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有些View经过Mask处理后，占用的区域应当比frame要小，这时候Gesture的响应区域也应该限制在Mask后的区域。
例如

简单来说，就是使用了一个代理函数来判断是否receive touch

class ViewController: UIViewController,UIGestureRecognizerDelegate {

    @IBOutlet weak var imageview: UIImageView!

    override func viewDidLoad() {
        super.viewDidLoad()

        self.imageview.userInteractionEnabled = true
        self.imageview.layer.cornerRadius = 100
        self.imageview.layer.masksToBounds = true
        let tapGesture = UITapGestureRecognizer(target: self,action: "tap:")
        tapGesture.delegate = self //Should set delegate to self
        self.imageview.addGestureRecognizer(tapGesture)

    }

    func tap(sender:UITapGestureRecognizer){
        println("Got tap")
    }
    //THis is key
    func gestureRecognizer(gestureRecognizer: UIGestureRecognizer,shouldReceiveTouch touch: UITouch) -> Bool {
        let touchPoint = touch.locationInView(self.imageview)
        if CGRectContainsPoint(self.imageview.bounds,touchPoint){
            let centerX = CGRectGetMidX(self.imageview.bounds)
            let centerY = CGRectGetMidY(self.imageview.bounds)
            let xy2 = pow(centerX - touchPoint.x,2.0) + pow(centerY - touchPoint.y,2.0)
            let radius2 = pow(CGRectGetWidth(self.imageview.frame)/2,2)
            if xy2 < radius2{
                return true
            }
        }
        return false
    }

}