Support for GraphQL subscriptions

In the articles "Overview" and "Abstract types", we looked in detail at the execution of GraphQL queries in the generated DSL. Execution of GraphQL mutations in the generated DSL is exactly the same as the execution of GraphQL queries (we just need to use context mutation function instead of query function). It remains to deal with GraphQL subscriptions. Let's define a GraphQL schema:

type Subscription {
    filmCreated: Film!
}

type Film {
    title: String!
}

According to this schema, we can subscribe to new films:

subscription {
    filmCreated {
        title
    }
}

To receive JSON messages that look like this:

{
  "data": {
    "filmCreated": {
      "title": "First"
    }
  }
}

Context

Let's take a look at the context interface generated by the schema:

public interface ExampleContext {
    public suspend fun query(__projection: QueryProjection.() -> Unit): Query

    public suspend fun mutation(__projection: MutationProjection.() -> Unit): Mutation

    public fun subscription(__projection: SubscriptionProjection.() -> Unit): ExampleSubscriber<Subscription>
}

public fun interface ExampleSubscriber<T> {
    public suspend fun subscribe(block: suspend ExampleReceiver<T>.() -> Unit): Unit
}

@ExampleDSL
public fun interface ExampleReceiver<out T> {
    public suspend fun receive(): T
}

The semantics of the subscription function is different from the semantics of the query and mutation functions. While the query and mutation functions take a projection argument to build a query and return the result of the query execution, the subscription function takes a projection but returns a ExampleSubscriber interface. The "subscriber" interface allows us to create a long-lived session to listen for incoming messages. The session lifetime is the same as the execution time of the subscribe function in the ExampleSubscriber interface. When we enter the subscribe function, a session is created, and when we exit it, the session is destroyed.

The subscribe function is executed in scope of ExampleReceiver interface, and we can call the receive function to receive the next message. Messages are usually listened to in an infinite loop with a call to the receive function inside the loop. Let's try this: