快速解决方案的拉伸填充，方面填补和方面适合

extension UIImage {
    enum ContentMode {
        case contentFill
        case contentAspectFill
        case contentAspectFit
    }
    
    func resize(withSize size: CGSize, contentMode: ContentMode = .contentAspectFill) -> UIImage? {
        let aspectWidth = size.width / self.size.width
        let aspectHeight = size.height / self.size.height
        
        switch contentMode {
        case .contentFill:
            return resize(withSize: size)
        case .contentAspectFit:
            let aspectRatio = min(aspectWidth, aspectHeight)
            return resize(withSize: CGSize(width: self.size.width * aspectRatio, height: self.size.height * aspectRatio))
        case .contentAspectFill:
            let aspectRatio = max(aspectWidth, aspectHeight)
            return resize(withSize: CGSize(width: self.size.width * aspectRatio, height: self.size.height * aspectRatio))
        }
    }
    
    private func resize(withSize size: CGSize) -> UIImage? {
        UIGraphicsBeginImageContextWithOptions(size, false, self.scale)
        defer { UIGraphicsEndImageContext() }
        draw(in: CGRect(x: 0.0, y: 0.0, width: size.width, height: size.height))
        return UIGraphicsGetImageFromCurrentImageContext()
    }
}

要使用，您可以执行以下操作:

let image = UIImage(named: "image.png")!
let newImage = image.resize(withSize: CGSize(width: 200, height: 150), contentMode: .contentAspectFill)

感谢abdullahselek最初的解决方案。

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)];

@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;}

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()
  }
}

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

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)
            })
        })
    }
}