React Native Academy · レッスン

リアルタイムサブスクリプション

supabase.channelでデータベースの変更をサブスクライブし、テーブルのINSERTとUPDATEイベントを監視します。ポーリングを使わず、ローカルUIの状態をリアクティブに更新します。

「リアルタイムサブスクリプション」はCoddyKit上の無料React Native Academyレッスンです。これはレッスン4/4です。下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。これはReact Native Academy学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 React Native Academyコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

What Are Real-Time Subscriptions?

Real-time subscriptions allow your React Native app to receive database changes as they happen, without polling. When another user inserts, updates, or deletes a row, Supabase pushes the change to all subscribed clients over a WebSocket connection.

This is ideal for chat apps, live feeds, collaborative tools, and any feature where multiple users interact with shared data. Supabase's real-time engine is built on PostgreSQL's logical replication and the supabase-js client handles the WebSocket lifecycle for you.

Enabling Real-Time on a Table

Before subscribing to changes, you must enable real-time replication for the table in Supabase. Go to the Supabase dashboard, open Database > Replication, and add the table to the supabase_realtime publication.

You can also do this with SQL: ALTER PUBLICATION supabase_realtime ADD TABLE your_table;. Without this step, your subscription will connect but receive no events, which is a common source of confusion when first setting up real-time.

-- Enable real-time for the 'messages' table
ALTER PUBLICATION supabase_realtime ADD TABLE messages;

-- Verify which tables have real-time enabled:
SELECT tablename FROM pg_publication_tables
WHERE pubname = 'supabase_realtime';

Creating a Channel and Subscribing

Subscriptions in Supabase v2 are created with supabase.channel('channel-name'). You attach one or more event listeners with .on('postgres_changes', filter, callback), then call .subscribe() to start the WebSocket connection.

The filter object specifies the event (INSERT, UPDATE, DELETE, or * for all), the schema, and the table. The callback receives a payload object with the changed data.

const channel = supabase
  .channel('messages-changes')
  .on(
    'postgres_changes',
    { event: '*', schema: 'public', table: 'messages' },
    (payload) => {
      console.log('Change received:', payload);
    }
  )
  .subscribe();

// Unsubscribe when done:
// supabase.removeChannel(channel);

The Payload Structure

The payload object passed to your callback contains the change details:

eventType — 'INSERT', 'UPDATE', or 'DELETE'
new — the new row data (available for INSERT and UPDATE)
old — the previous row data (available for UPDATE and DELETE)
schema and table — which table changed

For DELETE events, payload.new is an empty object and payload.old contains the deleted row. For INSERT, payload.old is empty.

supabase
  .channel('messages-changes')
  .on(
    'postgres_changes',
    { event: 'INSERT', schema: 'public', table: 'messages' },
    (payload) => {
      // payload.eventType === 'INSERT'
      // payload.new === { id: '...', text: 'Hello!', user_id: '...' }
      // payload.old === {}
      console.log('New message:', payload.new);
    }
  )
  .subscribe();

Updating State on Real-Time Events

The most common pattern is to update a state array when a real-time event arrives. For INSERT events, append the new row. For UPDATE, replace the matching row. For DELETE, filter the row out.

Be careful to use the functional form of the state setter (setMessages(prev => [...])) to always work with the latest state, since the subscription callback is a closure that captures the state at subscription time.

const [messages, setMessages] = useState([]);

useEffect(() => {
  const channel = supabase
    .channel('messages-live')
    .on('postgres_changes',
      { event: 'INSERT', schema: 'public', table: 'messages' },
      (payload) => {
        setMessages(prev => [payload.new, ...prev]);
      }
    )
    .on('postgres_changes',
      { event: 'DELETE', schema: 'public', table: 'messages' },
      (payload) => {
        setMessages(prev => prev.filter(m => m.id !== payload.old.id));
      }
    )
    .subscribe();

  return () => { supabase.removeChannel(channel); };
}, []);

Filtering Subscriptions by Column Value

To subscribe only to changes in rows relevant to the current user or room, add a filter string to the event options. The filter syntax is 'column_name=eq.value', similar to PostgREST query syntax.

This reduces unnecessary network traffic on the client and avoids processing changes that are not relevant to the current screen. For example, in a chat app, subscribe only to messages in the current room.

const roomId = 'room-42';

const channel = supabase
  .channel('room-messages')
  .on(
    'postgres_changes',
    {
      event: '*',
      schema: 'public',
      table: 'messages',
      filter: 'room_id=eq.' + roomId,
    },
    (payload) => {
      // Only fires for messages in room-42
      console.log('Room message event:', payload);
    }
  )
  .subscribe();

Broadcast: Real-Time Without a Database

Supabase channels also support Broadcast, which lets you send ephemeral messages to all clients subscribed to the same channel without writing to the database. This is perfect for typing indicators, cursor positions, or transient presence events.

