Apollo Elements Apollo Elements Guides API Blog Toggle darkmode

Usage: Subscriptions

Subscription components fetch data in real time via GraphQL subscriptions, which usually use websockets under the hood.

Example: Chat App

A chat app queries for a list of messages, then initiates a subscription for all incoming messages. When a new message comes in, we want to notify the users, as well as add the message to the bottom of the existing on-screen list.

Consider this query and subscription:

query MessagesQuery {
  messages {
    date
    message
    user
  }
}

subscription MessageSentSubscription {
  messageSent {
    date
    message
    user
  }
}

Let's define a component which performs the following tasks:

  1. queries for messages
  2. subscribes to any new messages
  3. when new messages arrive, integrate them with the cached messages from the query.

We'll accomplish this by calling subscribeToMore on our element once it's connected to the DOM, passing in an updateQuery function to define the merge for new data:

<apollo-client id="messages-client">
  <apollo-query id="messages-query">
    <script type="application/graphql" src="Messages.query.graphql"></script>
    <template>
      <link rel="stylesheet" href="messages.css"/>
      <ol>
        <template type="repeat" repeat="{{ data.messages ?? [] }}">
          <li>
            <h4>
              <time datetime="{{ date }}">{{ formatDate(item.date) }}</time>
              <span class="user">{{ item.user }} said:</span>
            </h4>
            <p>{{ item.message }}</p>
          </li>
        </template>
      </ol>
    </template>
  </apollo-query>
</apollo-client>
<script type="module">
  import '@apollo-elements/components';
  import { client } from './client';
  import { MessageSentSubscription } from './MessageSent.subscription.graphql.js';

  const element = document.getElementById('messages-query');

  element.formatDate = function formatDate(iso) {
    try { return new Date(iso).toDateString(); }
    catch { return ''; }
  };

  element.subscribeToMore({
    document: MessageSentSubscription
    updateQuery(prev, { subscriptionData }) {
      if (!subscriptionData.data) return prev;
      return {
        ...prev,
        messages: [...prev.messages, subscriptionData.data.messageSent]
      };
    },
  }) = subscribeToMore;

  document.getElementById('messages-client').client = client;
</script>

import { ApolloQueryMixin } from '@apollo-elements/mixins'
import { client } from './client';
import { MessageSentSubscription } from './MessageSent.subscription.graphql.js';
import { MessagesQuery } from './Messages.query.graphql.js';

const template = document.createElement('template');
      template.innerHTML = `
      <link rel="stylesheet" href="messages.css"/>
      <ol></ol>`;

const msgTpl = document.createElement('template');
      msgTpl.innerHTML = `
      <li>
        <h4>
          <time>{{ formatDate(item.date) }}</time>
          <span class="user">{{ item.user }} said:</span>
        </h4>
        <p>{{ item.message }}</p>
      </li>`;

class Messages extends ApolloQueryMixin(HTMLElement) {
  client = client;

  query = MessagesQuery;

  $ = selector => this.shadowRoot.querySelector(selector);

  constructor() {
    super();
    this
      .attachShadow({ mode: 'open' })
      .append(template.content.cloneNode(true))
  }

  connectedCallback() {
    super.connectedCallback();
    this.subscribeToMore({
      document: MessageSentSubscription
      updateQuery(prev, { subscriptionData }) {
        if (!subscriptionData.data) return prev;
        return {
          ...prev,
          messages: [...prev.messages, subscriptionData.data.messageSent]
        };
      },
    });
  }

  onData(data) {
    if (!data) return;
    for (const message of data.messages) {
      if (!this.$(`[datetime="${message.date}"]`))
        this.$('ol').append(Messages.stampMessage(message))
    }
  }

  static stampMessage({ date, message, user }) {
    const node = msgTpl.content.cloneNode(true);
    const time = node.querySelector('time')
          time.setAttribute('datetime', date);
          time.textContent = Messages.formatDate(date);
    const user = node.querySelector('.user');
          user.textContent = user;
    const p = node.querySelector('p');
          p.textContent = message;
    return node;
  }

  static formatDate(iso) {
    try { return new Date(iso).toDateString(); }
    catch { return ''; }
  }
}

customElements.define('messages-query', MessagesQuery);

import { ApolloQueryController } from '@apollo-elements/core';
import { LitElement, html } from 'lit';
import { customElement } from 'lit/decorators.js';

import { client } from './client';
import { MessageSentSubscription } from './MessageSent.subscription.graphql.js';
import { MessagesQuery } from './Messages.query.graphql.js';

import style from './messages.css.js';

