(UIImage *)squareImageWithImage:(UIImage *)image scaledToSize:(CGSize)newSize {
    double ratio;
    double delta;
    CGPoint offset;

    //make a new square size, that is the resized imaged width
    CGSize sz = CGSizeMake(newSize.width, newSize.width);

    //figure out if the picture is landscape or portrait, then
    //calculate scale factor and offset
    if (image.size.width > image.size.height) {
        ratio = newSize.width / image.size.width;
        delta = (ratio*image.size.width - ratio*image.size.height);
        offset = CGPointMake(delta/2, 0);
    } else {
        ratio = newSize.width / image.size.height;
        delta = (ratio*image.size.height - ratio*image.size.width);
        offset = CGPointMake(0, delta/2);
    }

    //make the final clipping rect based on the calculated values
    CGRect clipRect = CGRectMake(-offset.x, -offset.y,
                                 (ratio * image.size.width) + delta,
                                 (ratio * image.size.height) + delta);


    //start a new context, with scale factor 0.0 so retina displays get
    //high quality image
    if ([[UIScreen mainScreen] respondsToSelector:@selector(scale)]) {
        UIGraphicsBeginImageContextWithOptions(sz, YES, 0.0);
    } else {
        UIGraphicsBeginImageContext(sz);
    }
    UIRectClip(clipRect);
    [image drawInRect:clipRect];
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return newImage;
}

更新2014-05-28:我写这篇文章的时候，iOS 3左右还是热门的新事物，我相信现在有更好的方法来做到这一点，可能是内置的。正如很多人提到的，这种方法没有考虑到轮换;阅读一些额外的答案，并散布一些点赞的爱，让这个问题的回答对每个人都有帮助。

最初的反应:

我将把我对相同问题的回答复制/粘贴到其他地方:

没有一个简单的类方法可以做到这一点，但有一个函数可以使用来获得所需的结果:CGImageCreateWithImageInRect(CGImageRef, CGRect)将帮助您解决这个问题。

下面是一个简短的例子:

CGImageRef imageRef = CGImageCreateWithImageInRect([largeImage CGImage], cropRect);
// or use the UIImage wherever you like
[UIImageView setImage:[UIImage imageWithCGImage:imageRef]]; 
CGImageRelease(imageRef);

我不满意的其他解决方案，因为他们要么画了几次(使用更多的权力比必要的)或有问题的方向。下面是我从UIImage *图像中使用的缩放正方形croppedImage。

CGFloat minimumSide = fminf(image.size.width, image.size.height);
CGFloat finalSquareSize = 600.;

//create new drawing context for right size
CGRect rect = CGRectMake(0, 0, finalSquareSize, finalSquareSize);
CGFloat scalingRatio = 640.0/minimumSide;
UIGraphicsBeginImageContext(rect.size);

//draw
[image drawInRect:CGRectMake((minimumSide - photo.size.width)*scalingRatio/2., (minimumSide - photo.size.height)*scalingRatio/2., photo.size.width*scalingRatio, photo.size.height*scalingRatio)];

UIImage *croppedImage = UIGraphicsGetImageFromCurrentImageContext();

UIGraphicsEndImageContext();

在Swift中裁剪UIImage的最佳解决方案，在精度方面，像素缩放…:

private func squareCropImageToSideLength(let sourceImage: UIImage,
    let sideLength: CGFloat) -> UIImage {
        // input size comes from image
        let inputSize: CGSize = sourceImage.size

        // round up side length to avoid fractional output size
        let sideLength: CGFloat = ceil(sideLength)

        // output size has sideLength for both dimensions
        let outputSize: CGSize = CGSizeMake(sideLength, sideLength)

        // calculate scale so that smaller dimension fits sideLength
        let scale: CGFloat = max(sideLength / inputSize.width,
            sideLength / inputSize.height)

        // scaling the image with this scale results in this output size
        let scaledInputSize: CGSize = CGSizeMake(inputSize.width * scale,
            inputSize.height * scale)

        // determine point in center of "canvas"
        let center: CGPoint = CGPointMake(outputSize.width/2.0,
            outputSize.height/2.0)

        // calculate drawing rect relative to output Size
        let outputRect: CGRect = CGRectMake(center.x - scaledInputSize.width/2.0,
            center.y - scaledInputSize.height/2.0,
            scaledInputSize.width,
            scaledInputSize.height)

        // begin a new bitmap context, scale 0 takes display scale
        UIGraphicsBeginImageContextWithOptions(outputSize, true, 0)

        // optional: set the interpolation quality.
        // For this you need to grab the underlying CGContext
        let ctx: CGContextRef = UIGraphicsGetCurrentContext()
        CGContextSetInterpolationQuality(ctx, kCGInterpolationHigh)

        // draw the source image into the calculated rect
        sourceImage.drawInRect(outputRect)

        // create new image from bitmap context
        let outImage: UIImage = UIGraphicsGetImageFromCurrentImageContext()

        // clean up
        UIGraphicsEndImageContext()

        // pass back new image
        return outImage
}

调用此函数的指令:

let image: UIImage = UIImage(named: "Image.jpg")!
let squareImage: UIImage = self.squareCropImageToSideLength(image, sideLength: 320)
self.myUIImageView.image = squareImage

注意:最初的源代码灵感是用Objective-C写的，可以在Cocoanetics博客上找到。

CGSize size = [originalImage size];
int padding = 20;
int pictureSize = 300;
int startCroppingPosition = 100;
if (size.height > size.width) {
    pictureSize = size.width - (2.0 * padding);
    startCroppingPosition = (size.height - pictureSize) / 2.0; 
} else {
    pictureSize = size.height - (2.0 * padding);
    startCroppingPosition = (size.width - pictureSize) / 2.0;
}
// WTF: Don't forget that the CGImageCreateWithImageInRect believes that 
// the image is 180 rotated, so x and y are inverted, same for height and width.
CGRect cropRect = CGRectMake(startCroppingPosition, padding, pictureSize, pictureSize);
CGImageRef imageRef = CGImageCreateWithImageInRect([originalImage CGImage], cropRect);
UIImage *newImage = [UIImage imageWithCGImage:imageRef scale:1.0 orientation:originalImage.imageOrientation];
[m_photoView setImage:newImage];
CGImageRelease(imageRef);

我所见过的大多数响应只处理(x, y)的位置(0,0)。好吧，这是一种情况，但我希望我的裁剪操作居中。我花了一些时间才弄清楚WTF注释后面的那行。

让我们以纵向捕获的图像为例:

原始图像的高度高于它的宽度(哇，到目前为止还没有什么奇怪的!) CGImageCreateWithImageInRect方法在它自己的世界中想象的图像并不是一个真正的肖像，而是一个景观(这也是为什么如果你不使用imageWithCGImage构造函数中的方向参数，它将显示为180旋转)。 你可以把它想象成一个横向，(0,0)位置是图像的右上角。

希望这是有意义的!如果没有，尝试不同的值，你会发现在为你的cropRect选择正确的x、y、宽度和高度时，逻辑是颠倒的。

注意:所有这些答案都假设一个cgimage支持的图像对象。

的形象。CGImage可以返回nil，如果UIImage是由CIImage支持的，如果你用CIFilter创建了这个图像。

在这种情况下，您可能必须在新的上下文中绘制图像，并返回该图像(缓慢)。

UIImage* crop(UIImage *image, rect) {
    UIGraphicsBeginImageContextWithOptions(rect.size, false, [image scale]);
    [image drawAtPoint:CGPointMake(-rect.origin.x, -rect.origin.y)];
    cropped_image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return cropped_image;
}