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.
根據(jù)內(nèi)容容器的大小來重新調(diào)整內(nèi)容的大小唯笙，保持內(nèi)容的方向不變监徘。如果內(nèi)容不能完全填滿容器，內(nèi)容將會居中放置。
Declaration——聲明方法
let kCAGravityResizeAspect: String
