dependency injection with closures

import UIKit
import XCTest

struct LoggedInUser {
    let name: String
}

class LoginViewController: UIViewController {
    var login: (((LoggedInUser) -> Void) -> Void)?
    var user: String?
    func didTapLoginButton() {
        guard let login = self.login else {
            user = nil
            return
        }
        login { [weak self] user in
            self?.user = user.name
        }
    }
    
    func showUser() {
        print(user ?? "unknown")
    }
}

let loginVC = LoginViewController()
let user = LoggedInUser(name: "randy")

loginVC.login = { completion in
    completion(user)
}
loginVC.didTapLoginButton()
loginVC.showUser()
XCTAssertTrue(loginVC.user == "randy", "user value has wrong setting")

loginVC.login = { completion in
    completion(LoggedInUser(name:"second user"))
}
loginVC.didTapLoginButton()
loginVC.showUser()
XCTAssertTrue(loginVC.user == "second user", "user value has wrong setting")

loginVC.login = nil
loginVC.didTapLoginButton()
loginVC.showUser()
XCTAssertTrue(loginVC.user == nil, "user value still set")