Subscriptions

One feature of urql that was not mentioned in the "Basics" sections is urql's APIs and ability to handle GraphQL subscriptions.

The Subscription Exchange

To add support for subscriptions we need to add the subscriptionExchange to our Client.

import { Client, defaultExchanges, subscriptionExchange } from 'urql';

const client = new Client({
  url: '/graphql',
  exchanges: [
    ...defaultExchanges,
    subscriptionExchange({
      forwardSubscription,
    }),
  ],
});

Read more about Exchanges and how they work on the "Authoring Exchanges" page. or what they are on the "Architecture" page.

In the above example, we add the subscriptionExchange to the Client with the default exchanges add before it. The subscriptionExchange is a factory that accepts additional options and returns the actual Exchange function. It does not make any assumption over the transport protocol and scheme that is used. Instead, we need to pass a forwardSubscription function which is called with an "enriched" Operation every time the Client attempts to execute a GraphQL Subscription.

When we define this function it must return an "Observable-like" object, which needs to follow the Observable spec, which comes down to having an object with a .subscribe() method accepting an observer.

Setting up `subscriptions-transport-ws`

If your GraphQL API is using the Apollo Server, you'll be able to use Apollo's subscriptions-transport-ws package.

import { Client, defaultExchanges, subscriptionExchange } from 'urql';
import { SubscriptionClient } from 'subscriptions-transport-ws';

const subscriptionClient = new SubscriptionClient('ws://localhost/graphql', { reconnect: true });

const client = new Client({
  url: '/graphql',
  exchanges: [
    ...defaultExchanges,
    subscriptionExchange({
      forwardSubscription(operation) {
        return subscriptionClient.request(operation);
      },
    }),
  ],
});

In this example, we're creating a SubscriptionClient, are passing in a URL and some parameters, and are using the SubscriptionClient's request method to create a Subscription Observable, which we return to the subscriptionExchange inside forwardSubscription.

Setting up `graphql-ws`

If your GraphQL API server is using graphql-ws, you'll be able to use it here too!

import { createClient, defaultExchanges, subscriptionExchange } from 'urql';
import { createClient as createWSClient } from 'graphql-ws';

const wsClient = createWSClient({
  url: 'ws://localhost/graphql',
});

const client = createClient({
  url: '/graphql',
  exchanges: [
    ...defaultExchanges,
    subscriptionExchange({
      forwardSubscription(operation) {
        return {
          subscribe: sink => {
            const dispose = wsClient.subscribe(operation, sink);
            return {
              unsubscribe: dispose,
            };
          },
        };
      },
    }),
  ],
});

We create a WebSocket client with the necessary options and use the subscribe method from it to create a Subscription Observable, which we return to the subscriptionExchange inside forwardSubscription.

React & Preact

The useSubscription hooks comes with a similar API to useQuery, which we've learned about in the "Queries" page in the "Basics" section.

Its usage is extremely similar in that it accepts options, which may contain query and variables. However, it also accepts a second argument, which is a reducer function, similar to what you would pass to Array.prototype.reduce.

It receives the previous set of data that this function has returned or undefined. As the second argument, it receives the event that has come in from the subscription. You can use this to accumulate the data over time, which is useful for a list for example.

In the following example, we create a subscription that informs us of new messages. We will concatenate the incoming messages so that we can display all messages that have come in over the subscription across events.

import React from 'react';
import { useSubscription } from 'urql';

const newMessages = `
  subscription MessageSub {
    newMessages {
      id
      from
      text
    }
  }
`;

const handleSubscription = (messages = [], response) => {
  return [response.newMessages, ...messages];
};

const Messages = () => {
  const [res] = useSubscription({ query: newMessages }, handleSubscription);

  if (!res.data) {
    return <p>No new messages</p>;
  }

  return (
    <ul>
      {res.data.map(message => (
        <p key={message.id}>
          {message.from}: "{message.text}"
        </p>
      ))}
    </ul>
  );
};

As we can see, the result.data is being updated and transformed by the handleSubscription function. This works over time, so as new messages come in, we will append them to the list of previous messages.

Svelte

The subscription function in @urql/svelte comes with a similar API to query, which we've learned about in the "Queries" page in the "Basics" section.

Its usage is extremely similar in that it accepts an operationStore, which will typically contain our GraphQL subscription query. However, subscription also accepts a second argument, which is a reducer function, similar to what you would pass to Array.prototype.reduce.

<script>
  import { operationStore, subscription } from '@urql/svelte';

  const messages = operationStore(`
    subscription MessageSub {
      newMessages {
        id
        from
        text
      }
    }
  `);

  const handleSubscription = (messages = [], data) => {
    return [data.newMessages, ...messages];
  };

  subsription(messages, handleSubscription);
</script>

{#if !$result.data}
  <p>No new messages</p>
{:else}
  <ul>
    {#each $messages.data as message}
      <li>{message.from}: "{message.text}"</li>
    {/each}
  </ul>
{/if}

As we can see, the $result.data is being updated and transformed by the handleSubscription function. This works over time, so as new messages come in, we will append them to the list of previous messages.

Vue

The useSubscription API is very similar to useQuery, which we've learned about in the "Queries" page in the "Basics" section.

<template>
  <div v-if="error">
    Oh no... {{error}}
  </div>
  <div v-else>
    <ul v-if="data">
      <li v-for="msg in data">{{ msg.from }}: "{{ msg.text }}"</li>
    </ul>
  </div>
</template>

<script>
import { useSubscription } from '@urql/vue';

export default {
  setup() {
    const handleSubscription = (messages = [], response) => {
      return [response.newMessages, ...messages];
    };

    const result = useSubscription({
      query: `
        subscription MessageSub {
          newMessages {
            id
            from
            text
          }
        }
      `
    }, handleSubscription)

    return {
      data: result.data,
      error: result.error,
    };
  }
};
</script>

One-off Subscriptions

When you're using subscriptions directly without urql's framework bindings, you can use the Client's subscription method for one-off subscriptions. This method is similar to the ones for mutations and subscriptions that we've seen before on the "Core Package" page.

This method will always returns a Wonka stream and doesn't have a .toPromise() shortcut method, since promises won't return the multiple values that a subscription may deliver. Let's convert the above example to one without framework code, as we may use subscriptions in a Node.js environment.

import { pipe, subscribe } from 'wonka';

const newMessages = `
  subscription MessageSub {
    newMessages {
      id
      from
      text
    }
  }
`;

const { unsubscribe } = pipe(
  client.subscription(MessageSub),
  subscribe(result => {
    console.log(result); // { data: ... }
  })
);