swift 基础之UIDynamicAnimation

class ViewController: UIViewController, UICollisionBehaviorDelegate {        @IBOutlet weak var gravity: UIButton!    @IBOutlet weak var push: UIButton!    @IBOutlet weak var attachment: UIButton!        var collision: UICollisionBehavior!        @IBAction func gravity(sender: UIButton) {        animator.removeAllBehaviors()        //gravity, push and attachment        let gravity = UIGravityBehavior(items: [self.gravity,self.push,self.attachment])        animator.addBehavior(gravity)                collision = UICollisionBehavior(items: [self.push, self.gravity, self.attachment])        collision.translatesReferenceBoundsIntoBoundary = true        animator.addBehavior(collision)    }        @IBAction func push(sender: AnyObject) {        animator.removeAllBehaviors()        let push = UIPushBehavior(items: [self.gravity,self.push,self.attachment], mode:UIPushBehaviorMode.Instantaneous)        push.magnitude = 2                animator.addBehavior(push)                collision = UICollisionBehavior(items: [self.push, self.gravity, self.attachment])        collision.translatesReferenceBoundsIntoBoundary = true        animator.addBehavior(collision)    }        var attachmentBehavior: UIAttachmentBehavior? = nil        @IBAction func attachment(sender: AnyObject) {        animator.removeAllBehaviors()        let anchorPoint = CGPointMake(self.attachment.center.x, self.attachment.center.y)        attachmentBehavior = UIAttachmentBehavior(item: self.attachment, attachedToAnchor: anchorPoint)        attachmentBehavior!.frequency = 0.5        attachmentBehavior!.damping = 2        attachmentBehavior!.length = 20        animator.addBehavior(attachmentBehavior!)        collision = UICollisionBehavior(items: [self.push, self.gravity, self.attachment])        collision.translatesReferenceBoundsIntoBoundary = true        animator.addBehavior(collision)    }        @IBAction func handleAttachment(sender: UIPanGestureRecognizer) {        if((attachmentBehavior) != nil){            attachmentBehavior!.anchorPoint = sender.locationInView(self.view)        }    }        var animator = UIDynamicAnimator()    override func viewDidLoad() {        super.viewDidLoad()        animator = UIDynamicAnimator(referenceView: self.view)    }        override func viewDidAppear(animated: Bool) {        let max: CGRect = UIScreen.mainScreen().bounds        let snap1 = UISnapBehavior(item: self.gravity, snapToPoint: CGPointMake(max.size.width/2, max.size.height/2 - 50))        let snap2 = UISnapBehavior(item: self.push, snapToPoint: CGPointMake(max.size.width/2, max.size.height/2 ))        let snap3 = UISnapBehavior(item: self.attachment, snapToPoint: CGPointMake(max.size.width/2, max.size.height/2 + 50))        snap1.damping = 1        snap2.damping = 2        snap3.damping = 4        animator.addBehavior(snap1)        animator.addBehavior(snap2)        animator.addBehavior(snap3)    }        override func didReceiveMemoryWarning() {        super.didReceiveMemoryWarning()    }    }

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