UIImageView三种方式

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UIImageView *imgView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"图片"]];
    /*
     UIViewContentModeScaleToFill,
     UIViewContentModeScaleAspectFit,      // contents scaled to fit with fixed aspect. remainder is transparent
     UIViewContentModeScaleAspectFill,
     */
    imgView.contentMode = UIViewContentModeScaleAspectFit;

Fill(填充)，Fit(试用)，Aspect(方向)，Scale(比例)