import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var dynamicImageView: UIImageView!
    var dynamicAnimator = UIDynamicAnimator()

    override func viewDidLoad() {
        super.viewDidLoad()

        let img1 = UIImage(named:"littleYellowMan.jpg")
        dynamicImageView.image = img1

        dynamicAnimator = UIDynamicAnimator(referenceView: self.view)

        let gravityBehavior = UIGravityBehavior(items: [self.dynamicImageView])
        dynamicAnimator.addBehavior(gravityBehavior)

        let collisionBehavior = UICollisionBehavior(items: [self.dynamicImageView])
        collisionBehavior.translatesReferenceBoundsIntoBoundary = true
        dynamicAnimator.addBehavior(collisionBehavior)
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
}

运行结果：图片掉落下去，撞到屏幕的边界