如何使用UIPanGestureRecognizer设置拖动控件的边界？

Question

我有一个View，里面我正在添加一个Label然后我尝试使用PanGesture设置移动Label的边界。以下是我的代码：

@objc func handlePan(_ gestureRecognizer: UIPanGestureRecognizer) {
    if gestureRecognizer.state == .began || gestureRecognizer.state == .changed {
        let translation = gestureRecognizer.translation(in: self)
        if let view = gestureRecognizer.view {

            if (view.frame.origin.x + translation.x >= 0) && (view.frame.origin.y + translation.y >= 0) && (view.frame.origin.x + translation.x <= view.frame.width) && (view.frame.origin.y + translation.y <= view.frame.height)
            {
                view.center = CGPoint(x:view.center.x + translation.x,
                                       y:view.center.y + translation.y)
            }
        }
        gestureRecognizer.setTranslation(CGPoint.zero, in: self)
    }

}

我参考了这个答案：https://stackoverflow.com/a/49008808/9970928

但它不起作用，任何人都可以告诉我在这种情况下缺少什么？

Answer 1

代码并不是最直观的，但假设其他一切工作的问题在于它只会偏离右边和底部的边界。看看你的情况：

(view.frame.origin.x + translation.x <= view.frame.width)
(view.frame.origin.y + translation.y <= view.frame.height)

所以它说原点可能不大于你想要做的边界大小是检查内部视图的最大值：

(view.frame.maxX + translation.x <= view.frame.width)
(view.frame.maxY + translation.y <= view.frame.height)

但是这个程序一般可能会产生问题。想象一下，用户可以非常快速地向右滑动。并且最大的center.x可以是100。当前的center.x是50，用户将它拖到200的单帧中。你的病情会失败，你的标签将留在50而不是100。我会把框架夹紧到边界。

像下面这样的东西应该做：

func clampFrame(_ frame: CGRect, inBounds bounds: CGRect) -> CGRect {
    let center: CGPoint = CGPoint(x: max(bounds.minX + frame.width*0.5, min(frame.midX, bounds.maxX - frame.width*0.5)),
                                  y: max(bounds.minY + frame.height*0.5, min(frame.midY, bounds.maxY - frame.height*0.5)))
    return CGRect(x: center.x-frame.width*0.5, y: center.y-frame.height*0.5, width: frame.width, height: frame.height)
}

func moveFrame(_ frame: CGRect, by translation: CGPoint, constrainedTo bounds: CGRect) -> CGRect {
    var newFrame = frame
    newFrame.origin.x += translation.x
    newFrame.origin.y += translation.y
    return clampFrame(newFrame, inBounds: bounds)
}

使用“翻译”程序可能还有其他问题。我会去寻找一个位置。请参阅以下工作示例：

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        let myView = MyView(frame: CGRect(x: 100.0, y: 100.0, width: 200.0, height: 200.0))
        myView.backgroundColor = UIColor.green
        view.addSubview(myView)
        let label = UILabel(frame: .zero)
        label.backgroundColor = UIColor.blue.withAlphaComponent(0.2)
        label.font = UIFont.systemFont(ofSize: 50.0)
        label.text = "Hi!"
        label.sizeToFit()
        myView.addSubview(label)
        label.addGestureRecognizer(UIPanGestureRecognizer(target: myView, action: #selector(MyView.handlePan)))
        label.isUserInteractionEnabled = true
    }


}

class MyView: UIView {

    func clampFrame(_ frame: CGRect, inBounds bounds: CGRect) -> CGRect {
        let center: CGPoint = CGPoint(x: max(bounds.minX + frame.width*0.5, min(frame.midX, bounds.maxX - frame.width*0.5)),
                                      y: max(bounds.minY + frame.height*0.5, min(frame.midY, bounds.maxY - frame.height*0.5)))
        return CGRect(x: center.x-frame.width*0.5, y: center.y-frame.height*0.5, width: frame.width, height: frame.height)
    }

    func moveFrame(_ frame: CGRect, by translation: CGPoint, constrainedTo bounds: CGRect) -> CGRect {
        var newFrame = frame
        newFrame.origin.x += translation.x
        newFrame.origin.y += translation.y
        return clampFrame(newFrame, inBounds: bounds)
    }

    private var startLocation: CGPoint = .zero
    private var startFrame: CGRect = .zero

    @objc func handlePan(_ gestureRecognizer: UIPanGestureRecognizer) {
        guard let label = gestureRecognizer.view else { return }

        if gestureRecognizer.state == .began {
            startLocation = gestureRecognizer.location(in: self)
            startFrame = label.frame
        } else if gestureRecognizer.state == .changed {
            let newLocation = gestureRecognizer.location(in: self)
            let translation = CGPoint(x: newLocation.x-startLocation.x, y: newLocation.y-startLocation.y)

            label.frame = moveFrame(startFrame, by: translation, constrainedTo: self.bounds)
        }

    }

}