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)];

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()
  }
}

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)
            })
        })
    }
}

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))
}

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)];

以下是斯威夫特版的保罗·林奇的回答

func imageWithImage(image:UIImage, scaledToSize newSize:CGSize) -> UIImage{
    UIGraphicsBeginImageContextWithOptions(newSize, false, 0.0);
    image.drawInRect(CGRectMake(0, 0, newSize.width, newSize.height))
    let newImage:UIImage = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()
    return newImage
}

作为延伸:

public extension UIImage {
    func copy(newSize: CGSize, retina: Bool = true) -> UIImage? {
        // In next line, pass 0 to use the current device's pixel scaling factor (and thus account for Retina resolution).
        // Pass 1 to force exact pixel size.
        UIGraphicsBeginImageContextWithOptions(
            /* size: */ newSize,
            /* opaque: */ false,
            /* scale: */ retina ? 0 : 1
        )
        defer { UIGraphicsEndImageContext() }

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