GraphQL main functionality is to fetch and update data from the server. In addition to that subscriptions allow us to listen and to send messages from and to server in real-time. It is similar to regular queries, but the answer will be received when an event happens either on the server or on the client.
Apollo iOS SDK library supports subscriptions and it is powered by the Swift WebSocket library Starscream behind the scenes to connect to the server. Let's look at how to set it up and start using it.
Schema and code generation
Subscriptions are already supported once you download your schema file from the GraphQL server and perform the code generation. It will generate all the subscriptions using GraphQLSubscription protocol which allows passing parameters to subscription you want to implement.
Let’s imagine we have a chat application and GraphQL server can get messages for a specific thread as soon as they are sent. We need to create a GraphQL query in our .graphql file like this:
subscription messagesReceived($threadID: ID!) {
messagesReceived(threadID: $threadID) {
id
date
text
author
}
}
Once this is done we use Apollo iOS SDK to auto-generate type-safe Swift code. Which can be used in our app.
You can read more about using Apollo iOS SDK in my previous post.
Setting up the client
Setting up the Apollo client using subscriptions can be the trickiest step, so let's go through it.
First, we need to create WebSocketTransport instance to send GraphQL subscription operations to the server.
let webSocketTransport: WebSocketTransport = {
let url = URL(string: "ws://messaging.app/websocket")!
let request = URLRequest(url: url)
return WebSocketTransport(request: request)
}()
Then an HTTP transport instance will help us to use queries and mutations.
let httpTransport = HTTPNetworkTransport(url: URL(string: "http://messaging.app/graphql")!)
A split network transport allows the use of HTTP Transport and WebSocket transport protocols. That enables us to avoid any potential issues of having multiple client objects.
let splitNetworkTransport = SplitNetworkTransport(
httpNetworkTransport: httpTransport,
webSocketNetworkTransport: webSocketTransport
)
Then we create the Apollo client using network transport type which in this case is SplitNetworkTransport.
let apolloClient = Apollo(networkTransport: splitTransport)
Using the subscription
After we have created the Apollo client with subscription capabilities we can use it in our app to listen when a new message has been created to the specific thread.
let receivedMessagesSubscription = MessagesReceivedSubscription(id: threadID)
let messagesSubsription = apolloClient.subscribe(subscription: receivedMessagesSubscription) {[weak self] result in
switch result {
case .success(let graphQLResult):
if let message = graphQLResult.data.message {
print("Message received \(message)")
}
case .failure(let error):
print("Failed to subscribe \(error)")
}
}
Each time when a message has been sent, code inside closure is executed with new data.
TL;DR
GraphQL is not only for fetching data from the server and updating data on the server. You can be using GraphQL subscription feature to listen and send real-time messages using WebSockets.
Apollo iOS SDK library has the functionality to support subscriptions in your Swift applications. It is using WebSocket library Starscream behind the scenes to offload this cumbersome task. By leveraging code generation you can use strictly typed Swift code to interact with your GraphQL server in real-time in your iOS, iPadOS, macOS and tvOS apps.
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