这些层可以被动态的增加或删除，你也可以对这些层进行改动，因为每层都有一个轨道，你可以在轨道中的特定时间点加入关键帧到达改变动画的目的。

These layers and be added or removed dynamically and you can change each layer as it has a track that lets you change it as a function of time by inserting keys at certain frames in the track.

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正如书中所指出，为了设置定时器和确定动画的当前进度，需要编写大量必不可少却又一再重复的代码。

As the book illustrates, there is a lot of boiler-plate code required in order to setup timers and determine the current progress of the animation.

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AnimationManager 类控制定时器并向其 Animation 对象列表中的第一项发送动画消息。

The AnimationManager class controls the timer and sends animate messages out to the first item in its list of Animation objects.

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