[TDD] SessionManager Without Knowing the Storage Type

Instead of hardcoding UserDefaults or Keychain, we create a protocol that defines session storage behavior.

struct User: Codable {
    let id: String
    let name: String
    let email: String
}

protocol SessionStorage {
    func saveUser(_ user: User)
    func loadUser() -> User?
    func clearUser()
}

• SessionManager doesn’t depend on UserDefaults/Keychain directly.
• We can swap storage solutions easily later.
• We can mock the storage for unit testing.

Now, SessionManager doesn’t depend on UserDefaults directly. Instead, it uses any session storage that conforms to SessionStorage.

class SessionManager {
    private let storage: SessionStorage

    init(storage: SessionStorage) {
        self.storage = storage
    }

    func saveUser(_ user: User) {
        storage.saveUser(user)
    }

    func loadUser() -> User? {
        return storage.loadUser()
    }

    func clearUser() {
        storage.clearUser()
    }
}

• SessionManager doesn’t care whether storage is UserDefaults, Keychain, or something else.
• It’s testable because we can inject a mock storage for testing.

To test SessionManager, we mock storage instead of using real UserDefaults.

class MockSessionStorage: SessionStorage {
    var storedUser: User?

    func saveUser(_ user: User) {
        storedUser = user
    }

    func loadUser() -> User? {
        return storedUser
    }

    func clearUser() {
        storedUser = nil
    }
}

• We avoid real data storage during testing.
• We can manipulate test data easily.

Now, we test SessionManager with MockSessionStorage instead of real storage.

import XCTest

class SessionManagerTests: XCTestCase {
    var sessionManager: SessionManager!
    var mockStorage: MockSessionStorage!

    override func setUp() {
        super.setUp()
        mockStorage = MockSessionStorage()
        sessionManager = SessionManager(storage: mockStorage)
    }

    func testSaveAndLoadUser() {
        let testUser = User(id: "123", name: "John Doe", email: "john@example.com")
        sessionManager.saveUser(testUser)

        let loadedUser = sessionManager.loadUser()
        XCTAssertNotNil(loadedUser, "User should be saved and loaded")
        XCTAssertEqual(loadedUser?.id, testUser.id, "User ID should match")
        XCTAssertEqual(loadedUser?.name, testUser.name, "User name should match")
        XCTAssertEqual(loadedUser?.email, testUser.email, "User email should match")
    }

    func testClearUser() {
        sessionManager.saveUser(User(id: "123", name: "John Doe", email: "john@example.com"))
        sessionManager.clearUser()

        let loadedUser = sessionManager.loadUser()
        XCTAssertNil(loadedUser, "User should be cleared")
    }
}

• MockSessionStorage mimics real storage but doesn’t use UserDefaults.
• We test saving, loading, and clearing a user.
• Tests run fast and reliably without modifying actual user data.