- (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];
}

注释部分的代码是不作为property出现的，上下两段代码除了一个是property一个不是其它的都一毛一样，为什么只有当animator作为property的时候这个add进去的snap动画才生效呢？？看了官方文档也没找到这方面的解释，而且之前忘记是用什么其它的东西也是遇到了这种情况。跪求大神给解释一下ORZ