Use .on('broadcast', { event: 'typing' }, callback) to receive broadcast messages and channel.send to send them. Broadcast messages are not stored and do not trigger RLS policies.

const channel = supabase.channel('typing-indicators');

// Listen for typing events
channel.on('broadcast', { event: 'typing' }, (payload) => {
  console.log(payload.payload.username, 'is typing...');
}).subscribe();

// Send a typing event
async function sendTypingIndicator(username: string) {
  await channel.send({
    type: 'broadcast',
    event: 'typing',
    payload: { username },
  });
}

Presence: Tracking Online Users

Presence is a Supabase feature that lets you track which users are currently online and share state between them. Each client tracks its own presence, and the channel aggregates the presence of all connected clients in real time.

This is ideal for showing an online indicator next to user avatars in a chat list or a participants sidebar in a collaborative document.

const channel = supabase.channel('online-users');

channel
  .on('presence', { event: 'sync' }, () => {
    const state = channel.presenceState();
    console.log('Currently online:', Object.keys(state));
  })
  .subscribe(async (status) => {
    if (status === 'SUBSCRIBED') {
      await channel.track({ user_id: currentUserId, online_at: new Date().toISOString() });
    }
  });

// Untrack when leaving the screen
// await channel.untrack();

Cleaning Up Subscriptions in useEffect

Real-time channels use WebSocket connections. It is critical to clean up by calling supabase.removeChannel(channel) when the component unmounts. If you forget to clean up, you will accumulate WebSocket connections and leak memory.

Return a cleanup function from useEffect that removes the channel. React calls this cleanup automatically when the component unmounts or before the effect re-runs due to dependency changes.

useEffect(() => {
  const channel = supabase
    .channel('my-channel')
    .on('postgres_changes',
      { event: '*', schema: 'public', table: 'messages' },
      handleChange
    )
    .subscribe();

  // Cleanup: remove channel on unmount
  return () => {
    supabase.removeChannel(channel);
  };
}, []); // Run once on mount, clean up on unmount

Building a Real-Time Chat Screen

A real-time chat screen combines an initial useEffect fetch for existing messages with a real-time subscription for new ones. Fetch the last 50 messages on mount, then append new messages as they arrive via INSERT events.

Use an inverted FlatList (inverted prop) so the newest message always appears at the bottom without needing to scroll. Render each message as a row with the sender's avatar and text.

export function ChatScreen({ roomId }: { roomId: string }) {
  const [messages, setMessages] = useState([]);

  useEffect(() => {
    // 1. Fetch existing messages
    supabase
      .from('messages')
      .select('*, sender:profiles(username)')
      .eq('room_id', roomId)
      .order('created_at', { ascending: false })
      .limit(50)
      .then(({ data }) => setMessages(data ?? []));

    // 2. Subscribe to new messages
    const channel = supabase
      .channel('room-' + roomId)
      .on('postgres_changes',
        { event: 'INSERT', schema: 'public', table: 'messages',
          filter: 'room_id=eq.' + roomId },
        (payload) => setMessages(prev => [payload.new, ...prev])
      )
      .subscribe();

    return () => { supabase.removeChannel(channel); };
  }, [roomId]);

  return <FlatList inverted data={messages} renderItem={({ item }) =>
    <MessageRow message={item} />
  } />;
}

Subscription Status and Error Handling

The .subscribe() method accepts an optional callback that receives the subscription status. You can use this to show connection indicators in your UI or retry logic when the subscription fails.

Possible status values include 'SUBSCRIBED' (active), 'TIMED_OUT', 'CLOSED', and 'CHANNEL_ERROR'. Always handle error states to give users feedback when real-time functionality is unavailable.

const channel = supabase
  .channel('live-feed')
  .on('postgres_changes',
    { event: '*', schema: 'public', table: 'posts' },
    handleChange
  )
  .subscribe((status) => {
    if (status === 'SUBSCRIBED') {
      setConnectionStatus('Live');
    } else if (status === 'CHANNEL_ERROR') {
      setConnectionStatus('Disconnected');
    } else if (status === 'TIMED_OUT') {
      setConnectionStatus('Reconnecting...');
    }
  });

Quick Check

Test your understanding of React Native Mobile Development concepts from this lesson.

Lesson Recap

In this lesson you learned: how to subscribe to Postgres changes with supabase.channel, how to filter subscriptions by column value for targeted updates, and how to properly clean up channels on component unmount to avoid WebSocket leaks. Next up we explore integrating Firebase Auth and Firestore as an alternative backend for React Native apps.

よくある質問

「リアルタイムサブスクリプション」レッスンは無料ですか？

はい。「リアルタイムサブスクリプション」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、React Native Academyコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 React Native Academyコースには全4レッスンが含まれています。

「リアルタイムサブスクリプション」で何を学びますか？

supabase.channelでデータベースの変更をサブスクライブし、テーブルのINSERTとUPDATEイベントを監視します。ポーリングを使わず、ローカルUIの状態をリアクティブに更新します。ブラウザで直接実行するハンズオンコードでReact Native Academyを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

React Native Academyを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのReact Native Academyは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。これはレッスン4/4です。

「リアルタイムサブスクリプション」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このReact Native Academyレッスンでコードを書いて実行できますか？

はい。すべてのReact Native Academyレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

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