以下是基于面条回答的Swift 3更新版本

func cropping(to rect: CGRect) -> UIImage? {

    if let cgCrop = cgImage?.cropping(to: rect) {
        return UIImage(cgImage: cgCrop)
    }
    else if let ciCrop = ciImage?.cropping(to: rect) {
        return UIImage(ciImage: ciCrop)
    }

    return nil
}

斯威夫特5:

extension UIImage {
    func cropped(rect: CGRect) -> UIImage? {
        guard let cgImage = cgImage else { return nil }

        UIGraphicsBeginImageContextWithOptions(rect.size, false, 0)
        let context = UIGraphicsGetCurrentContext()

        context?.translateBy(x: 0.0, y: self.size.height)
        context?.scaleBy(x: 1.0, y: -1.0)
        context?.draw(cgImage, in: CGRect(x: rect.minX, y: rect.minY, width: self.size.width, height: self.size.height), byTiling: false)


        let croppedImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()

        return croppedImage
    }
}

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。

这是我的UIImage作物实现，它遵从imageOrientation属性。所有方向都经过了彻底的测试。

inline double rad(double deg)
{
    return deg / 180.0 * M_PI;
}

UIImage* UIImageCrop(UIImage* img, CGRect rect)
{
    CGAffineTransform rectTransform;
    switch (img.imageOrientation)
    {
        case UIImageOrientationLeft:
            rectTransform = CGAffineTransformTranslate(CGAffineTransformMakeRotation(rad(90)), 0, -img.size.height);
            break;
        case UIImageOrientationRight:
            rectTransform = CGAffineTransformTranslate(CGAffineTransformMakeRotation(rad(-90)), -img.size.width, 0);
            break;
        case UIImageOrientationDown:
            rectTransform = CGAffineTransformTranslate(CGAffineTransformMakeRotation(rad(-180)), -img.size.width, -img.size.height);
            break;
        default:
            rectTransform = CGAffineTransformIdentity;
    };
    rectTransform = CGAffineTransformScale(rectTransform, img.scale, img.scale);

    CGImageRef imageRef = CGImageCreateWithImageInRect([img CGImage], CGRectApplyAffineTransform(rect, rectTransform));
    UIImage *result = [UIImage imageWithCGImage:imageRef scale:img.scale orientation:img.imageOrientation];
    CGImageRelease(imageRef);
    return result;
}

关注@Arne的回答。 我只是固定到类别函数。把它放在UIImage的Category中。

-(UIImage*)cropImage:(CGRect)rect{

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

看起来有点奇怪，但工作得很好，并考虑到图像方向:

var image:UIImage = ...

let img = CIImage(image: image)!.imageByCroppingToRect(rect)
image = UIImage(CIImage: img, scale: 1, orientation: image.imageOrientation)