拖拽手势提供的函数

解决思路

1.创建拖拽手势实例，并依附于视图

 let v = UIView.init(frame: .init(x: 100, y: 300, width: 100, height: 100))
  v.backgroundColor = .red
        
 let p = UIPanGestureRecognizer.init(target: self, action: #selector(handlePan(pan:)))
  v.addGestureRecognizer(p)

2.实现手势的回调方法

    @objc func handlePan(pan: UIPanGestureRecognizer) {
        if pan.state == .changed || pan.state == .ended {
            let view = pan.view
            var viewRect = view?.frame
            let offset = pan.translation(in: view)
            print(offset.x)
            //例子中只演示仅竖直方向的手势处理，水平方向逻辑与之类似
            viewRect = changeYWith(frame: viewRect!, offset: offset)
            //更新视图的frame，已达到移动的视觉效果
            view?.frame = viewRect!
            //清零视图的偏移量记录
            pan.setTranslation(.zero, in: view)
        }
    }

3.实现更新视图坐标的方法（水平移动只更新x轴坐标，竖直方向移动则只更新y轴坐标)

    func changeYWith(frame: CGRect, offset: CGPoint) -> CGRect {
        var rect = frame
        var viewRect = rect.origin
        viewRect.y = offset.y + viewRect.y
         //移动至屏幕顶端，则不再继续向上
        if viewRect.y <= 0 {
            viewRect.y = 0
        }
        //移动至屏幕底部，则不再向下
        if viewRect.y >= self.view.frame.maxY - rect.height {
            viewRect.y = self.view.frame.maxY - rect.height
        }
        rect.origin = viewRect
        return rect
    }