@customElement('messages-query')
class Messages extends LitElement {
  static styles = [style];

  query = new ApolloQueryController(this, MessagesQuery);

  static formatDate(iso) {
    try { return new Date(iso).toDateString(); }
    catch { return ''; }
  }

  connectedCallback() {
    super.connectedCallback();
    this.query.subscribeToMore({
      document: MessageSentSubscription,
      updateQuery(prev, { subscriptionData }) {
        if (!subscriptionData.data) return prev;
        return {
          ...prev,
          messages: [...prev.messages, subscriptionData.data.messageSent]
        };
      },
    });
  }

  render() {
    const messages = this.query.data || [];
    return html`
      <ol>${messages.map(({ date, user, message }) => html`
        <li>
          <h4>
            <time datetime="${date}">${Messages.formatDate(date)}</time>
            <span class="user">${user} said:</span>
          </h4>
          <p>${message}</p>
        </li>`)}
      </ol>
    `;
  }
}

import { FASTElement, customElement, html, repeat } from '@microsoft/fast-element';
import { ApolloQueryBehavior } from '@apollo-elements/fast';

import { client } from './client';
import { MessageSentSubscription } from './MessageSent.subscription.graphql.js';
import { MessagesQuery } from './Messages.query.graphql.js';

import style from './messages.css.js';

const template: ViewTemplate<Messages> = html`
  <ol>${repeat(x => x.query.data?.messages ?? [], ({ date, user, message }) => html`
    <li>
      <h4>
        <time datetime="${date}">${Message.formatDate(date)}</time>
        <span class="user">${user} said:</span>
      </h4>
      <p>${message}</p>
    </li>` as ViewTemplate<Message>)}
  </ol>
`;

@customElement({ name: 'messages-query', style template })
class Messages extends ApolloQuery<typeof MessagesQuery> {
  query = new ApolloQueryBehavior(this, MessagesQuery);

  static formatDate(iso) {
    try { return new Date(iso).toDateString(); }
    catch { return ''; }
  }

  connectedCallback() {
    super.connectedCallback();
    this.query.subscribeToMore({
      document: MessageSentSubscription,
      updateQuery(prev, { subscriptionData }) {
        if (!subscriptionData.data) return prev;
        return {
          ...prev,
          messages: [...prev.messages, subscriptionData.data.messageSent]
        };
      },
    });
  }
}

import { useQuery, component, html } from '@apollo-elements/haunted';

import { client } from './client';
import { MessageSentSubscription } from './MessageSent.subscription.graphql.js';
import { MessagesQuery } from './Messages.query.graphql.js';

function formatDate(iso) {
  try { return new Date(iso).toDateString(); }
  catch { return ''; }
}

function Messages() {
  const { data, subscribeToMore } = useQuery(MessagesQuery, { client });

  useEffect(() => {
    subscribeToMore({
      document: MessageSentSubscription,
      updateQuery(prev, { subscriptionData }) {
        if (!subscriptionData.data) return prev;
        return {
          ...prev,
          messages: [...prev.messages, subscriptionData.data.messageSent]
        };
      },
    });
  }, []);

  const messages = this.query.data || [];

  return html`
    <ol>
      ${messages.map(({ date, user, message }) => html`
      <li>
        <h4>
          <time datetime="${date}">${formatDate(date)}</time>
          <span class="user">${user} said:</span>
        </h4>
        <p>${message}</p>
      </li>
      `)}
    </ol>
  `;
}

customElements.define('messages-query', component(Messages));

import { useQuery, c } from '@apollo-elements/atomico';

import { client } from './client';
import { MessageSentSubscription } from './MessageSent.subscription.graphql.js';
import { MessagesQuery } from './Messages.query.graphql.js';

function formatDate(iso) {
  try { return new Date(iso).toDateString(); }
  catch { return ''; }
}

function Messages() {
  const { data, subscribeToMore } = useQuery(MessagesQuery, { client });

