有时你的图像的比例大于1，所以调整图像大小会使图像意想不到。这就是这种情况下的解。

extension UIImage {
    func resizeTo(newSize: CGSize) -> UIImage {
        // Important thing here
        let format = UIGraphicsImageRendererFormat()
        format.scale = 1
        
        let image = UIGraphicsImageRenderer(size: newSize, format: format).image { _ in
            draw(in: CGRect(origin: .zero, size: newSize))
        }
            
        return image.withRenderingMode(renderingMode)
    }
}

Swift 2.0:

let image = UIImage(named: "imageName")
let newSize = CGSize(width: 10, height: 10)

UIGraphicsBeginImageContextWithOptions(newSize, false, 0.0)
image?.drawInRect(CGRectMake(0, 0, newSize.width, newSize.height))
let imageResized = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()

(兼容Swift 4) iOS 10+和iOS < 10解决方案(如果可能，使用UIGraphicsGetImageFromCurrentImageContext)

/// Resizes an image
///
/// - Parameter newSize: New size
/// - Returns: Resized image
func scaled(to newSize: CGSize) -> UIImage {
    let rect = CGRect(origin: .zero, size: newSize)

    if #available(iOS 10, *) {
        let renderer = UIGraphicsImageRenderer(size: newSize)
        return renderer.image { _ in
            self.draw(in: rect)
        }
    } else {
        UIGraphicsBeginImageContextWithOptions(newSize, false, 0.0)
        self.draw(in: rect)
        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return newImage!
    }
}

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)];

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

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)