kCAGravityResizeAspect
The content is resized to fit the bounds
rectangle, preserving the aspect of the content. If the content does
not completely fill the bounds rectangle, the content is centered in the
partial axis.
根据内容容器的大小来重新调整内容的大小，保持内容的方向不变。如果内容不能完全填满容器，内容将会居中放置。
Declaration——声明方法
let kCAGravityResizeAspect: String
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