ios - 平移手势设置边界

Question

我使用UIPanGesture，在目标方法中，监听state，我设置了可以移动的最大值和最小值；在 changed 状态下，当我检测到 View 的 view.origin.y 超过 myOrigin.y 时，view.origin .y = myOrigin.y。当缓慢拖动 View 时，似乎没有问题，但是当我快速拖动 View 时， View 会超出这个边界，然后立即变成我设置的边界，看起来像在闪烁。如果我设置动画 UIView.animation;它不会眨眼，但很明显我超出了界限并返回到这个 Action 。我想知道如何避免这种情况。不管拖动的速度有多快，它都会停在我设定的边界处，不会超过边界然后返回。至少看起来是这样的

```

  let point = pan.translation(in: view)
  switch pan.state {
    case .began:
        break
    case .changed:
        view.frame.origin.y = view.frame.origin.y + point.y
        if view.frame.origin.y > 200{
          view.frame.origin.y = 200
        }
        pan.setTranslation(.zero, in: view)
    default ：
    break
  }

```

Answer 1

您可以通过约束来做到这一点 - 这更简单、更灵活。

此代码将创建一个 100 x 100 View Controller 中心的蓝色 View ，允许拖动它(通过 PanGesture )，并将其垂直移动限制为距 View 顶部 200 点(距底部 20 点，因此您可以不要将其拖离屏幕)。

关键是使用centerYAnchor优先级为 750 ( .defaultHigh ) 的约束，以及 >= topAnchor和<= bottomAnchor限制垂直定位的约束。

//
//  PanLimitViewController.swift
//
//  Created by Don Mag on 8/1/18.
//

import UIKit

class PanLimitViewController: UIViewController {

    // blue view for panning (dragging)
    var panView: UIView = {
        let v = UIView()
        v.translatesAutoresizingMaskIntoConstraints = false
        v.backgroundColor = .blue
        return v
    }()

    // the panView's center y constraint
    var panCenterYConstraint: NSLayoutConstraint!

    // var to track the current center y
    var currentCenterYConstant: CGFloat = 0.0

    // pan gesture recognizer
    var panGesture  = UIPanGestureRecognizer()

    override func viewDidLoad() {
        super.viewDidLoad()

        // add the panView
        view.addSubview(panView)

        // constrain width to 100, height equal to width
        panView.widthAnchor.constraint(equalToConstant: 100.0).isActive = true
        panView.heightAnchor.constraint(equalTo: panView.widthAnchor).isActive = true

        // constrain to center horizontally
        panView.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true

        // initialize panView's center y constraint to center of view
        panCenterYConstraint = NSLayoutConstraint(
            item: panView,
            attribute: .centerY,
            relatedBy: .equal,
            toItem: view,
            attribute: .centerY,
            multiplier: 1.0,
            constant: 0.0)

        // set panView's center y constraint priority to 750
        // required to allow dragging
        panCenterYConstraint.priority = .defaultHigh

        // activate panView's center y constraint
        panCenterYConstraint.isActive = true

        // set panView's topAnchor to >= view's top + 200
        // this will prevent dragging it higher than 200 pts from the top
        panView.topAnchor.constraint(greaterThanOrEqualTo: view.topAnchor, constant: 200.0).isActive = true

        // set panView's bottomAnchor to <= view's bottom
        // this will prevent dragging it below the bottom
        panView.bottomAnchor.constraint(lessThanOrEqualTo: view.bottomAnchor, constant: -20.0).isActive = true

        // create pan gesture and add to panView
        panGesture = UIPanGestureRecognizer(target: self, action: #selector(self.panGestureHandler(_:)))
        panView.addGestureRecognizer(panGesture)

    }

    @objc func panGestureHandler(_ recognizer: UIPanGestureRecognizer){

        switch recognizer.state {
        case .began:
            // save the current center y constant value
            currentCenterYConstant = panCenterYConstraint.constant
            break

        case .changed:
            // update panView's center y, based on drag
            let translation = recognizer.translation(in: self.view)
            panCenterYConstraint.constant = currentCenterYConstant + translation.y
            break

        case .ended, .cancelled:
            // update panView's center y when drag is finished
            panCenterYConstraint.constant = panView.center.y - view.center.y
            break

        default:
            break
        }

    }

}