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))

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()

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))
}

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)

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!
}

我发现在你的Swift 3项目中很难找到一个开箱即用的答案。其他答案的主要问题是他们不尊重图像的alpha通道。这是我在我的项目中使用的技巧。

extension UIImage {

    func scaledToFit(toSize newSize: CGSize) -> UIImage {
        if (size.width < newSize.width && size.height < newSize.height) {
            return copy() as! UIImage
        }

        let widthScale = newSize.width / size.width
        let heightScale = newSize.height / size.height

        let scaleFactor = widthScale < heightScale ? widthScale : heightScale
        let scaledSize = CGSize(width: size.width * scaleFactor, height: size.height * scaleFactor)

        return self.scaled(toSize: scaledSize, in: CGRect(x: 0.0, y: 0.0, width: scaledSize.width, height: scaledSize.height))
    }

    func scaled(toSize newSize: CGSize, in rect: CGRect) -> UIImage {
        if UIScreen.main.scale == 2.0 {
            UIGraphicsBeginImageContextWithOptions(newSize, !hasAlphaChannel, 2.0)
        }
        else {
            UIGraphicsBeginImageContext(newSize)
        }

        draw(in: rect)
        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()

        return newImage ?? UIImage()
    }

    var hasAlphaChannel: Bool {
        guard let alpha = cgImage?.alphaInfo else {
            return false
        }
        return alpha == CGImageAlphaInfo.first ||
            alpha == CGImageAlphaInfo.last ||
            alpha == CGImageAlphaInfo.premultipliedFirst ||
            alpha == CGImageAlphaInfo.premultipliedLast
    }
}

用法示例:

override func viewDidLoad() {
    super.viewDidLoad()

    let size = CGSize(width: 14.0, height: 14.0)
    if let image = UIImage(named: "barbell")?.scaledToFit(toSize: size) {
        let imageView = UIImageView(image: image)
        imageView.center = CGPoint(x: 100, y: 100)
        view.addSubview(imageView)
    }
}

这段代码重写了Apple的扩展，增加了对有和没有alpha通道的图像的支持。

作为进一步的阅读，我建议查看这篇文章，了解不同的图像调整技术。目前的方法提供了不错的性能，它操作高级api，易于理解。我建议坚持使用它，除非您发现图像大小调整是性能的瓶颈。