Iphone CATTransition动画关键点始终“始终”；“过渡”；

有一个基本的分类。工作

        animationKey = @"fadeIn";

        [self.icon setImage:self.iconSelected];
        [self.shadow setAlpha:1.0];
        CATransition *transitionIn = [CATransition animation];
        transitionIn.removedOnCompletion = NO;
        transitionIn.delegate = self;
        transitionIn.duration = 1.2;
        transitionIn.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseInEaseOut];
        transitionIn.type = kCATransitionFade;

        [self.icon.layer addAnimation:transitionIn forKey:animationKey];
        [self.shadow.layer addAnimation:transitionIn forKey:animationKey];
        [CATransaction commit];

这是关键：

        [self.icon.layer animationForKey:animationKey]

始终返回NULL

当我这样做时：

        [self.icon.layer animationKeys]

我得到：

        icon keys: (
transition
         )

这里我遗漏了什么？

说“特殊键KCATRANSION自动用于过渡动画。”