swift 图片的缩放功能

//缩放功能的方法
    func scaleImage(image:UIImage, newSize:CGSize) -> UIImage{
        
        //获得原图像的 大小 宽  高
        let imageSize = image.size
        let width = imageSize.width
        let height = imageSize.height
        
        //计算图像新尺寸与旧尺寸的宽高比例
        let widthFactor = newSize.width/width
        let heightFactor = newSize.height/height
        //获取最小的比例
        let scalerFactor = (widthFactor < heightFactor) ? widthFactor : heightFactor
        
        //计算图像新的高度和宽度，并构成标准的CGSize对象
        let scaledWidth = width * scalerFactor
        let scaledHeight = height * scalerFactor
        let targetSize = CGSize(width: scaledWidth, height: scaledHeight)
        
        //创建绘图上下文环境
        UIGraphicsBeginImageContext(targetSize)
        image.draw(in: CGRect(x: 0, y: 0, width: scaledWidth, height: scaledHeight))
        
        // 获取上下文里的内容，将视图写入到新的图像对象
        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        return newImage!
        
    }

let img = UIImageView()
img.image = scaleImage(image:UIImage(named: "message.jpg")!,newSize:CGSize(width: 40, height: 40))