React Native Academy · 课时

实时订阅

使用 supabase.channel 订阅数据库变更，监听表上的 INSERT 和 UPDATE 事件，并在不轮询的情况下响应式地更新本地界面状态。

实时订阅 是 CoddyKit 上的免费 React Native Academy 课时。这是第 4 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 React Native Academy 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 React Native Academy 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

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.

常见问题解答

「实时订阅」课时是免费的吗？

是的 — 「实时订阅」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 React Native Academy 课程的其余内容，请升级到 CoddyKit PRO。 React Native Academy 课程共包含 4 节课。

「实时订阅」这节课中我会学到什么？

使用 supabase.channel 订阅数据库变更，监听表上的 INSERT 和 UPDATE 事件，并在不轮询的情况下响应式地更新本地界面状态。你通过在浏览器中直接运行的动手代码来练习 React Native Academy，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 React Native Academy 需要有经验吗？

无需任何先前经验。CoddyKit 上的 React Native Academy 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 4 节课，共 4 节。

「实时订阅」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 React Native Academy 课中编写并运行代码吗？

能。每节 React Native Academy 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

此课程中的所有课时

← 返回 React Native Academy