UIViewContentMode各类型效果

UIViewContentMode如下：

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,

};

注意以上几个常量，凡是没有带Scale的，当图片尺寸超过
ImageView尺寸时，只有部分显示在ImageView中。UIViewContentModeScaleToFill属性会导致图片变形。UIViewContentModeScaleAspectFit会保证图片比例不变，而且全部显示在ImageView中，这意味着ImageView会有部分空白。UIViewContentModeScaleAspectFill也会证图片比例不变，但是是填充整个ImageView的，可能只有部分图片显示出来。