This code allows for testing UNNotificationCenter and UNNotificationSettings

CountdownTests.swift

func testNotifications() {
        // map all authorizationStatus with expected Result
        let authorizationStatusMap: [UNAuthorizationStatus: Int] = [.authorized: 1, .denied: 0, .notDetermined: 0, .provisional: 1]
        UNNotificationSettings.swizzleAuthorizationStatus()
        
        authorizationStatusMap.forEach { (key: UNAuthorizationStatus, value: Int) in
            UNNotificationSettings.fakeAuthorizationStatus = key
            
            let mockCenter = UserNotificationCenterMock()
            let mockCoder = MockNSCoder()
            mockCenter.settingsCoder = mockCoder
            let mockDelegate = MockCountdownDelegate()
            let defaults = MockUserDefaults()
            let timer = Countdown(delegate: mockDelegate, defaults: defaults, userNotificationCenter: mockCenter)
            
            XCTAssertEqual(mockCenter.pendingNotifications.count, 0)
            
            let mockContent = UNMutableNotificationContent()
            mockContent.title = "title"
            mockContent.body = "body"
            
            timer.startCountdown(with: Date().addingTimeInterval(1), with: mockContent)
            XCTAssertEqual(mockCenter.pendingNotifications.count, value)
            
            timer.skipRunningCountdown()
            XCTAssertEqual(mockCenter.pendingNotifications.count, 0)
        }
    }

extension UNNotificationSettings {
    static var fakeAuthorizationStatus: UNAuthorizationStatus = .authorized
    
    static func swizzleAuthorizationStatus() {
        let originalMethod = class_getInstanceMethod(self, #selector(getter: authorizationStatus))!
        let swizzledMethod = class_getInstanceMethod(self, #selector(getter: swizzledAuthorizationStatus))!
        
        method_exchangeImplementations(originalMethod, swizzledMethod)
    }
    
    @objc var swizzledAuthorizationStatus: UNAuthorizationStatus {
        return Self.fakeAuthorizationStatus
    }
}

class UserNotificationCenterMock: UserNotificationCenter {
    
    var pendingNotifications = [UNNotificationRequest]()
    var settingsCoder = MockNSCoder()
    
    func getNotificationSettings(completionHandler: @escaping (UNNotificationSettings) -> Void) {
        let settings = UNNotificationSettings(coder: settingsCoder)!
        completionHandler(settings)
    }
    
    func removeAllPendingNotificationRequests() {
        pendingNotifications.removeAll()
    }
    
    func add(_ request: UNNotificationRequest, withCompletionHandler completionHandler: ((Error?) -> Void)?) {
        pendingNotifications.append(request)
        completionHandler?(nil)
    }
}

class MockNSCoder: NSCoder {
    var authorizationStatus = UNAuthorizationStatus.authorized.rawValue
    
    override func decodeInt64(forKey key: String) -> Int64 {
        return Int64(authorizationStatus)
    }
    
    override func decodeBool(forKey key: String) -> Bool {
        return true
    }
}

UserNotificationProtocol.swift

protocol UserNotificationCenter {
    func getNotificationSettings(completionHandler: @escaping (UNNotificationSettings) -> Void)
    func removeAllPendingNotificationRequests()
    func add(_ request: UNNotificationRequest, withCompletionHandler completionHandler: ((Error?) -> Void)?)
}

extension UNUserNotificationCenter: UserNotificationCenter {}

Hi @Pavel-Laukhin, this is an example from production code of a unit test mocking UNNotificationCenter. The Countdown is the class under test but ultimately it doesn't matter what it does.
The interesting parts are how to setup the UserNotificationCenterMock and the UNNotificationSettings extensions where the fake authorization state will be returned using method swizzling.
However as this code is already quite old I am not sure whether this even still works.

Hi @Pavel-Laukhin, this is an example from production code of a unit test mocking UNNotificationCenter. The Countdown is the class under test but ultimately it doesn't matter what it does. The interesting parts are how to setup the UserNotificationCenterMock and the UNNotificationSettings extensions where the fake authorization state will be returned using method swizzling. However as this code is already quite old I am not sure whether this even still works.

Thank you!