iOS在屏幕上拖拽图片

    override func viewDidLoad() {
        super.viewDidLoad()
        addPanGesture()
    }

    fileprivate func addPanGesture() {
        let tap = UIPanGestureRecognizer(target: self, action: #selector(dragOnView(_:)))
        draggedView.isUserInteractionEnabled = true
        draggedView.addGestureRecognizer(tap)
    }

    @objc fileprivate func dragOnView(_ drag: UIPanGestureRecognizer) {
        if drag.state == .began || drag.state == .changed {
            //手指移动了多少距离
            var newPosition = drag.translation(in: drag.view)
            //视图应该移动到哪个位置
            newPosition.x += drag.view!.center.x
            newPosition.y += drag.view!.center.y
            drag.view?.center = newPosition
            //将视图的位置设为新位置
            drag.setTranslation(CGPoint.zero, in: drag.view)
        }
    }