以编程方式将iphone中的UIImage比率更改为fb cover

我正在从图像选取器中选取一张图像，然后我想更改该图像作为facebook封面的比例。我有一张图片，假设它的分辨率是640宽，480高，我想把它换成facebook封面（851像素宽，315像素高），我将如何在iphone上编程实现这一点 查看此页了解封面图片的详细信息 谢谢

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在contentMode中我将传递什么？UIViewContentMode指定视图在其大小更改时如何调整其内容。类型定义枚举{UIViewContentModeScaleToFill、UIViewContentModeScaleSpectFit、UIViewContentModeScaleSpectFill、UIViewContentModerDraw、UIViewContentModeCenter、UIViewContentModeTop、UIViewContentModeBottom、UIViewContentModeLeft、UIViewContentModerTopLeft、UIViewContentModerTopRight、UIViewContentModerModeBottomLeft，UIViewContentModeBottomRight，}此处的apple文档：我将在contentMode中传递什么？UIViewContentMode指定视图在其大小更改时如何调整其内容。typedef enum{UIViewContentModeScaleToFill、UIViewContentModeScaleSpectFit、UIViewContentModeScaleSpectFill、UIViewContentModerDraw、UIViewContentModeCenter、UIViewContentModeTop、UIViewContentModeBottom、UIViewContentModeLeft、UIViewContentModerTopLeft、UIViewContentModerTopRight、UIViewContentModerModeBottomLeft，UIViewContentModeBottomRight，}苹果文档在此：

 Use this Method to Resize the image according to the given content mode, taking into account the image's orientation...

- (UIImage *)resizedImageWithContentMode:(UIViewContentMode)contentMode bounds:(CGSize)bounds
                interpolationQuality:(CGInterpolationQuality)quality {

CGFloat horizontalRatio = bounds.width / self.size.width;
CGFloat verticalRatio = bounds.height / self.size.height;
CGFloat ratio;

switch (contentMode) {
    case UIViewContentModeScaleAspectFill:
        ratio = MAX(horizontalRatio, verticalRatio);
        break;

    case UIViewContentModeScaleAspectFit:
        ratio = MIN(horizontalRatio, verticalRatio);
        break;

    default:
        [NSException raise:NSInvalidArgumentException format:@"Unsupported content mode:  %d", contentMode];
}

CGSize newSize = CGSizeMake(self.size.width * ratio, self.size.height * ratio);

return [self resizedImage:newSize interpolationQuality:quality];