这里有一个简单的方法:

    UIImage * image = [UIImage imageNamed:@"image"];
    CGSize sacleSize = CGSizeMake(10, 10);
    UIGraphicsBeginImageContextWithOptions(sacleSize, NO, 0.0);
    [image drawInRect:CGRectMake(0, 0, sacleSize.width, sacleSize.height)];
    UIImage * resizedImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

resizedImage是一个新的图像。

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;
}

这是一个与Swift 3和Swift 4兼容的UIImage扩展，它可以将图像缩放到给定的大小

extension UIImage {

    func scaledImage(withSize size: CGSize) -> UIImage {
        UIGraphicsBeginImageContextWithOptions(size, false, 0.0)
        defer { UIGraphicsEndImageContext() }
        draw(in: CGRect(x: 0.0, y: 0.0, width: size.width, height: size.height))
        return UIGraphicsGetImageFromCurrentImageContext()!
    }

    func scaleImageToFitSize(size: CGSize) -> UIImage {
        let aspect = self.size.width / self.size.height
        if size.width / aspect <= size.height {
            return scaledImage(withSize: CGSize(width: size.width, height: size.width / aspect))
        } else {
            return scaledImage(withSize: CGSize(width: size.height * aspect, height: size.height))
        }
    }

}

示例使用

let image = UIImage(named: "apple")
let scaledImage = image.scaleImageToFitSize(size: CGSize(width: 45.0, height: 45.0))

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 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
}

有时你的图像的比例大于1，所以调整图像大小会使图像意想不到。这就是这种情况下的解。

extension UIImage {
    func resizeTo(newSize: CGSize) -> UIImage {
        // Important thing here
        let format = UIGraphicsImageRendererFormat()
        format.scale = 1
        
        let image = UIGraphicsImageRenderer(size: newSize, format: format).image { _ in
            draw(in: CGRect(origin: .zero, size: newSize))
        }
            
        return image.withRenderingMode(renderingMode)
    }
}