正确的Swift 3.0 iOS 10+解决方案:使用ImageRenderer和闭包语法:

func imageWith(newSize: CGSize) -> UIImage {
    let image = UIGraphicsImageRenderer(size: newSize).image { _ in
        draw(in: CGRect(origin: .zero, size: newSize))
    }
        
    return image.withRenderingMode(renderingMode)
}

这是Objective-C版本:

@implementation UIImage (ResizeCategory)
- (UIImage *)imageWithSize:(CGSize)newSize
{
    UIGraphicsImageRenderer *renderer = [[UIGraphicsImageRenderer alloc] initWithSize:newSize];
    UIImage *image = [renderer imageWithActions:^(UIGraphicsImageRendererContext*_Nonnull myContext) {
        [self drawInRect:(CGRect) {.origin = CGPointZero, .size = newSize}];
    }];
    return [image imageWithRenderingMode:self.renderingMode];
}
@end

下面是iwasrob对上述类别的修改。它保持原始图像的纵横比，而不是扭曲它。

- (UIImage*)scaleToSizeKeepAspect:(CGSize)size {
    UIGraphicsBeginImageContext(size);

    CGFloat ws = size.width/self.size.width;
    CGFloat hs = size.height/self.size.height;

    if (ws > hs) {
        ws = hs/ws;
        hs = 1.0;
    } else {
        hs = ws/hs;
        ws = 1.0;
    }

    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextTranslateCTM(context, 0.0, size.height);
    CGContextScaleCTM(context, 1.0, -1.0);

    CGContextDrawImage(context, CGRectMake(size.width/2-(size.width*ws)/2,
        size.height/2-(size.height*hs)/2, size.width*ws,
        size.height*hs), self.CGImage);

    UIImage* scaledImage = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    return scaledImage;
}

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()
  }
}

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)];

有效的方法，不拉伸图像Swift 4

// Method to resize image
func resize(image: UIImage, toScaleSize:CGSize) -> UIImage {
                UIGraphicsBeginImageContextWithOptions(toScaleSize, true, image.scale)
                        image.draw(in: CGRect(x: 0, y: 0, width: toScaleSize.width, height: toScaleSize.height))
                        let scaledImage = UIGraphicsGetImageFromCurrentImageContext()
                        UIGraphicsEndImageContext()
                        return scaledImage!
                }

//调用方法

    let resizedImage = self.resize(image: UIImage(named: "YourImageName")!, toScaleSize: CGSize(width: 290, height: 390))