class MyWindow: public WithMyCustomLayout<TopWindow> {
...
   Callback WhenMinimize; // callback for minimize

   // Overriding default WindowProc
   LRESULT WindowProc(UINT message, WPARAM wParam, LPARAM lParam);
...
};

// implementation of WndProc
LRESULT MyWindow::WindowProc(UINT message, WPARAM wParam, LPARAM lParam)
{
	if (message == WM_SYSCOMMAND && wParam == SC_MINIMIZE)
	{
		WhenMinimize(); // callback
		return 0;
	}
	return TopWindow::WindowProc(message, wParam, lParam);
}

// in window constructor or any other suitable method, 
// assign custom function to callback
MyWindow::MyWindow()
{
...
   WhenMinimize = THISBACK(MyCustomFunction);
...
}

// implementation of WndProc
LRESULT MyWindow::WindowProc(UINT message, WPARAM wParam, LPARAM lParam)
{
	if (message == WM_SYSCOMMAND && wParam == SC_MINIMIZE)
	{
		My_Custom_Function(); // -------------------------------------------------------this does  not have to be a CallBack but any function can well do.
		return 0;  //-----------------------------------------------------add this line if you want to ignore the actual minimizing else remove it.
	}
	return TopWindow::WindowProc(message, wParam, lParam);
}

virtual void Close()
{
  ... // custom code
  ...
  ...
  TopWindow::Close(); // call this if you want to actually close window after custom processing
}

my_window_app::my_window_app()
{
...
WhenClose = THISBACK(custom_action_function);  // WhenClose is a member of your baseclass already do not re-declare it ..
...
}


........

my_window_app::custom_action_function()
{
  ... // custom code
  ...
  ...
  Close(); // the TopWindow Close would be called as Close() is not re-defined in this case
}