Scale 图片会缩放。
Aspect 保持比例

UIViewContentModeScaleToFill
缩放 不用保证比例 那就拉伸

UIViewContentModeScaleAspectFit
缩放 又保证比例 那么缩小咯 露出黑边

UIViewContentModeScaleAspectFill
缩放 又保证比例 那么放大咯 显示不全

https://www.jianshu.com/p/6f51b22ef678

typedef NS_ENUM(NSInteger, UIViewContentMode) {
    UIViewContentModeScaleToFill,
    UIViewContentModeScaleAspectFit,      // contents scaled to fit with fixed aspect. remainder is transparent
    UIViewContentModeScaleAspectFill,     // contents scaled to fill with fixed aspect. some portion of content may be clipped.
    UIViewContentModeRedraw,              // redraw on bounds change (calls -setNeedsDisplay)
    UIViewContentModeCenter,              // contents remain same size. positioned adjusted.
    UIViewContentModeTop,
    UIViewContentModeBottom,
    UIViewContentModeLeft,
    UIViewContentModeRight,
    UIViewContentModeTopLeft,
    UIViewContentModeTopRight,
    UIViewContentModeBottomLeft,
    UIViewContentModeBottomRight,
};

@property(nonatomic) UIViewContentMode contentMode; // default is UIViewContentModeScaleToFill

视图的contentMode属性 默认UIViewContentModeScaleToFill

先上些图

UIViewContentModeScaleToFill.png

UIViewContentModeScaleAspectFit.png

UIViewContentModeScaleAspectFill.png

UIViewContentModeCenter.png