Hub

Amplify has a local eventing system called Hub. It is a lightweight implementation of Publisher-Subscriber pattern, and is used to share data between modules and components in your app. Amplify uses Hub for different categories to communicate with one another when specific events occur, such as authentication events like a user sign-in or notification of a file download.

Working with the API

Listening for messages

You can use Amplify.Hub.listen(to: HubChannel) {...} (listener) or Amplify.Hub.publisher(for: HubChannel).sink {...} (Combine) to listen for messages that have been dispatched. Additionally, you can pass isIncluded: HubFilter or eventName: HubPayloadEventName to the listen method to further filter Hub events. You can add multiple listeners to your application for different combinations of channels, filters, and events.

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view.
    // Assumes `unsubscribeToken` is declared as an instance variable in your view
    unsubscribeToken = Amplify.Hub.listen(to: .auth) { payload in
        switch payload.eventName {
        case HubPayload.EventName.Auth.signedIn:
            print("User signed in")
            // Update UI
        case HubPayload.EventName.Auth.sessionExpired:
            print("Session expired")
            // Re-authenticate the user
        case HubPayload.EventName.Auth.signedOut:
            print("User signed out")
            // Update UI
        case HubPayload.EventName.Auth.userDeleted:
            print("User deleted")
            // Update UI
        default:
            break

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view.
    // Assumes `sink` is declared as an instance variable in your view controller
    sink = Amplify.Hub
        .publisher(for: .auth)
        .sink { payload in
            switch payload.eventName {
            case HubPayload.EventName.Auth.signedIn:
                print("User signed in")
                // Update UI
            case HubPayload.EventName.Auth.sessionExpired:
                print("Session expired")
                // Re-authenticate the user
            case HubPayload.EventName.Auth.signedOut:
                print("User signed out")
                // Update UI
            case HubPayload.EventName.Auth.userDeleted:
                print("User deleted")
                // Update UI
            default:
                break

Stop Listening

Hub provides a way to stop listening for messages. This is useful if you no longer need to receive messages in your application flow.

// Assumes `unsubscribeToken` references the token
// returned by Amplify.Hub.listen(...)
Amplify.Hub.removeListener(unsubscribeToken)

// Assumes `sink` references the sink
// returned by Amplify.Hub.publisher(...).sink
sink.cancel()