  useEffect(() => {
    subscribeToMore({
      document: MessageSentSubscription,
      updateQuery(prev, { subscriptionData }) {
        if (!subscriptionData.data) return prev;
        return {
          ...prev,
          messages: [...prev.messages, subscriptionData.data.messageSent]
        };
      },
    });
  }, []);

  const messages = data?.messages ?? [];

  return (
    <host shadowDom>
      <ol>
        {messages.map(({ date, user, message }) => (
        <li>
          <h4>
            <time datetime={date}>{formatDate(date)}</time>
            <span class="user">{user} said:</span>
          </h4>
          <p>{message}</p>
        </li>
        ))}
      </ol>
    </host>
  );
}

customElements.define('messages-query', c(Messages));

import { define, query, html } from '@apollo-elements/hybrids';

import { client } from './client';

import { MessageSentSubscription } from './MessageSent.subscription.graphql.js';
import { MessagesQuery } from './Messages.query.graphql.js';

function formatDate(iso) {
  try { return new Date(iso).toDateString(); }
  catch { return ''; }
}

function connect(host) {
  host.query.subscribeToMore({
    document: MessageSentSubscription,
    updateQuery(prev, { subscriptionData }) {
      if (!subscriptionData.data) return prev;
      return {
        ...prev,
        messages: [...prev.messages, subscriptionData.data.messageSent]
      };
    },
  });
}

define('chat-query', {
  query: query(MessagesQuery, { client }),
  _subscribeToMore: { connect },
  render: host => {
    const messages = host.query.data || [];
    return html`
      <ol>
        ${messages.map(({ date, user, message }) => html`
        <li>
          <h4>
            <time datetime="${date}">${formatDate(date)}</time>
            <span class="user">${user} said:</span>
          </h4>
          <p>${message}</p>
        </li>
        `)}
      </ol>
    `;
  },
});

ApolloSubscriptionController

The first approach of calling subscribeToMore suits our requirement of updating the list of messages, but we still have to notify our users. Let's use a subscription controller and run our notification side effect using its lifecycle events.

<apollo-client id="messages-client">
  <apollo-query id="messages-query">
    <script type="application/graphql" src="Messages.query.graphql"></script>
    <template>
      <link rel="stylesheet" href="messages.css"/>
      <ol>
        <template type="repeat" repeat="{{ data.messages ?? [] }}">
          <li>
            <h4>
              <time datetime="{{ date }}">{{ formatDate(item.date) }}</time>
              <span class="user">{{ item.user }} said:</span>
            </h4>
            <p>{{ item.message }}</p>
          </li>
        </template>
      </ol>
    </template>
  </apollo-query>

  <apollo-subscription id="messages-subscription">
    <script type="application/graphql" src="MessageSent.subscription.graphql"></script>
    <template>
      <mwc-snackbar labeltext="{{ data.messagesSent.user }} sent a message!"></mwc-snackbar>
    </template>
  </apollo-subcription>

</apollo-client>
<script type="module">
  import '@apollo-elements/components';
  import '@material/mwc-snackbar';

  import { client } from './client';

  const q = document.getElementById('messages-query');
  const s = document.getElementById('messages-subscription');

  q.formatDate = function formatDate(iso) { /*...*/ };

  s.addEventListener('apollo-subscription-result', async event => {
    // update the query element
    const { client, subscriptionResult: { data: { messageSent } } } = event.detail;
    const { query } = q;
    const cached = client.readQuery({ query });
    client.writeQuery({ query, data: { messages: [...cached.messages, messageSent] } });

    // Display the notification
    await s.updateComplete;
    s.$('mwc-snackbar').show();
  });

  document.getElementById('messages-client').client = client;
</script>

import { ApolloQueryMixin } from '@apollo-elements/mixins';
import { ApolloSubscriptionController } from '@apollo-elements/core';
import { client } from './client';
import { MessageSentSubscription } from './MessageSent.subscription.graphql.js';
import { MessagesQuery } from './Messages.query.graphql.js';

import '@material/mwc-snackbar';

const template = document.createElement('template');
      template.innerHTML = `
      <link rel="stylesheet" href="messages.css"/>
      <mwc-snackbar></mwc-snackbar>
      <ol></ol>`;

