多点触摸

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event;
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event;
- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event;
- (void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event
当一个或多个手指触碰屏幕时，发送touchesBegan:withEvent:消息。
当一个或多个手指在屏幕上移动时，发送touchesMoved:withEvent:消息。
当一个或多个手指离开屏幕时，发送touchesEnded:withEvent:消息。