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))

下面是iwasrob对上述类别的修改。它保持原始图像的纵横比，而不是扭曲它。

- (UIImage*)scaleToSizeKeepAspect:(CGSize)size {
    UIGraphicsBeginImageContext(size);

    CGFloat ws = size.width/self.size.width;
    CGFloat hs = size.height/self.size.height;

    if (ws > hs) {
        ws = hs/ws;
        hs = 1.0;
    } else {
        hs = ws/hs;
        ws = 1.0;
    }

    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextTranslateCTM(context, 0.0, size.height);
    CGContextScaleCTM(context, 1.0, -1.0);

    CGContextDrawImage(context, CGRectMake(size.width/2-(size.width*ws)/2,
        size.height/2-(size.height*hs)/2, size.width*ws,
        size.height*hs), self.CGImage);

    UIImage* scaledImage = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    return scaledImage;
}

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)];

有效的方法，不拉伸图像Swift 4

// Method to resize image
func resize(image: UIImage, toScaleSize:CGSize) -> UIImage {
                UIGraphicsBeginImageContextWithOptions(toScaleSize, true, image.scale)
                        image.draw(in: CGRect(x: 0, y: 0, width: toScaleSize.width, height: toScaleSize.height))
                        let scaledImage = UIGraphicsGetImageFromCurrentImageContext()
                        UIGraphicsEndImageContext()
                        return scaledImage!
                }

//调用方法

    let resizedImage = self.resize(image: UIImage(named: "YourImageName")!, toScaleSize: CGSize(width: 290, height: 390))

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。