const msgTpl = document.createElement('template');
      msgTpl.innerHTML = `
      <li>
        <h4>
          <time>{{ formatDate(item.date) }}</time>
          <span class="user">{{ item.user }} said:</span>
        </h4>
        <p>{{ item.message }}</p>
      </li>`;

class Messages extends ApolloQueryMixin(HTMLElement) {
  client = client;

  query = MessagesQuery;

  subscription = new ApolloSubscriptionController(this, MessageSentSubscription, {
    onData: async ({ client, subscriptionResult: { data: { messageSent } } }) => {
      // update the query element
      const cached = client.readQuery({ query: MessagesQuery });
      client.writeQuery({
        query: MessagesQuery,
        data: { messages: [...cached.messages, messageSent] }
      });

      // Display the notification
      await this.updateComplete;
      this.$('mwc-snackbar').labelText = `${messageSent.user} sent a message!`;
      this.$('mwc-snackbar').show();
    }
  });

  $ = selector => this.shadowRoot.querySelector(selector);

  constructor() { /*...*/ }

  onData(data) { /*...*/ }

  static stampMessage({ date, message, user }) { /*...*/ }

  static formatDate(iso) { /*...*/ }
}

customElements.define('messages-query', MessagesQuery);

import { ApolloQueryController, ApolloSubscriptionController } from '@apollo-elements/core';
import { LitElement, html } from 'lit';
import { customElement } from 'lit/decorators.js';

import { client } from './client';
import { MessageSentSubscription } from './MessageSent.subscription.graphql.js';
import { MessagesQuery } from './Messages.query.graphql.js';

import style from './messages.css.js';

import { Snackbar } from '@material/mwc-snackbar';

@customElement('messages-query')
class Messages extends LitElement {
  static styles = [style];

  query = new ApolloQueryController(this, MessagesQuery);

  @query('mwc-snackbar') snackbar: Snackbar;

  subscription = new ApolloSubscriptionController(this, MessageSentSubscription, {
    onData: async ({ client, subscriptionResult: { data: { messageSent } } }) => {
      // update the query element
      const cached = client.readQuery({ query: MessagesQuery });
      client.writeQuery({
        query: MessagesQuery,
        data: { messages: [...cached.messages, messageSent] }
      });

      // Display the notification
      await this.updateComplete;
      this.snackbar.labelText = `${messageSent.user} sent a message!`
      this.snackbar.show();
    }
  });


  static formatDate(iso) { /*...*/ }

  render() { /*...*/ }
}

import { FASTElement, customElement, html, ref, ViewTemplate } from 'lit';
import { ApolloQueryBehavior, ApolloSubscriptionBehavior } from '@apollo-elements/fast';

import { client } from './client';
import { MessageSentSubscription } from './MessageSent.subscription.graphql.js';
import { MessagesQuery } from './Messages.query.graphql.js';

import styles from './messages.css.js';

import { Snackbar } from '@material/mwc-snackbar';

const template: ViewTemplate<Messages> = html`...`;

@customElement({ name: 'messages-query', styles, template })
class Messages extends LitElement {
  query = new ApolloQueryBehavior(this, MessagesQuery);

  subscription = new ApolloSubscriptionController(this, MessageSentSubscription, {
    onData: async ({ client, subscriptionResult: { data: { messageSent } } }) => {
      // update the query element
      const cached = client.readQuery({ query: MessagesQuery });
      client.writeQuery({
        query: MessagesQuery,
        data: { messages: [...cached.messages, messageSent] }
      });

      // Display the notification
      await this.updateComplete;
      this.snackbar.labelText = `${messageSent.user} sent a message!`
      this.snackbar.show();
    }
  });


  static formatDate(iso) { /*...*/ }
}

import { useQuery, useSubscription, useEffect, component, html } from '@apollo-elements/haunted';
import { client } from './client';
import { MessageSentSubscription } from './MessageSent.subscription.graphql.js';
import { MessagesQuery } from './Messages.query.graphql.js';

import '@material/mwc-snackbar';

function formatDate(iso) {
  try { return new Date(iso).toDateString(); }
  catch { return ''; }
}

function Messages() {
  const { data } = useQuery(MessagesQuery, { client });
  const { data: subscriptionData } = useSubscription(MessageSentSubscription, { client,
    onData: async ({ subscriptionResult: { data: { messageSent } } }) => {
      // update the query element
      const cached = client.readQuery({ query: MessagesQuery });
      client.writeQuery({
        query: MessagesQuery ,
        data: { messages: [...cached.messages, messageSent] }
      });
    }
  });

