UIImage+Scale

The final interface is as simple as this:

image.scaled(to: size)
image.scaled(to: size, scalingMode: .aspectFill)
image.scaled(to: size, scalingMode: .aspectFit)
.. and is implemented like this:

// MARK: - Image Scaling.
extension UIImage {
    
    /// Represents a scaling mode
    enum ScalingMode {
        case aspectFill
        case aspectFit

        /// Calculates the aspect ratio between two sizes
        /// 
        /// - parameters:
        ///     - size:      the first size used to calculate the ratio
        ///     - otherSize: the second size used to calculate the ratio
        /// 
        /// - return: the aspect ratio between the two sizes
        func aspectRatio(between size: CGSize, and otherSize: CGSize) -> CGFloat {
            let aspectWidth  = size.width/otherSize.width
            let aspectHeight = size.height/otherSize.height

            switch self {
            case .aspectFill:
               return max(aspectWidth, aspectHeight)
            case .aspectFit:
                return min(aspectWidth, aspectHeight)
            }
        }
    }

    /// Scales an image to fit within a bounds with a size governed by the passed size. Also keeps the aspect ratio.
    ///
    /// - parameter:
    ///     - newSize:     the size of the bounds the image must fit within.
    ///     - scalingMode: the desired scaling mode
    ///
    /// - returns: a new scaled image.
    func scaled(to newSize: CGSize, scalingMode: UIImage.ScalingMode = .aspectFill) -> UIImage {
        
        let aspectRatio = scalingMode.aspectRatio(between: newSize, and: size)
        
        /* Build the rectangle representing the area to be drawn */
        var scaledImageRect = CGRect.zero

        scaledImageRect.size.width  = size.width * aspectRatio
        scaledImageRect.size.height = size.height * aspectRatio
        scaledImageRect.origin.x    = (newSize.width - size.width * aspectRatio) / 2.0
        scaledImageRect.origin.y    = (newSize.height - size.height * aspectRatio) / 2.0

        /* Draw and retrieve the scaled image */
        UIGraphicsBeginImageContext(newSize)

        draw(in: scaledImageRect)
        let scaledImage = UIGraphicsGetImageFromCurrentImageContext()

        UIGraphicsEndImageContext()

        return scaledImage!
    }
}