An alternative example of a simple user name storage using UserDefaults that includes basic tests. This is in reference to https://www.youtube.com/watch?v=8IiRB28N5OI

AlternativeExampleUserNameStorage.swift

import Foundation

protocol CredentialKeeper {
    func store(userName: String)
    func getUserName() -> String?
}

class UserCredentialStorage: CredentialKeeper {

    var userDefaults: UserDefaults = .standard
    var storageKey: String = "foo.user-name-store"

    func store(userName: String) {
        userDefaults.set(userName, forKey: storageKey)
    }

    func getUserName() -> String? {
        userDefaults.string(forKey: storageKey)
    }
}

class ViewModel {

    private let userCredentialStorage: UserCredentialStorage

    init(userCredentialStorage: UserCredentialStorage) {
        self.userCredentialStorage = userCredentialStorage
    }

    var userName: String {
        userCredentialStorage.getUserName() ?? "Unknown"
    }

    func save(userName: String) {
        userCredentialStorage.store(userName: userName)
    }
}

// MARK: Tests

import XCTest

class UserCredentialStorageTests: XCTestCase {

    func testSample() {
        let sut = UserCredentialStorage()
        sut.userDefaults = UserDefaults()
        sut.storageKey = "mock.user-cred-store-test"

        let expectedUserName = "Luke Skywalker"
        sut.store(userName: expectedUserName)

        let storedUserName = sut.getUserName()
        XCTAssertEqual(storedUserName, expectedUserName)
    }
}

class ViewModelTests: XCTestCase {

    func testSample() {
        let storage = UserCredentialStorage()
        storage.userDefaults = UserDefaults()
        storage.storageKey = "mock.view-model-test"

        let sut = ViewModel(userCredentialStorage: storage)

        let expectedUserName = "Han Solo"
        sut.save(userName: expectedUserName)

        XCTAssertEqual(sut.userName, expectedUserName)
    }
}