Working with Subscriptions

import { useServer } from "graphql-ws/lib/use/ws";
import { WebSocketServer } from "ws";

const wsServer = new WebSocketServer({ server: httpServer, path: "/graphql" });
useServer({ schema, context: ctxFn }, wsServer);

import { PubSub } from "graphql-subscriptions";
const pubsub = new PubSub();

const resolvers = {
  Subscription: {
    messageAdded: {
      subscribe: (_, { channelId }) => pubsub.asyncIterableIterator(["MESSAGE_ADDED:" + channelId])
    }
  }
};

Mutation: {
  postMessage: async (_, { channelId, body }, { db }) => {
    const message = await db.message.create({ data: { channelId, body } });
    pubsub.publish("MESSAGE_ADDED:" + channelId, { messageAdded: message });
    return message;
  }
}

import { withFilter } from "graphql-subscriptions";

Subscription: {
  messageAdded: {
    subscribe: withFilter(
      () => pubsub.asyncIterableIterator(["MESSAGE_ADDED"]),
      (payload, vars, ctx) => payload.messageAdded.channelId === vars.channelId && ctx.user.canRead(payload)
    )
  }
}