An example of using #function for user defaults properties, and a test that guards against property name changes

OnboardingManager.swift

import UIKit

class OnboardingManager {
    private let userDefaults: UserDefaults

    init(userDefaults: UserDefaults = .standard) {
        self.userDefaults = userDefaults
    }

    func presentOnboardingControllerIfNeeded(in viewController: UIViewController) {
        guard !userDefaults.onboardingCompleted else {
            return
        }

        // Present view controller
        
        userDefaults.onboardingCompleted = true
    }
}

private extension UserDefaults {
    var onboardingCompleted: Bool {
        get { return bool(forKey: #function) }
        set { set(newValue, forKey: #function) }
    }
}

OnboardingManagerTests.swift

import XCTest

class OnboardingManagerTests: XCTestCase {
    private var manager: OnboardingManager!
    private var userDefaults: UserDefaults!

    override func setUp() {
        super.setUp()

        userDefaults = UserDefaults(suiteName: "test")
        userDefaults.removePersistentDomain(forName: "test")
        manager = OnboardingManager(userDefaults: userDefaults)
    }

    func testPresentingOnboardingViewController() {
        let viewController = UIViewController()
        manager.presentOnboardingControllerIfNeeded(in: viewController)
        XCTAssertNotNil(viewController.presentedViewController)
    }

    func testNoOnboardingViewControllerPresentedIfOnboardingCompleted() {
        // Here we use a string literal (the property isn't even accessible), which guards against name changes
        userDefaults.set(true, forKey: "onboardingCompleted")

        let viewController = UIViewController()
        manager.presentOnboardingControllerIfNeeded(in: viewController)
        XCTAssertNil(viewController.presentedViewController)
    }
}

Thanks for the example, John!

This is a neat way to use #function for string literals.