UIImage Scaling Extension

Posted 03/15/2016 7:36 PM by Corey Klass

The iPhone app I'm working on in my spare time lets the user capture photos as part of its functionality. After I receive the image, I need to resize it so that it's a thumbnail size.

To make this easier, I've written a UIImage Swift extension that provides you two functions: scaleToMaxSize() scales an image to the maximum size specified, scaleToSize() scales the image to exactly the specified size. scaleToMaxSize() works in conjunction with scaleToSize().

    //  UIImage+CKScaling.swift
    //  BlackBook
    //  Created by Corey Klass on 2/28/16.
    //  Copyright © 2016 Corey Klass. All rights reserved.

    import Foundation
    import UIKit

    extension UIImage {

        func scaleToMaxSize(maxSize: CGSize) -> UIImage {
            // get the scale values for the width and height
            let xScale = maxSize.width / self.size.width
            let yScale = maxSize.height / self.size.height

            // we'll need one of these scales, whichever fits
            let xScaleSize = CGSizeMake(self.size.width * xScale, self.size.height * xScale)
            let yScaleSize = CGSizeMake(self.size.width * yScale, self.size.height * yScale)

            var newSize: CGSize

            // if the x scale is too large, use the y scale
            if ((xScaleSize.width > maxSize.width) || (xScaleSize.height > maxSize.height)) {
                newSize = yScaleSize

            // otherwise use the x scale
            else {
                newSize = xScaleSize

            // resize and return the image
            let image = self.scaleToSize(newSize)

            return image

        func scaleToSize(size: CGSize) -> UIImage {
            // Create a bitmap graphics context
            // This will also set it as the current context

            // Draw the scaled image in the current context
            self.drawInRect(CGRectMake(0.0, 0.0, size.width, size.height))

            // Create a new image from current context
            let scaledImage = UIGraphicsGetImageFromCurrentImageContext()

            // Pop the current context from the stack

            // Return our new scaled image
            return scaledImage