override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
    for touch in (touches) {
        let location = touch.location(in: self)
        if (ball.frame.contains(location)) {
            stickActive = true
        }
        else {
            stickActive = false
        }
    }
}

override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
    for touch in (touches) {
        var location = touch.location(in: self)
        let selfCenter = CGPoint(x: self.frame.width/2, y: self.frame.height/2)
        let selfRadius = self.frame.size.width/2
        let distance = distanceBetweenTwoPoints(point1: location, point2: selfCenter)
        if (distance > selfRadius) {
            let vX = location.x - selfCenter.x
            let vY = location.y - selfCenter.y
            location.x = selfCenter.x + ((vX / distance) * selfRadius)
            location.y = selfCenter.y + ((vY / distance) * selfRadius)
        }
        ball.center = location
    }
}

override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent?) {
    if (stickActive==true) {
        UIView.animate(withDuration: 0.3, animations: {
            self.ball.center = CGPoint(x: self.frame.width/2, y: self.frame.height/2)
        })
    }
}

func distanceBetweenTwoPoints(point1:CGPoint,point2:CGPoint)-> CGFloat {
    let dx = point2.x - point1.x
    let dy = point2.y - point1.y
    return sqrt(dx * dx + dy * dy)
}