/** per registrare un azione in base ad un evento:
*/
    // Erase the view when recieving a notification named "shake" from the NSNotificationCenter object
	// The "shake" nofification is posted by the PaintingWindow object when user shakes the device
	[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(eraseView) name:@"shake" object:nil];
 
/** INVIO dell'evento
*/
 
	// User was shaking the device. Post a notification named "shake".
	[[NSNotificationCenter defaultCenter] postNotificationName:@"shake" object:self];