还有另一种调整UIImage大小的方法:

// Resize to height = 100 points.
let originalImage = UIImage(named: "MyOriginalImage")!
let resizingFactor = 100 / originalImage.size.height
let newImage = UIImage(cgImage: originalImage.cgImage!, scale: originalImage.scale / resizingFactor, orientation: .up)

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)];

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;
}

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!
}

@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;}

Swift 3.0带有故障安全选项(在错误的情况下返回原始图像):

func resize(image: UIImage, toSize size: CGSize) -> UIImage{
    UIGraphicsBeginImageContextWithOptions(size,false,1.0)
    image.draw(in: CGRect(x: 0, y: 0, width: size.width, height: size.height))
    if let resizedImage = UIGraphicsGetImageFromCurrentImageContext() {
        UIGraphicsEndImageContext()
        return resizedImage
    }
    UIGraphicsEndImageContext()
    return image
}