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))
}

@Paul Lynch的回答很好，但它会改变图像比例。 如果你不想改变图像比例，并且仍然希望新的图像适合新的大小，试试这个。

+ (UIImage *)imageWithImage:(UIImage *)image scaledToSize:(CGSize)newSize {

// calculate a new size which ratio is same to original image
CGFloat ratioW = image.size.width / newSize.width;
CGFloat ratioH = image.size.height / newSize.height;

CGFloat ratio = image.size.width / image.size.height;

CGSize showSize = CGSizeZero;
if (ratioW > 1 && ratioH > 1) { 

    if (ratioW > ratioH) { 
        showSize.width = newSize.width;
        showSize.height = showSize.width / ratio;
    } else {
        showSize.height = newSize.height;
        showSize.width = showSize.height * ratio;
    }

} else if (ratioW > 1) {

    showSize.width = showSize.width;
    showSize.height = showSize.width / ratio;

} else if (ratioH > 1) {

    showSize.height = showSize.height;
    showSize.width = showSize.height * ratio;

}

//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(showSize, NO, 0.0);
[image drawInRect:CGRectMake(0, 0, showSize.width, showSize.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))

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。

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)];

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)
            })
        })
    }
}