更快嘅測試，你可以 真正值得信賴。

final class AuthSessionStore {

    private let keychain: KeychainClient

    func restore() throws -> AuthSession? {

        guard let data = try keychain.read(.session) else {

            return nil

        }

        return try decoder.decode(AuthSession.self, from: data)

    }

        let session = try decoder.decode(AuthSession.self, from: data)

        guard session.expiresAt > Date.now else {

            try keychain.delete(.session)

            return nil

        }

        return session

    }

    func persist(_ session: AuthSession) throws {

        let data = try encoder.encode(session)

        try keychain.write(data, for: .session)

    }

}