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))

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()

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()
    }
}

迅速回答:

func scaleDown(image: UIImage, withSize: CGSize) -> UIImage {
    let scale = UIScreen.main.scale
    UIGraphicsBeginImageContextWithOptions(withSize, false, scale)
    image.draw(in: CGRect(x: 0, y: 0, width: withSize.width, height: withSize.height))
    let 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

Rogerio Chaves的回答是一个快速扩展

func scaledTo(size: CGSize) -> UIImage{
    UIGraphicsBeginImageContextWithOptions(size, false, 0.0);
    self.draw(in: CGRect(x: 0, y: 0, width: size.width, height: size.height))
    let newImage:UIImage = UIGraphicsGetImageFromCurrentImageContext()!
    UIGraphicsEndImageContext()
    return newImage
}

还有额外的好处

func scaledTo(height: CGFloat) -> UIImage{
    let width = height*self.size.width/self.size.height
    return scaledTo(size: CGSize(width: width, height: height))
}