UIView有一個屬性：contentMode，用來表示如何顯示內容，取值引數為：

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,
};
 

_photoView = [UIImageView new];
_photoView.frame = CGRectMake(10, kMarginTop+5,
                              kScreenWidth - 10*2, 240);
_photoView.backgroundColor = [UIColor greenColor];
NSString *str = @"http://i5.hexunimg.cn/2014-09-18/168589835.jpg";
_photoView setImageURLStr:str placeholder:nil]
_photoView.contentMode = UIViewContentModeScaleToFill;

[self.view addSubview:_photoView];
 

附效果圖，使用引數見標題