UIViewContentMode

// 但凡在设置图片模式的枚举中包含Scale这个单词的值,都会对原有的图片进行缩放
typedef NS_ENUM(NSInteger, UIViewContentMode) {

// 默认属性,按照"UIImageView"的宽高比缩放图片至图片填充整个UIImageView
    UIViewContentModeScaleToFill,

// 按照"图片的宽高"比例缩放图片至图片的宽度或者高度和UIImageView一样,并且让整个图片都在UIImageView中,然后居中显示
    UIViewContentModeScaleAspectFit,      // contents scaled to fit with fixed aspect. remainder is transparent

// 按照"图片的宽高"比例缩放图片至图片的宽度和高度填充整个UIImageView,然后居中显示
    UIViewContentModeScaleAspectFill,     // contents scaled to fill with fixed aspect. some portion of content may be clipped.

// 调用setNeedsDisplay方法时,就会重新渲染图片
    UIViewContentModeRedraw,              // redraw on bounds change (calls -setNeedsDisplay)
    UIViewContentModeCenter,              // contents remain same size. positioned adjusted.
    UIViewContentModeTop,
    UIViewContentModeBottom,
    UIViewContentModeLeft,
    UIViewContentModeRight,
    UIViewContentModeTopLeft,
    UIViewContentModeTopRight,
    UIViewContentModeBottomLeft,
    UIViewContentModeBottomRight,
};

Original Picture

horizontalImage.png

verticalImage.png

Demo

UIViewContentModeScaleToFill

_leftImageView.contentMode = UIViewContentModeScaleToFill;
_rightImageView.contentMode = UIViewContentModeScaleToFill;

UIViewContentModeScaleToFill.png

默认填充模式，将图片拉伸或者压缩至充满整个容器。

UIViewContentModeScaleAspectFit

_leftImageView.contentMode = UIViewContentModeScaleAspectFit;
_rightImageView.contentMode = UIViewContentModeScaleAspectFit;

UIViewContentModeScaleAspectFit.png

图片并没有变形，等比例缩放至父容器完全装下图片为止。

UIViewContentModeScaleAspectFill

_leftImageView.contentMode = UIViewContentModeScaleAspectFill;
_rightImageView.contentMode = UIViewContentModeScaleAspectFill;
    
// 推荐:将超出容器范围的切除掉
//  _leftImageView.clipsToBounds = YES;
//  _rightImageView.clipsToBounds = YES;