  useEffect(() => {
    this.shadowRoot.querySelector('mwc-snackbar').labelText =
      `${messageSent.user} sent a message!`
    this.shadowRoot.querySelector('mwc-snackbar').show();
  }, [subscriptionData])

  const messages = this.query.data || [];

  return html`
    <mwc-snackbar></mwc-snackbar>
    <ol>
      ${messages.map(({ date, user, message }) => html`
      <li>
        <h4>
          <time datetime="${date}">${formatDate(date)}</time>
          <span class="user">${user} said:</span>
        </h4>
        <p>${message}</p>
      </li>
      `)}
    </ol>
  `;
}
customElements.define('messages-query', component(Messages));

import { useQuery, useSubscription, useEffect, useHost, c } from '@apollo-elements/atomico';
import { client } from './client';
import { MessageSentSubscription } from './MessageSent.subscription.graphql.js';
import { MessagesQuery } from './Messages.query.graphql.js';

import '@material/mwc-snackbar';

function formatDate(iso) {
  try { return new Date(iso).toDateString(); }
  catch { return ''; }
}

function Messages() {
  const ref = useHost();
  const { data } = useQuery(MessagesQuery, { client });
  const { data: subscriptionData } = useSubscription(MessageSentSubscription, { client,
    onData: async ({ subscriptionResult: { data: { messageSent } } }) => {
      // update the query element
      const cached = client.readQuery({ query: MessagesQuery });
      client.writeQuery({
        query: MessagesQuery ,
        data: { messages: [...cached.messages, messageSent] }
      });
    }
  });

  useEffect(() => {
    ref.current.shadowRoot.querySelector('mwc-snackbar').labelText =
      `${messageSent.user} sent a message!`
    ref.current.shadowRoot.querySelector('mwc-snackbar').show();
  }, [subscriptionData])

  const messages = data?.messages ?? [];

  return (
    <host shadowDom>
      <mwc-snackbar></mwc-snackbar>
      <ol>
        {messages.map(({ date, user, message }) => (
        <li>
          <h4>
            <time datetime={date}>{formatDate(date)}</time>
            <span class="user">{user} said:</span>
          </h4>
          <p>{message}</p>
        </li>
        ))}
      </ol>
    </host>
  );
}
customElements.define('messages-query', c(Messages));

import { define, query, html } from '@apollo-elements/hybrids';

import { client } from './client';

import { MessageSentSubscription } from './MessageSent.subscription.graphql.js';
import { MessagesQuery } from './Messages.query.graphql.js';

import '@material/mwc-snackbar';

function formatDate(iso) {/*...*/}

define('chat-query', {
  query: query(MessagesQuery, { client }),
  subscription: subscription(MessageSentSubscription, { client,
    onData: async ({ subscriptionResult: { data: { messageSent } } }) => {
      // update the query element
      const cached = client.readQuery({ query: MessagesQuery });
      client.writeQuery({
        query: MessagesQuery,
        data: { messages: [...cached.messages, messageSent] }
      });

      const el = document.querySelector('chat-query')
      el.shadowRoot.querySelector('mwc-snackbar').labelText =
        `${messageSent.user} sent a message!`
      el.shadowRoot.querySelector('mwc-snackbar').show();
    }
  }),
  render: host => {
    const messages = host.query.data || [];
    return html`
      <mwc-snackbar></mwc-snackbar>
      <ol>
        ${messages.map(({ date, user, message }) => html`
        <li>
          <h4>
            <time datetime="${date}">${formatDate(date)}</time>
            <span class="user">${user} said:</span>
          </h4>
          <p>${message}</p>
        </li>
        `)}
      </ol>
    `;
  },
});

Subscription Component

We could create on a separate <chat-subscription> component to handle fetching the subscription side. See the HTML example in the previous section for an example of listening for the subscription's lifecycle events to update the query

Next Steps

See this simple chat-app demo which demonstrates building custom elements which subscribe to a GraphQL server over websockets: #leeway

Dive into the ApolloSubscription API and component lifecycle or continue on to the managing local state guide.