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)
            })
        })
    }
}

给我的萨玛瑞恩同胞们，这是萨玛琳。iOS c#版本的@Paul Lynch回答。

private UIImage ResizeImage(UIImage image, CGSize newSize) 
{
    UIGraphics.BeginImageContextWithOptions(newSize, false, 0.0f);
    image.Draw(new CGRect(0, 0, newSize.Width, newSize.Height));
    UIImage newImage = UIGraphics.GetImageFromCurrentImageContext();
    UIGraphics.EndImageContext();
    return newImage;
}

当使用iOS 15或更新版本时，你可以使用UIImage的新的prepareThumbnail方法:

sourceImage.prepareThumbnail(of: thumbnailSize) { thumbnail in
    // Do something with the resized image
    DispatchQueue.main.async {
        cell.imageView?.image = thumbnail
    }
}

更多信息: https://developer.apple.com/documentation/uikit/uiimage/3750845-preparethumbnail

最简单的方法是设置UIImageView的框架，并将contentMode设置为其中一个调整大小的选项。

或者你可以使用这个实用工具方法，如果你真的需要调整图像的大小:

+ (UIImage *)imageWithImage:(UIImage *)image scaledToSize:(CGSize)newSize {
    //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(newSize, NO, 0.0);
    [image drawInRect:CGRectMake(0, 0, newSize.width, newSize.height)];
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();    
    UIGraphicsEndImageContext();
    return newImage;
}

使用示例:

#import "MYUtil.h"
…
UIImage *myIcon = [MYUtil imageWithImage:myUIImageInstance scaledToSize:CGSizeMake(20, 20)];

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。

(兼容Swift 4) iOS 10+和iOS < 10解决方案(如果可能，使用UIGraphicsGetImageFromCurrentImageContext)

/// Resizes an image
///
/// - Parameter newSize: New size
/// - Returns: Resized image
func scaled(to newSize: CGSize) -> UIImage {
    let rect = CGRect(origin: .zero, size: newSize)

    if #available(iOS 10, *) {
        let renderer = UIGraphicsImageRenderer(size: newSize)
        return renderer.image { _ in
            self.draw(in: rect)
        }
    } else {
        UIGraphicsBeginImageContextWithOptions(newSize, false, 0.0)
        self.draw(in: rect)
        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return newImage!
    }
}