Refactoring Massive App Delegate using Mediator pattern. See blog post for more details: https://www.vadimbulavin.com/refactoring-massive-app-delegate

RefactoringAppDelegate-Mediator.swift

// MARK: - AppLifecycleListener

protocol AppLifecycleListener {
    func onAppWillEnterForeground()
    func onAppDidEnterBackground()
    func onAppDidFinishLaunching()
}

// MARK: - Mediator

class AppLifecycleMediator: NSObject {
    private let listeners: [AppLifecycleListener]

    init(listeners: [AppLifecycleListener]) {
        self.listeners = listeners
        super.init()
        subscribe()
    }

    deinit {
        NotificationCenter.default.removeObserver(self)
    }

    private func subscribe() {
        NotificationCenter.default.addObserver(self, selector: #selector(onAppWillEnterForeground), name: .UIApplicationWillEnterForeground, object: nil)
        NotificationCenter.default.addObserver(self, selector: #selector(onAppDidEnterBackground), name: .UIApplicationDidEnterBackground, object: nil)
        NotificationCenter.default.addObserver(self, selector: #selector(onAppDidFinishLaunching), name: .UIApplicationDidFinishLaunching, object: nil)
    }

    @objc private func onAppWillEnterForeground() {
        listeners.forEach { $0.onAppWillEnterForeground() }
    }

    @objc private func onAppDidEnterBackground() {
        listeners.forEach { $0.onAppDidEnterBackground() }
    }

    @objc private func onAppDidFinishLaunching() {
        listeners.forEach { $0.onAppDidFinishLaunching() }
    }
}

extension AppLifecycleMediator {
    static func makeDefaultMediator() -> AppLifecycleMediator {
        let listener1 = ...
        let listener2 = ...
        return AppLifecycleMediator(listeners: [listener1, listener2])
    }
}