KA Engineering

KA Engineering

We're the engineers behind Khan Academy. We're building a free, world-class education for anyone, anywhere.


Upcoming fortnightly post

Migrating to a Mobile Monorepo for React Native

by Jared Forsyth on May 29

Latest posts

Memcached-Backed Content Infrastructure

Ben Kraft on May 15

Profiling App Engine Memcached

Ben Kraft on May 1

App Engine Flex Language Shootout

Amos Latteier on April 17

What's New in OSS at Khan Academy

Brian Genisio on April 3

Automating App Store Screenshots

Bryan Clark on March 27

It's Okay to Break Things: Reflections on Khan Academy's Healthy Hackathon

Kimerie Green on March 6

Interning at Khan Academy: from student to intern

Shadaj Laddad on December 12

Prototyping with Framer

Nick Breen on October 3

Evolving our content infrastructure

William Chargin on September 19

Building a Really, Really Small Android App

Charlie Marsh on August 22

A Case for Time Tracking: Data Driven Time-Management

Oliver Northwood on August 8

Time Management at Khan Academy

Several Authors on July 25

Hackathons Can Be Healthy

Tom Yedwab on July 11

Ensuring transaction-safety in Google App Engine

Craig Silverstein on June 27

The User Write Lock: an Alternative to Transactions for Google App Engine

Craig Silverstein on June 20

Khan Academy's Engineering Principles

Ben Kamens on June 6

Minimizing the length of regular expressions, in practice

Craig Silverstein on May 23

Introducing SwiftTweaks

Bryan Clark on May 9

The Autonomous Dumbledore

Evy Kassirer on April 25

Engineering career development at Khan Academy

Ben Eater on April 11

Inline CSS at Khan Academy: Aphrodite

Jamie Wong on March 29

Starting Android at Khan Academy

Ben Komalo on February 29

Automating Highly Similar Translations

Kevin Barabash on February 15

The weekly snippet-server: open-sourced

Craig Silverstein on February 1

Stories from our latest intern class

2015 Interns on December 21

Kanbanning the LearnStorm Dev Process

Kevin Dangoor on December 7

Forgo JS packaging? Not so fast

Craig Silverstein on November 23

Switching to Slack

Benjamin Pollack on November 9

Receiving feedback as an intern at Khan Academy

David Wang on October 26

Schrödinger's deploys no more: how we update translations

Chelsea Voss on October 12

i18nize-templates: Internationalization After the Fact

Craig Silverstein on September 28

Making thumbnails fast

William Chargin on September 14

Copy-pasting more than just text

Sam Lau on August 31

No cheating allowed!!

Phillip Lemons on August 17

Fun with slope fields, css and react

Marcos Ojeda on August 5

Khan Academy: a new employee's primer

Riley Shaw on July 20

How wooden puzzles can destroy dev teams

John Sullivan on July 6

Babel in Khan Academy's i18n Toolchain

Kevin Barabash on June 22

tota11y - an accessibility visualization toolkit

Jordan Scales on June 8


Automating App Store Screenshots

by Bryan Clark on March 27

Automating App Store Screenshots

Khan Academy in the App Store

One of the more tedious design tasks is keeping your App Store promo images up-to-date. As you add localization support and new features, it can mean taking a few days of a designer's time to update these design assets.

Our iOS app is currently available in 6 languages. Recently, we improved our iPhone app’s downloading feature, and changed its name from “Your List” to “Bookmarks”. A minor change - a few strings and a tab bar icon - but it meant we needed to update lots of screenshots in the App Store!

5 screenshots * 4 phone sizes * 6 locales = 180 images

If you’re an iOS designer, this is a major headache:

  • If your App Store screenshots are really handmade Sketch artboards, then you need to manually update 180 artboards - and make sure that the other features in the app’s design are kept up to date! Localization is a big pain here.
  • If your App Store screenshots are real screenshots, then you’re looking at a long week of loading up different user profiles in different languages to take screenshots.

Either way, that’s probably a tedious few days' worth of somebody’s time - which is a hard expense to justify if you’re shipping every few weeks! The result: your App Store images are either out of date, or your team is spending too much effort on this task.

A couple of months ago, our mobile team spent time automating our screenshots for the App Store and the Play Store. Now, when we want to update our App Store screenshots, this task now takes about an hour instead of a week - and almost all of that time is waiting for automated tests to compile and run.

To pull this off, we used Fastlane’s snapshot tool, and a homegrown Sketch file. Let’s dig in!

Step One: Write UI Tests

UI testing in Xcode is awesome - the tests are quite easy to write (they can literally write themselves with a "record me" feature) When you run ‘em, your app flies through its paces in the simulator. Then, at the right moment, snapshot records a screenshot and files it away.

Let’s take a look at the UI test that takes a screenshot of our Bookmarks tab:

    func testPhoneBookmarks() {
        // Skip the test if snapshot's locale is empty
        guard !locale.isEmpty else { return }

        // Tell the iOS app we'd like to populate the Bookmarks tab 
        // with test content
        let environment = AppStoreScreenshotUITests.screenshotEnvironment(plus: [
            KHAUITestingConstants.showAppStoreBookmarksEnvironmentVariable: KHAUITestingConstants.enableKey,
            KHAUITestingConstants.localeCodeEnvironmentVariable: locale 

        // Launch the app in the simulator
        launch(with: environment)

        // Tap on the Bookmarks tab (tab #3)
        XCUIApplication().tabBars.buttons.element(boundBy: 2).tap()

        // Take a screenshot!

