本例的运行结果是一个图片可以自由落下，点击鼠标第一次，图片粘附到鼠标点击的位置，点击鼠标第二次，图片自由落下，重复该过程。

需要注意的是，gravity和snap会又些冲突，所以需要以下这一行代码：

dynamicAnimator.removeAllBehaviors()

import UIKit

class ViewController: UIViewController {

    @IBAction func tapped(sender: AnyObject) {
        let tap = sender as! UITapGestureRecognizer
        
        if tap.state == UIGestureRecognizerState.Ended{
            let point = tap.locationInView(tap.view)
            //print(point)
            self.snap = UISnapBehavior(item: self.dynamicImageView,snapToPoint: point)
            if (snap != nil){
                dynamicAnimator.removeAllBehaviors()
                dynamicAnimator.addBehavior(self.snap!)
                if fall==0{
                    fall=1
                }else{
                    addBehaviorSelf()
                    fall=0
                }
            }
        }
    }
    @IBOutlet weak var dynamicImageView: UIImageView!
    
    var fall = 0
    
    var dynamicAnimator = UIDynamicAnimator()
    var snap: UISnapBehavior?
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let img1 = UIImage(named:"littleYellowMan.jpg")
        dynamicImageView.image = img1
        
        dynamicAnimator = UIDynamicAnimator(referenceView: self.view)
        addBehaviorSelf()
        
    }

    func addBehaviorSelf(){
        dynamicAnimator.removeAllBehaviors()
        
        let gravityBehavior = UIGravityBehavior(items: [self.dynamicImageView])
        dynamicAnimator.addBehavior(gravityBehavior)
        
        let collisionBehavior = UICollisionBehavior(items: [self.dynamicImageView])
        collisionBehavior.translatesReferenceBoundsIntoBoundary = true
        dynamicAnimator.addBehavior(collisionBehavior)
    }
}