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。

斯威夫特5:

func imageWithImage(_ image: UIImage?, scaledToSize newSize: CGSize) -> UIImage? {
    UIGraphicsBeginImageContextWithOptions(newSize, false, 0.0);
    image?.draw(in: CGRect(x: 0.0, y: 0.0, width: newSize.width, height: newSize.height))
    let newImage: UIImage? = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()
    return newImage
}

用法:

let img: UIImage? = imageWithImage(UIImage(named: "DefaultAvatar"), scaledToSize:CGSize(width: 20.0, height: 20.0))

以下是斯威夫特版的保罗·林奇的回答

func imageWithImage(image:UIImage, scaledToSize newSize:CGSize) -> UIImage{
    UIGraphicsBeginImageContextWithOptions(newSize, false, 0.0);
    image.drawInRect(CGRectMake(0, 0, newSize.width, newSize.height))
    let newImage:UIImage = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()
    return newImage
}

作为延伸:

public extension UIImage {
    func copy(newSize: CGSize, retina: Bool = true) -> UIImage? {
        // In next line, pass 0 to use the current device's pixel scaling factor (and thus account for Retina resolution).
        // Pass 1 to force exact pixel size.
        UIGraphicsBeginImageContextWithOptions(
            /* size: */ newSize,
            /* opaque: */ false,
            /* scale: */ retina ? 0 : 1
        )
        defer { UIGraphicsEndImageContext() }

        self.draw(in: CGRect(origin: .zero, size: newSize))
        return UIGraphicsGetImageFromCurrentImageContext()
    }
}

如果你只想要一个更小的图像，不关心确切的大小:

+ (UIImage *)imageWithImage:(UIImage *)image scaledToScale:(CGFloat)scale
{
    UIGraphicsBeginImageContextWithOptions(self.size, YES, scale);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetInterpolationQuality(context, kCGInterpolationHigh);
    [self drawInRect:CGRectMake(0, 0, self.size.width, self.size.height)];
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return newImage;
}

将比例设置为0.25f将从800万相机中获得816 * 612的图像。

这里有一个UIImage+Scale的分类。

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)
    }
}

使用这个扩展，如果你需要调整宽度/高度仅与纵横比。

extension UIImage {
    func resize(to size: CGSize) -> UIImage {
        return UIGraphicsImageRenderer(size: size).image { _ in
            draw(in: CGRect(origin: .zero, size: size))
        }
    }
    func resize(width: CGFloat) -> UIImage {
        return resize(to: CGSize(width: width, height: width / (size.width / size.height)))
    }
    func resize(height: CGFloat) -> UIImage {
        return resize(to: CGSize(width: height * (size.width / size.height), height: height))
    }
}