui/gfx/win/singleton_hwnd.h
diff --git a/ui/gfx/win/singleton_hwnd.h b/ui/gfx/win/singleton_hwnd.h
index 5dbfe90d2171b7964c90c5cc947759961ef4313a..1e10af118223b57c82c8e5d70128b2fc08c17cc0 100644
--- a/ui/gfx/win/singleton_hwnd.h
+++ b/ui/gfx/win/singleton_hwnd.h
@@ -9,6 +9,7 @@
 #include <vector>
 
 #include "base/basictypes.h"
+#include "base/callback.h"
 #include "base/callback_forward.h"
 #include "base/observer_list.h"
 #include "ui/gfx/win/window_impl.h"
@@ -21,20 +22,31 @@ namespace gfx {
 // notifications.
 class GFX_EXPORT SingletonHwnd : public WindowImpl {
  public:
-  static SingletonHwnd* GetInstance();
-
-  // Observer interface for receiving Windows WM_* notifications.
-  class Observer {
+  // Singleton lifetime management is tricky. This observer handles the correct
+  // cleanup if either the SingletonHwnd or forwarded object is destroyed first.
+  // To use this class, make sure this object is bound to your object's lifetime
+  // and use SetWndProc to set your WndProc.
+  class GFX_EXPORT Observer {

[sky, 2015/04/24 20:58:23]

Put this into its own header. (From the style guide: "Prefer putting delegate
classes in their own header files").

[robliao, 2015/04/24 21:49:03]

Done.

    public:
-    virtual void OnWndProc(HWND hwnd,
-                           UINT message,
-                           WPARAM wparam,
-                           LPARAM lparam) = 0;
+    typedef base::Callback<void(HWND, UINT, WPARAM, LPARAM)> WndProc;
+
+    Observer();
+    ~Observer();
+
+    void SetWndProc(const WndProc& wndProc);

[sky, 2015/04/24 20:58:23]

wnd_proc

[robliao, 2015/04/24 21:49:03]

Done.

+    void ClearWndProc();
+
+   private:
+    friend class SingletonHwnd;
+
+    void OnWndProc(HWND hwnd, UINT message, WPARAM wparam, LPARAM lparam);
+
+    WndProc wndProc_;

[sky, 2015/04/24 20:58:23]

wm_proc_.

[robliao, 2015/04/24 21:49:03]

Done.

+
+    DISALLOW_COPY_AND_ASSIGN(Observer);
   };
 
-  // Add/remove observer to receive WM_* notifications.
-  void AddObserver(Observer* observer);
-  void RemoveObserver(Observer* observer);
+  static SingletonHwnd* GetInstance();
 
   // Windows callback for WM_* notifications.
   BOOL ProcessWindowMessage(HWND window,
@@ -50,6 +62,10 @@ class GFX_EXPORT SingletonHwnd : public WindowImpl {
   SingletonHwnd();
   ~SingletonHwnd() override;
 
+  // Add/remove Observer to forward WM_* notifications.
+  void AddObserver(Observer* observer);
+  void RemoveObserver(Observer* observer);
+
   // List of registered observers.
   ObserverList<Observer> observer_list_;