iOS开发系列-UIImageView的contentMode

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,
};

UIViewContentModeScaleAspectFit：会保证图片比例不变，等比缩放，使得图片能够完全显示在UIImageView，会导致UIImageView有空白
UIViewContentModeScaleAspectFill：会保证图片比例不变，等比缩放，使得UIImageView被图片填充，会导致图片显示不完全。

官方给了一组图片如下：