What's up with that last line, though?

Screenshot is a little enum that handles telling snapshot when to take a picture, and what the image should be called. It looks like this:

/// Identifiers for screenshots that we want to take.
/// The App Store cases are uniquely identified, and numbered by their display order in the App Store.
/// Use the `.test(string)` case for one-off test shots, e.g. `.test("ProfileUnauthenticated")`.
internal enum Screenshot {
    case appStoreExplore, appStoreVideo, appStoreBookmarks, appStoreExercise, appStoreUserProfile
    case test(String)

    var imageName: String {
        switch self {
        case .appStoreExplore:
            return "AppStore_01_Explore"
        case .appStoreExercise:
            return "AppStore_02_Exercise"
        case .appStoreBookmarks:
            return "AppStore_03_Bookmarks"
        case .appStoreVideo:
            return "AppStore_04_Video"
        case .appStoreUserProfile:
            return "AppStore_05_UserProfile"
        case .test(let screenshotName):
            return "Test_" + screenshotName

    func take() {
        // this is a global function from `snapshot`

Step Two: Create your Snapfile

The Snapfile is where you configure snapshot’s run. Ours looks like this:

# Check out this link for what to put in this file:
# https://github.com/fastlane/fastlane/blob/master/snapshot/README.md#snapfile

scheme "Khan Academy.UITests"
output_directory "./app-store-screenshots/snapshot-output"
clear_previous_screenshots true
reinstall_app true
localize_simulator true
workspace "./Khan Academy.xcworkspace"
app_identifier "org.khanacademy.Khan-Academy"

  "iPhone 7",
  "iPhone 7 Plus",
  "iPhone SE"


Step Three: Render mock data in your app

So, we’ve got UI tests written, and snapshot is able to grab screenshots at the right moment. The problem is - how do you get beautiful content in there?

One tricky thing about UI tests: you have a very limited ability to pass information into your iOS app. This is intentional - UI tests would be pretty weird if you could write code that directly manipulated views in your app! To send information into the running app, we use environment variables - which are basically just strings that the app can then read when it launches.

Remember the bit above with KHAUITestingConstants? When the app is running, we check to see if the environment contains a given variable, like so:

/// Checks NSProcessInfo for environment variables used by UI Testing.
/// If there are environment variables for a content item, the app will navigate to that location.
@objc func openUITestingContentIfApplicable() {
    let environment = NSProcessInfo.processInfo().environment

    let shouldOpenContent = environment[KHAUITestingConstants.shouldOpenContentEnvironmentVariable] == KHAUITestingConstants.enableKey

    if shouldOpenContent {
            let contentSlug = environment[KHAUITestingConstants.contentItemSlugVariable],
            let contentTypeString = environment[KHAUITestingConstants.contentItemTypeVariable],
            let contentType = URLTarget.contentTypeFromURLComponent(contentTypeString)
            // Here's where our app navigates to the desired content.
            let urlTarget = URLTarget.ContentItem(contentType: contentType, slug: contentSlug)

We’ve also got structs that contain good-looking content for each locale. For example, this struct populates our app’s bookmark tab with the right content for each locale:

/// Contains good-looking topics and videos for the Bookmarks tab in our automated App Store screenshots.
internal struct AppStoreScreenshotBookmarks {

    /// Returns a list of video slugs for App Store screenshots.
    internal static func contentSlugsForLocale(locale: ScreenshotLocale) -> [String] {
        switch locale {
        case .en_US, .tr, .es_ES:
            return [
        // etc.

    /// Returns a list of topic slugs for App Store screenshots.
    internal static func topicSlugsForLocale(locale: ScreenshotLocale) -> [String] {
        switch locale {
        case .en_US, .tr, .fr, .pt_BR:
            return [
        // etc.

Step Four: Custom Layouts

Now you’ve got great content in your screenshots for each locale - what do you want to do with them?

For Khan Academy’s app, we want to inset the phone in a device, and have some text above it, like so:

Screenshot of our app in French

Fastlane offers a tool called frameit that gets you pretty close - but we wanted more control over the layout and background color, so I created a Sketch File that nearly-automates this step.

I won’t go too in-depth into breaking down the Sketch file, but here’s a quick overview of how it’s built: - It uses a Sketch plugin called Sketch Replace Images, which updates image layers in the Sketch file to match similarly-named images in our snapshot output. - It uses Symbols and Shared Styles to keep the design consistent across all 180 screenshots. - We used the beautiful Devices sketch files from Facebook Design, and stripped out the shadows, textures, and colors to create our stylized white devices - then scaled them down and pixel-snapped the edges.

Our Sketch file has one-page-per-device, and looks like this: Our Sketch File

Putting it all together

Now, when we want to update our screenshots, we can type in a Terminal command, and wait about 30 minutes (Swift compilation is responsible for the vast majority of that time; our Android app only takes a few minutes to do this part). Then, we open our Sketch file, run the Sketch Replace Images plugin, and export our screens - that's it!

Want to learn more?