Create Subscription and Render Result

So let's define the graphql subscription to be used.

Open components/OnlineUsers/OnlineUsersWrapper.js and add the following code, below the other imports.

components/OnlineUsers/OnlineUsersWrapper.js

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- import React, { useEffect, useState } from "react";
+ import React, { useEffect, Fragment, useState } from "react";
- import { useMutation } from "@apollo/client";
+ import { useMutation, useSubscription, gql } from "@apollo/client";

We are importing the useSubscription React hook from @apollo/client and the graphql subscription query we defined above to fetch the online user data.

Now, we will use the useSubscription React hook passing the subscription query:

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+ const { loading, error, data } = useSubscription(
+     gql`
+       subscription getOnlineUsers {
+         online_users(order_by: { user: { name: asc } }) {
+           id
+           user {
+             name
+           }
+         }
+       }
+     `
+   );
+ 
+   if (loading) {
+     return <span>Loading...</span>;
+   }
+   if (error) {
+     console.error(error);
+     return <span>Error!</span>;
+   }
+   if (data) {
+     onlineUsersList = data.online_users.map(u => (
+       <OnlineUser key={u.id} user={u.user} />
+     ));
+   }
+ 
+   return (
+     <div className="onlineUsersWrapper">
+       <Fragment>
+         <div className="sliderHeader">
+           Online users - {onlineUsersList.length}
+         </div>
+         {onlineUsersList}
+       </Fragment>
+     </div>
+   );
+ };

export default OnlineUsersWrapper;

Now that we have the real data, let's remove the mock online user state

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const OnlineUsersWrapper = () => {
-  const onlineUsers = [{ name: "someUser1" }, { name: "someUser2" }];
-
-  const onlineUsersList = [];
-  onlineUsers.forEach((user, index) => {
-    onlineUsersList.push(<OnlineUser key={index} index={index} user={user} />);
-  });
-
-  return (
-    <div className="onlineUsersWrapper">
-      <div className="sliderHeader">Online users - {onlineUsers.length}</div>
-      {onlineUsersList}
-    </div>
-  );
};

How does this work?

We are using the useSubscription React hook which returns properties (similar to useQuery and useMutation React hooks). The data property gives the result of the realtime data for the query we have made.

Refresh your app and see yourself online! Don't be surprised; There could be other users online as well.

Awesome! You have completed implementations of a GraphQL Query, Mutation and Subscriptions.