Launch GraphOS Studio

13. Write your first subscription

In this section, you will use subscriptions to get notified whenever someone books a flight 🚀! Subscriptions allow you to be notified in real time whenever an event happens on your server. The fullstack backend supports based on WebSockets.

Write your subscription

Open your Sandbox back up, click on the Schema tab at the far left. In addition to queries and mutations, you will see a third type of , subscriptions. Click on to see the tripsBooked :

The definition of tripsBooked in the schema

This doesn't take any and returns a single named tripsBooked. Since you can book multiple trips at once, tripsBooked is an Int. It will contain the number of trips booked at once or -1 if a trip has been cancelled.

Click the play button to the far right of tripsBooked to open the in Explorer. Open a new tab, then check the tripsBooked button to have the added:

The initial definition of the TripsBooked subscription

Again, rename your so it's easier to find:

The subscription after rename

Click the Submit button, and your will start listening to events. You can tell it's up and running because a panel will pop up at the lower right where subscription data will come in:

The UI showing that it's listening for subscription updates

Test your subscription

Open a new tab in Explorer. In this new tab, add code to book a trip like on step 8, but with a hard-coded ID:

mutation BookTrip {
bookTrips(launchIds: ["93"]){

Do not forget to include the authentication header. At the bottom of the Sandbox Explorer pane where you add , there's a Headers section:

Adding a login token to explorer

Click the Submit button. If everything went well, you just booked a trip! At the top of the right panel, you'll see the success JSON for your your BookTrip , and below it, updated JSON for the TripsBooked :

Subscription success in Explorer

Continue booking and/or canceling trips, you will see events coming in the panel in real time. After some time, the server might close the connection and you'll have to restart your subscription to keep receiving events.

Add the subscription to your project

Now that your is working, add it to your project. Create an empty file named TripsBooked.graphql next to your other files and paste the contents of the . The process is similar to what you've already done for queries and :

subscription TripsBooked {

Now run code generation in Terminal to generate the code for your .

Configure your ApolloClient to use subscription

In Network.swift, you'll need to set up a transport which supports in addition to general network usage. In practice, this means adding a WebSocketTransport which will allow real-time communication with your server.

First, at the top of the file, add an import for the ApolloWebSocket framework to get access to the classes you'll need:

import ApolloWebSocket

Next, in the lazy declaration of the apollo , immediately after transport is declared, set up what you need to add support to your client:

private(set) lazy var apollo: ApolloClient = {
let client = URLSessionClient()
let cache = InMemoryNormalizedCache()
let store = ApolloStore(cache: cache)
let provider = NetworkInterceptorProvider(client: client, store: store)
let url = URL(string: "")!
let transport = RequestChainNetworkTransport(interceptorProvider: provider, endpointURL: url)
let webSocket = WebSocket(
url: URL(string: "wss://")!,
protocol: .graphql_ws
let webSocketTransport = WebSocketTransport(websocket: webSocket)
let splitTransport = SplitNetworkTransport(
uploadingNetworkTransport: transport,
webSocketNetworkTransport: webSocketTransport
return ApolloClient(networkTransport: splitTransport, store: store)

What's happening here?

  1. You've created a web socket with the server's web socket URL - wss:// is the protocol for a secure web socket.
  2. You've created a WebSocketTransport, which allows the Apollo SDK to communicate with the web socket.
  3. You've created a SplitNetworkTransport, which can decide whether to use a web socket or not automatically, with both the RequestChainNetworkTransport you had previously set up, and the WebSocketTransport you just set up.
  4. You're now passing the splitTransport into the ApolloClient, so that it's the main transport being used in your ApolloClient.

Now, you're ready to actually use your !

Use your subscription

To use the you created, go to LaunchListViewModel.swift and start by adding a new to hold the :

@Published var launches = [LaunchListQuery.Data.Launches.Launch]()
@Published var lastConnection: LaunchListQuery.Data.Launches?
@Published var activeRequest: Cancellable?
var activeSubscription: Cancellable?
@Published var appAlert: AppAlert?
@Published var notificationMessage: String?

Now, replace the TODO in the startSubscription() method with the following code:

func startSubscription() {
activeSubscription = Network.shared.apollo.subscribe(subscription: TripsBookedSubscription()) { [weak self] result in
guard let self = self else {
switch result {
case .success(let graphQLResult):
if let tripsBooked = {
self.handleTripsBooked(value: tripsBooked)
if let errors = graphQLResult.errors {
self.appAlert = .errors(errors: errors)
case .failure(let error):
self.appAlert = .errors(errors: [error])

Next, update the TODO in the init() method to the following:

init() {

Notice in LaunchListView.swift there is already a line to handle displaying a view from the :

.notificationView(message: $viewModel.notificationMessage)

Test your subscription

Build and run the application, now whenever a trip is booked or cancelled (from either in the app detail view or from the Sandbox Explorer) you should see a small notification pop up at the bottom of the screen:

Subscription notification view

Congratulations, you've completed the tutorial!

More resources

There are way more things you can do with the SDK, and the rest of this documentation includes info on more advanced topics like:

Feel free to ask questions by either joining our Discord server or joining the Apollo GraphQL Forum.

12. Define additional mutations
Client Directives
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