complete clear the drawing Button, and  do you notice the difference between         button.addTarget(self, action: "clearButtonTapped:", forControlEvents: UIControlEvents.TouchUpInside)
   and           button.addTarget(self, action: "clearButtonTapped", forControlEvents: UIControlEvents.TouchUpInside) ,  which confused me about 30 minutes
the colon ":" is very import,    button.addTarget(self, action: "clearButtonTapped:", forControlEvents: UIControlEvents.TouchUpInside)