// these need to be strong references.
self.credentials = Credentials(username: /* username */, password: /* password */, verifyBy: .text)
self.handler = APIHandler()
handler.authenticate(with: .user(credentials)) {
    switch $0 {
    case .success(let response, _):
        print("Login successful.")
        // persist cache safely in the keychain for logging in again in the future.
        guard let key = response.persist() else { return print("`Authentication.Response` could not be persisted.") }
        // store the `key` wherever you want, so you can access the `Authentication.Response` later.
        // `UserDefaults` is just an example.
        UserDefaults.standard.set(key, forKey: "current.account")
        UserDefaults.standard.synchronize()
    case .failure(let error):
        if error.requiresInstagramCode {
            /* update interface to ask for code */
        } else {
            /* notify the user */
        }
    }
}

self.credentials.code = /* the code */

let login = LoginWebViewController { controller, result in
    controller.dismiss(animated: true, completion: nil)
    // deal with authentication response.
    guard let (response, _) = try? result.get() else { return print("Login failed.") }
    print("Login successful.")
    // persist cache safely in the keychain for logging in again in the future.
    guard let key = response.persist() else { return print("`Authentication.Response` could not be persisted.") }
    // store the `key` wherever you want, so you can access the `Authentication.Response` later.
    // `UserDefaults` is just an example.
    UserDefaults.standard.set(key, forKey: "current.account")
    UserDefaults.standard.synchronize()
}
if #available(iOS 13, *) {
    present(login, animated: true, completion: nil) // just swipe down to dismiss.
} else {
    present(UINavigationController(rootViewController: login),  // already adds a `Cancel` button to dismiss it.
            animated: true,
            completion: nil)
}

// recover the `key` returned by `Authentication.Response.persist()`.
// in our example, we stored it in `UserDefaults`.
guard let key = UserDefaults.standard.string(forKey: "current.account") else { return print("`key` not found.") }
// recover the safely persisted `Authentication.Response`.
guard let cache = Authentication.Response.persisted(with: key) else { return print("`Authentication.Response` not found.") }
// log in.
let handler = APIHandler()
handler.authenticate(with: .cache(cache)) { _ in
    /* do something here */
}

let handler: APIHandler = /* a valid, authenticated handler */
// for instance you can…
// …fetch your inbox.
handler.messages.inbox(with: .init(maxPagesToLoad: .max),
                       updateHandler: nil,
                       completionHandler: { _, _ in /* do something */ })
// …fetch all your followers.
handler.users.following(user: .me,
                        with: .init(maxPagesToLoad: .max),
                        updateHandler: nil,
                        completionHandler: { _, _ in /* do something */ })