iOS ImageView不规则图片自适应缩放

        //  retina屏幕图片显示问题
        [_imageview setContentScaleFactor:[[UIScreen mainScreen] scale]];
        //  不规则图片显示
        _imageview.contentMode =  UIViewContentModeScaleAspectFill;
        _imageview.autoresizingMask = UIViewAutoresizingFlexibleHeight;
        //  图片大于或小于显示区域
        _imageview.clipsToBounds  = YES;