Swift 2.0:

let image = UIImage(named: "imageName")
let newSize = CGSize(width: 10, height: 10)

UIGraphicsBeginImageContextWithOptions(newSize, false, 0.0)
image?.drawInRect(CGRectMake(0, 0, newSize.width, newSize.height))
let imageResized = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()

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)
            })
        })
    }
}

我发现在你的Swift 3项目中很难找到一个开箱即用的答案。其他答案的主要问题是他们不尊重图像的alpha通道。这是我在我的项目中使用的技巧。

extension UIImage {

    func scaledToFit(toSize newSize: CGSize) -> UIImage {
        if (size.width < newSize.width && size.height < newSize.height) {
            return copy() as! UIImage
        }

        let widthScale = newSize.width / size.width
        let heightScale = newSize.height / size.height

        let scaleFactor = widthScale < heightScale ? widthScale : heightScale
        let scaledSize = CGSize(width: size.width * scaleFactor, height: size.height * scaleFactor)

        return self.scaled(toSize: scaledSize, in: CGRect(x: 0.0, y: 0.0, width: scaledSize.width, height: scaledSize.height))
    }

    func scaled(toSize newSize: CGSize, in rect: CGRect) -> UIImage {
        if UIScreen.main.scale == 2.0 {
            UIGraphicsBeginImageContextWithOptions(newSize, !hasAlphaChannel, 2.0)
        }
        else {
            UIGraphicsBeginImageContext(newSize)
        }

        draw(in: rect)
        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()

        return newImage ?? UIImage()
    }

    var hasAlphaChannel: Bool {
        guard let alpha = cgImage?.alphaInfo else {
            return false
        }
        return alpha == CGImageAlphaInfo.first ||
            alpha == CGImageAlphaInfo.last ||
            alpha == CGImageAlphaInfo.premultipliedFirst ||
            alpha == CGImageAlphaInfo.premultipliedLast
    }
}

用法示例:

override func viewDidLoad() {
    super.viewDidLoad()

    let size = CGSize(width: 14.0, height: 14.0)
    if let image = UIImage(named: "barbell")?.scaledToFit(toSize: size) {
        let imageView = UIImageView(image: image)
        imageView.center = CGPoint(x: 100, y: 100)
        view.addSubview(imageView)
    }
}

这段代码重写了Apple的扩展，增加了对有和没有alpha通道的图像的支持。

作为进一步的阅读，我建议查看这篇文章，了解不同的图像调整技术。目前的方法提供了不错的性能，它操作高级api，易于理解。我建议坚持使用它，除非您发现图像大小调整是性能的瓶颈。

Swift 4和iOS 10+的更紧凑版本:

extension UIImage {
    func resized(to size: CGSize) -> UIImage {
        return UIGraphicsImageRenderer(size: size).image { _ in
            draw(in: CGRect(origin: .zero, size: size))
        }
    }
}

用法:

let resizedImage = image.resized(to: CGSize(width: 50, height: 50))

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;
}

正确的Swift 3.0 iOS 10+解决方案:使用ImageRenderer和闭包语法:

func imageWith(newSize: CGSize) -> UIImage {
    let image = UIGraphicsImageRenderer(size: newSize).image { _ in
        draw(in: CGRect(origin: .zero, size: newSize))
    }
        
    return image.withRenderingMode(renderingMode)
}

这是Objective-C版本:

@implementation UIImage (ResizeCategory)
- (UIImage *)imageWithSize:(CGSize)newSize
{
    UIGraphicsImageRenderer *renderer = [[UIGraphicsImageRenderer alloc] initWithSize:newSize];
    UIImage *image = [renderer imageWithActions:^(UIGraphicsImageRendererContext*_Nonnull myContext) {
        [self drawInRect:(CGRect) {.origin = CGPointZero, .size = newSize}];
    }];
    return [image imageWithRenderingMode:self.renderingMode];
}
@end