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.
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.
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.
动画的一个基本定义是通过在一定时间内改变或移动图像而产生的幻觉。
A basic definition of animation is the illusion of motion, or change, created by changing an image over time.
你必须界定时间从耳朵到脚趾，你的动画。
You must define the timing from ears to toes in your animation.
一定时间内帧数更多会让体育节目及动作电影中的快速移动画面看上去更为流畅。
More frames in a given time make fast-moving images in sports programs and action movies look seamless.
因此，为了实现动画，我们需要一些可以定时执行重绘的方法。
The requestAnimationFramemethod provides a smoother and more efficient way for animating by calling the animation frame when the system is ready to paint the frame.
因此，为了实现动画，我们需要一些可以定时执行重绘的方法。
The requestAnimationFramemethod provides a smoother and more efficient way for animating by calling the animation frame when the system is ready to paint the frame.
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