ImageView – Content Mode中Scale Fill、Aspect Fit 和 Aspect Fill 的区别：
Scale Fill：缩放（改变图片比例）以填满整个Image View
Aspect Fit：保持图片原比例，尽可能让图片全部显示出来，Image View 剩下空出来的部分是透明的
Aspect Fill：保持图片原比例，填满整个Image View（图片可能会被裁剪掉）
图片示例（只好牺牲这只可爱的小喵咪啦～）：
Scale Fill
Aspect Fit
Aspect Fill
UIViewContentModeScaleToFill：Scales the content to fit the size of itself by changing the aspect ratio of the content if necessary.
UIViewContentModeScaleAspectFit：Scales the content to fit the size of the view by maintaining the aspect ratio. Any remaining area of the view’s bounds is transparent.
UIViewContentModeScaleAspectFill：Scales the content to fill the size of the view. Some portion of the content may be clipped to fill the view’s bounds.
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