在Swift上用重力拖动图像

Question

我正在尝试使用Xcode编写代码来拖动图像，以便可以四处移动图像，当我放下图像时，可能会产生重力效果。我为图像创建了一个名为DraggedImageView的新类（继承自UIImageView），并进行了以下更改：

 override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
        startLocation = touches.first?.location(in: self)
    }

    override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
        let currentLocation = touches.first?.location(in: self)
        let dx = currentLocation!.x - startLocation!.x
        let dy = currentLocation!.y - startLocation!.y
        self.center = CGPoint(x: self.center.x+dx, y: self.center.y+dy)
}

在我的ViewController中，我有：

import UIKit

class ViewController: UIViewController {
    // Moving background items
    @IBOutlet weak var groundView: UIImageView!
    @IBOutlet weak var background1View: UIImageView!
    @IBOutlet weak var background2View: UIImageView!
    @IBOutlet weak var background3View: UIImageView!
    @IBOutlet weak var cloud1View: UIImageView!
    @IBOutlet weak var cloud2View: UIImageView!

    // Main character items
    @IBOutlet weak var marioView: DraggedImageView!

    // Behavior items
    var dynamicAnimator: UIDynamicAnimator!
    var collisionBehavior: UICollisionBehavior!
    var gravityBehavior: UIGravityBehavior!

    override func viewDidLoad() {
        super.viewDidLoad()

        self.view.addSubview(background1View)
        self.view.addSubview(background2View)
        self.view.addSubview(background3View)
        self.view.addSubview(groundView)
        self.view.addSubview(cloud1View)
        self.view.addSubview(cloud2View)
        self.view.addSubview(marioView)

        dynamicAnimator = UIDynamicAnimator(referenceView: self.view)

        gravityBehavior = UIGravityBehavior(items: [marioView])
        dynamicAnimator.addBehavior(gravityBehavior)
        gravityBehavior.magnitude = 0.5

        collisionBehavior = UICollisionBehavior(items: [marioView])
        collisionBehavior.translatesReferenceBoundsIntoBoundary = true
        dynamicAnimator.addBehavior(collisionBehavior)
    }

问题在于主角只掉落一次，当我尝试再次将其拖上时，gravityBehaviour无法正常工作。同样，根据我执行代码的模拟器，冲突的边界也不正确。

我只是希望它具有正确的行为，但是我已经尝试了很多方法，但我做不到。