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。

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

对于Swift 5:

extension UIImage {
  func resized(to newSize: CGSize) -> UIImage? {
    UIGraphicsBeginImageContextWithOptions(newSize, false, 0)
    defer { UIGraphicsEndImageContext() }

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

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))

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;
}