Test different behaviours in AppDelegate for each or separated unit and integration tests

Question

I want to test my application's behavior that decided on app launch. For example: In a tab bar controller, how many and which tabs will be created is been decided on app launch where the root window has been created so I want to test these behaviors for each test case.

This new feature is set via A/B service and the value retrieved only during app launching. Based on that value, the tab bar's view controllers are set.

For example:

var viewControllers: [UIViewController] = [ tabOne, tabTwo]
if Config.isNewFeatureEnabled {
    viewControllers.append(self._menuCoordinator.rootViewController)
} else {
    viewControllers.append(self._anotherTabBarController)
    viewControllers.append(self._anotherCoordinator.rootViewController)
    viewControllers.append(self._someOtherCoordinator.rootViewController)
}
_tabBarController.viewControllers = viewControllers

Let me put in code, in order to make tests easy I created a protocol (not necessarily but better approach for injection)

protocol FeatureFlag {
    var isNewFeatureEnabled: Bool { get set }
}

// Implementation
class FeatureFlagService: FeatureFlag {
   var isNewFeatureEnabled = false
   // Bunch of other feature flags
}

In my test cases I want to switch the config with out effecting other side of the app. Something like this:

class NewFeatureVisibilityTests: XCTestCase {
    func test_TabBar_has_threeTabs_when_NewFeature_isEnabled() {
        // Looking for a way to inject the config

        let tabBar = getKeyWindow()?.rootViewController as? UITabBarController

        guard let tabBar = appDel.currentWindow?.rootViewController as? UITabBarController else {
            return XCTFail("Expected root view controller to be a tab bar controller")
        }

        XCTAssertEqual(tabBar.viewControllers?.count, 3)
    }

    func test_TabBar_has_fiveTabs_when_NewFeature_isDisabled() {
        // Looking for a way to inject the config

        let tabBar = getKeyWindow()?.rootViewController as? UITabBarController

        guard let tabBar = appDel.currentWindow?.rootViewController as? UITabBarController else {
            return XCTFail("Expected root view controller to be a tab bar controller")
        }

        XCTAssertEqual(tabBar.viewControllers?.count, 5)
    }
}

What I want is set application's behaviour through injection (a config etc) for each test case.

One test the feature will be enabled, other test will assert the feature disabled state.

Answer 1

Create a config property in AppDelegate using existing type of FeatureFlag along with a default value on override init.

extension UIApplication {
    var currentWindow: UIWindow {
        return (connectedScenes
            .filter({$0.activationState == .foregroundActive})
            .compactMap({$0 as? UIWindowScene})
            .first?.windows
            .filter({$0.isKeyWindow}).first!)!
    }
}

@main
class AppDelegate: UIResponder, UIApplicationDelegate {
    
    var window: UIWindow?
    let config: FeatureFlag!
    
    override init() {
        config = FeatureFlagService()
    }
    
    init(config: FeatureFlag!) {
        self.config = config
    }
    
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        // Override point for customization after application launch.
        
        // Create a tabBar with 3 tabs
        let tabBarController = UITabBarController()
        let firstViewController = UIViewController()
        let secondViewController = UIViewController()
        let thirdViewController = UIViewController()
        let fourthViewController = UIViewController()
        let fifthViewController = UIViewController()
        
        firstViewController.tabBarItem = UITabBarItem(tabBarSystemItem: .favorites, tag: 0)
        secondViewController.tabBarItem = UITabBarItem(tabBarSystemItem: .downloads, tag: 1)
        thirdViewController.tabBarItem = UITabBarItem(tabBarSystemItem: .more, tag: 2)
        fourthViewController.tabBarItem = UITabBarItem(tabBarSystemItem: .bookmarks, tag: 3)
        fifthViewController.tabBarItem = UITabBarItem(tabBarSystemItem: .contacts, tag: 4)
        
        var viewControllers = [firstViewController, secondViewController]
        if config.isNewFeatureEnabled {
            viewControllers.append(thirdViewController)
        } else {
            viewControllers.append(fourthViewController)
            viewControllers.append(fifthViewController)
        }
        
        tabBarController.viewControllers = viewControllers
        
        // Create a window and set the root view controller
        let window = UIWindow(frame: UIScreen.main.bounds)
        window.rootViewController = tabBarController
        window.makeKeyAndVisible()
        self.window = window
        
        return true
    }
}

And in tests, I set my config, create an instance of AppDelegate, inject the config, and launching the application through appDelegate.application(UIApplication.shared, didFinishLaunchingWithOptions: nil) function of AppDelegate.

let appDelegate = AppDelegate(config: config)

        // This is the key function
        _ = appDelegate.application(UIApplication.shared, didFinishLaunchingWithOptions: nil)

Tests:

import XCTest
@testable import ExampleApp


final class NewFeatureVisibilityTests: XCTestCase {
   
    func test_app_can_start_with_isNewFeatureEnabled(){
        let config = FeatureFlagService()
        config.isNewFeatureEnabled = true
        let appDelegate = AppDelegate(config: config)

        // This is the key function
        _ = appDelegate.application(UIApplication.shared, didFinishLaunchingWithOptions: nil)
        
        guard let rootVC = UIApplication.shared.currentWindow.rootViewController as? UITabBarController else {
            return XCTFail("RootViewController is nil")
        }
        
        XCTAssertEqual(rootVC.viewControllers?.count, 3)
    }
    
    func test_app_can_start_with_isNewFeatureDisabled(){
        let config = FeatureFlagService()
        config.isNewFeatureEnabled = false
        let appDelegate = AppDelegate(config: config)

        // This is the key function
        _ = appDelegate.application(UIApplication.shared, didFinishLaunchingWithOptions: nil)
        
        guard let rootVC = UIApplication.shared.currentWindow.rootViewController as? UITabBarController else {
            return XCTFail("RootViewController is nil")
        }
        
        XCTAssertEqual(rootVC.viewControllers?.count, 4)
    }
}