How to set Dark Mode in XCUIApplication in swift UITests?

I was wondering if there is a way to set dark mode in-code for the XCUIApplication within a swift UITests project.

I have a need to launch the app in light mode and dark mode in the same test. Setting this in the scheme as a hard coded value will not work, or hacking the simulator from the outside will not work either (for performance & maintainability reasons among others).

Currently I set launch arguments like so:

    let app = XCUIApplication()
    var launchArguments: [AnyHashable] = []
    launchArguments.append("-AppleLanguages")
    launchArguments.append(langCode)
    launchArguments.append("-AppleLocale")
    launchArguments.append(localeCode)
    app.launchArguments = launchArguments
    app.launch()

And it works great.

How do I set Dark Mode for a XCUIApplication instance?

What I've done:

Extensive search on Apple Development Docs.
StackOverflow only shows how to hard-code this in the scheme within Xcode, or how to hack the simulator from the outside by killing the simulator, erasing it, and hacking a plist value.

Thanks for any help!

Solution

In macOS, from the Terminal.app, you can issue this command

defaults read NSGlobalDomain AppleInterfaceStyle

Which responds with

Dark

In your XCTestCase, this should work

    func testAppleInterfaceStyleDark() {
        let app = XCUIApplication()
        var launchArguments: [AnyHashable] = []

        launchArguments.append("-AppleInterfaceStyle")
        launchArguments.append("Dark")
        app.launchArguments = launchArguments as! [String]
        app.launch()
    }

Update as of Xcode 11.4 Beta

You can now toggle the appearance in the Simulator. This is a great way to test maps and other Dark mode features.

From the Simulator Menu Item > Features > Toggle Appearance, or shift⌘A

Simulator supports toggling appearance for iOS simulators (13.0 and later). From within the app select Debug > Toggle Appearance. From the command line use the simctl ui subcommand, e.g. to set dark appearance

xcrun simctl ui <device> appearance dark