iOSUIDynamicAnimator实例对象为什么不作为property出现的时候不好使？？

- (void)shake

{

    if (!self.animator) {

        self.animator = [[UIDynamicAnimator alloc]initWithReferenceView:self.view];

    }

   

    [self.animator removeAllBehaviors];

   

    CGPoint newPos = CGPointMake(arc4random()%320, arc4random()%568);

    UISnapBehavior *snap = [[UISnapBehavior alloc]initWithItem:_btn snapToPoint: newPos];

    snap.damping = 0.3;

    [self.animator addBehavior:snap];

   

//    UIDynamicAnimator *animator = [[UIDynamicAnimator alloc]initWithReferenceView:self.view];

//    

//    [animator removeAllBehaviors];

//    

//    CGPoint newPos = CGPointMake(arc4random()%320, arc4random()%568);

//    UISnapBehavior *snap = [[UISnapBehavior alloc]initWithItem:_btn snapToPoint: newPos];

//    snap.damping = 0.3;

//    [animator addBehavior:snap];

}