Subscriptions

Subscriptions are long-lived GraphQL read operations that can update their response over time, enabling clients to receive new data as it becomes available.

Apollo iOS supports subscriptions over the following protocols:

• WebSocket, including multiple subprotocols
• HTTP, using chunked multipart responses (version 1.1.0 and later)

You must use whichever protocol is supported by your GraphQL endpoint.

Enabling support

To support GraphQL subscriptions, you need to initialize your ApolloClient instance with a NetworkTransport that supports subscriptions. See below for applicable classes for each subscription protocol.

WebSocket

To use Apollo iOS with WebSocket-based subscriptions, you need to install the optional ApolloWebSocket library. This library includes two classes that conform to the NetworkTransport protocol:

• WebSocketTransport sends all operations (including queries and mutations) over WebSocket.
• SplitNetworkTransport maintains a WebSocketTransport instance, along with an UploadingNetworkTransport instance (usually RequestChainNetworkTransport). This provides a single network transport that uses HTTP for queries and mutations and WebSocket for subscriptions.

SplitNetworkTransport is recommended for most use cases, because it enables you to retain a single NetworkTransport setup that avoids issues caused by using multiple client objects.

Here's an example of setting up an ApolloClient that uses a SplitNetworkTransport to support all operation types:

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/// A common store to use for `httpTransport` and `webSocketTransport`.
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let store = ApolloStore()
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/// A web socket transport to use for subscriptions
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let webSocketTransport: WebSocketTransport = {
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  let url = URL(string: "ws://localhost:8080/websocket")!
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  let webSocketClient = WebSocket(url: url, protocol: .graphql_transport_ws)
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  return WebSocketTransport(websocket: webSocketClient)
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}()
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/// An HTTP transport to use for queries and mutations
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let httpTransport: RequestChainNetworkTransport = {
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  let url = URL(string: "http://localhost:8080/graphql")!
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  return RequestChainNetworkTransport(interceptorProvider: DefaultInterceptorProvider(store: store), endpointURL: url)
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}()
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/// A split network transport to allow the use of both of the above
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/// transports through a single `NetworkTransport` instance.
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let splitNetworkTransport = SplitNetworkTransport(
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  uploadingNetworkTransport: httpTransport,
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  webSocketNetworkTransport: webSocketTransport
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)
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/// Create a client using the `SplitNetworkTransport`.
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let client = ApolloClient(networkTransport: splitNetworkTransport, store: store)

WebSocket subprotocols

Apollo iOS supports the following WebSocket subprotocols for subscriptions:

• graphql-ws, which is implemented in the subscriptions-transport-ws and AWS AppSync libraries. (⚠️ This protocol is not actively maintained!)
• graphql-transport-ws, which is implemented in the graphql-ws library.

Providing authorization tokens

In a standard HTTP operation, if authentication is necessary an Authorization header is often sent with requests. However, with WebSocket, this can't be sent with every payload because a persistent connection is required.

For WebSocket, the connectingPayload provides the parameters you would typically specify in your request headers.

Note that this must be set when the WebSocketTransport is created. If you need to update the connectingPayload, you need to recreate the client using a new webSocketTransport.

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let webSocketTransport: WebSocketTransport = {
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  let url = URL(string: "ws://localhost:8080/websocket")!
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  let webSocketClient = WebSocket(url: url, protocol: .graphql_transport_ws)
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  let authPayload: JSONEncodableDictionary = ["authToken": myAuthToken]
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  let config = WebSocketTransport.Configuration(connectingPayload: authPayload)
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  return WebSocketTransport(websocket: webSocketClient, config: config)
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}()

HTTP

The default NetworkTransport for Apollo iOS is the RequestChainNetworkTransport. In Apollo iOS 1.1.0 and later, this transport supports subscriptions over HTTP, with no additional configuration required.

Generating and executing

Apollo iOS supports subscriptions via code generation. Similar to queries, subscriptions are represented by instances of generated classes, conforming to the GraphQLSubscription protocol.

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subscription ReviewAdded {
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  reviewAdded {
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    id
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    stars
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  }
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}

After you generate these classes, you can execute subscriptions using ApolloClient.subscribe(subscription:) with a subscription-supporting NetworkTransport. If you do, your client continues to receive updated data until the subscription is canceled.

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let subscription = client.subscribe(subscription: ReviewAddedSubscription()) { result in
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  guard let data = try? result.get().data else { return }
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  print(data.reviews.map { $0.stars })
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}

Canceling a subscription

It's important to cancel a subscription connection whenever you're done with it. As long as a subscription is active, it maintains a connection to the server, and its resultHandler completion block is retained. This can create memory leaks and reduce your application's performance.

When you call ApolloClient.subscribe(subscription:) an opaque Cancellable is returned. You can cancel the subscription by calling cancel() on the returned Cancellable. This terminates the connection to the server and releases the resultHandler completion block.

A subscription's cancellation object does not cancel itself when it's deallocated! Make sure to cancel() it yourself. A class can ensure any subscriptions it manages are canceled when it's released by using its deinitializer:

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class ReviewViewController {
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  let client: ApolloClient!
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  private var subscription: Cancellable?
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  func subscribeToReviews() {
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    // Keep a reference to the subscription's cancellation object.
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    self.subscription = client.subscribe(subscription: ReviewAddedSubscription()) { [weak self] result in
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      // Handle each update from the subscription.
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    }
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  }
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  deinit {
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    // Make sure the subscription is cancelled, if it exists, when this object is deallocated.
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    self.subscription?.cancel()
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  }
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}