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()
    }
}

我也见过这样做(我在UIButtons的正常和选定状态下使用，因为按钮不会调整大小来适应)。功劳归于原作者。

首先创建一个空的。h和。m文件，分别命名为uiimageresize .h和uiimageresize .m

// Put this in UIImageResizing.h
@interface UIImage (Resize)
- (UIImage*)scaleToSize:(CGSize)size;
@end

// Put this in UIImageResizing.m
@implementation UIImage (Resize)

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

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

CGContextDrawImage(context, CGRectMake(0.0f, 0.0f, size.width, size.height), self.CGImage);

UIImage* scaledImage = UIGraphicsGetImageFromCurrentImageContext();

UIGraphicsEndImageContext();

return scaledImage;
}

@end

将.h文件包含在你将要使用该函数的.m文件中，然后像这样调用它:

UIImage* image = [UIImage imageNamed:@"largeImage.png"];
UIImage* smallImage = [image scaleToSize:CGSizeMake(100.0f,100.0f)];

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()

Trevor Howard有一些UIImage类别，可以很好地处理大小调整。如果没有其他问题，您可以使用代码作为示例。

注意:从iOS 5.1开始，这个答案可能无效。见下面的评论。

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;
}

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))
    }
}