使用这个扩展

extension UIImage {
    public func resize(size:CGSize, completionHandler:(resizedImage:UIImage, data:NSData?)->()) {
        dispatch_async(dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0), { () -> Void in
            let newSize:CGSize = size
            let rect = CGRectMake(0, 0, newSize.width, newSize.height)
            UIGraphicsBeginImageContextWithOptions(newSize, false, 1.0)
            self.drawInRect(rect)
            let newImage = UIGraphicsGetImageFromCurrentImageContext()
            UIGraphicsEndImageContext()
            let imageData = UIImageJPEGRepresentation(newImage, 0.5)
            dispatch_async(dispatch_get_main_queue(), { () -> Void in
                completionHandler(resizedImage: newImage, data:imageData)
            })
        })
    }
}

对Paul代码的改进将使你在带有视网膜显示屏的iPhone上获得清晰的高分辨率图像。否则按比例缩小时就会模糊。

+ (UIImage *)imageWithImage:(UIImage *)image scaledToSize:(CGSize)newSize {
if ([[UIScreen mainScreen] respondsToSelector:@selector(scale)]) {
    if ([[UIScreen mainScreen] scale] == 2.0) {
        UIGraphicsBeginImageContextWithOptions(newSize, YES, 2.0);
    } else {
        UIGraphicsBeginImageContext(newSize);
    }
} else {
    UIGraphicsBeginImageContext(newSize);
}
[image drawInRect:CGRectMake(0, 0, newSize.width, newSize.height)];
UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();    
UIGraphicsEndImageContext();
return newImage;
}

[cf Chris]调整大小到所需的大小:

UIImage *after = [UIImage imageWithCGImage:before.CGImage
                                     scale:CGImageGetHeight(before.CGImage)/DESIREDHEIGHT
                               orientation:UIImageOrientationUp];

或者，等效地，替换CGImageGetWidth(…)/DESIREDWIDTH

最简单的方法是设置UIImageView的框架，并将contentMode设置为其中一个调整大小的选项。

或者你可以使用这个实用工具方法，如果你真的需要调整图像的大小:

+ (UIImage *)imageWithImage:(UIImage *)image scaledToSize:(CGSize)newSize {
    //UIGraphicsBeginImageContext(newSize);
    // In next line, pass 0.0 to use the current device's pixel scaling factor (and thus account for Retina resolution).
    // Pass 1.0 to force exact pixel size.
    UIGraphicsBeginImageContextWithOptions(newSize, NO, 0.0);
    [image drawInRect:CGRectMake(0, 0, newSize.width, newSize.height)];
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();    
    UIGraphicsEndImageContext();
    return newImage;
}

使用示例:

#import "MYUtil.h"
…
UIImage *myIcon = [MYUtil imageWithImage:myUIImageInstance scaledToSize:CGSizeMake(20, 20)];

这里是我的一些冗长的Swift代码

func scaleImage(image:UIImage,  toSize:CGSize) -> UIImage {
    UIGraphicsBeginImageContextWithOptions(toSize, false, 0.0);

    let aspectRatioAwareSize = self.aspectRatioAwareSize(image.size, boxSize: toSize, useLetterBox: false)


    let leftMargin = (toSize.width - aspectRatioAwareSize.width) * 0.5
    let topMargin = (toSize.height - aspectRatioAwareSize.height) * 0.5


    image.drawInRect(CGRectMake(leftMargin, topMargin, aspectRatioAwareSize.width , aspectRatioAwareSize.height))
    let retVal = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()
    return retVal
}

func aspectRatioAwareSize(imageSize: CGSize, boxSize: CGSize, useLetterBox: Bool) -> CGSize {
    // aspect ratio aware size
    // http://stackoverflow.com/a/6565988/8047
    let imageWidth = imageSize.width
    let imageHeight = imageSize.height
    let containerWidth = boxSize.width
    let containerHeight = boxSize.height

    let imageAspectRatio = imageWidth/imageHeight
    let containerAspectRatio = containerWidth/containerHeight

    let retVal : CGSize
    // use the else at your own risk: it seems to work, but I don't know 
    // the math
    if (useLetterBox) {
        retVal = containerAspectRatio > imageAspectRatio ? CGSizeMake(imageWidth * containerHeight / imageHeight, containerHeight) : CGSizeMake(containerWidth, imageHeight * containerWidth / imageWidth)
    } else {
        retVal = containerAspectRatio < imageAspectRatio ? CGSizeMake(imageWidth * containerHeight / imageHeight, containerHeight) : CGSizeMake(containerWidth, imageHeight * containerWidth / imageWidth)
    }

    return retVal
}

Swift 3.0带有故障安全选项(在错误的情况下返回原始图像):

func resize(image: UIImage, toSize size: CGSize) -> UIImage{
    UIGraphicsBeginImageContextWithOptions(size,false,1.0)
    image.draw(in: CGRect(x: 0, y: 0, width: size.width, height: size.height))
    if let resizedImage = UIGraphicsGetImageFromCurrentImageContext() {
        UIGraphicsEndImageContext()
        return resizedImage
    }
    UIGraphicsEndImageContext()
    return image
}