Mastering React's experimental_useSubscription: A Global Guide to External Data Synchronization

            const unsubscribe = subscribe(callback);

            

              
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            const currentValue = getSnapshot();

            

              
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            import mitt from 'mitt'; // A lightweight event emitter library

const emitter = mitt();

let count = 0;

export const increment = () => {
  count++;
  emitter.emit('countChange', count);
};

export const getCount = () => count;

export const subscribeToCount = (callback) => {
  emitter.on('countChange', callback);
  // Return an unsubscribe function
  return () => {
    emitter.off('countChange', callback);
  };
};

            

              
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            import React from 'react';
import { experimental_useSubscription } from 'react-experimental'; // Assuming this is available
import { subscribeToCount, getCount, increment } from './store';

function CounterDisplay() {
  // The getSnapshot function should synchronously return the current value
  const currentCount = experimental_useSubscription(
    (callback) => subscribeToCount(callback),
    getCount
  );

  return (
    

      
Current Count: {currentCount}
      Increment Global Count
    
  );
}

export default CounterDisplay;

            

              
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            const listeners = new Set();
let websocket;

function connectWebSocket(url) {
  if (websocket && websocket.readyState === WebSocket.OPEN) {
    return;
  }

  websocket = new WebSocket(url);

  websocket.onopen = () => {
    console.log('WebSocket Connected');
    // You might want to send initial messages here
  };

  websocket.onmessage = (event) => {
    const data = JSON.parse(event.data);
    // Notify all listeners with the new data
    listeners.forEach(listener => listener(data));
  };

  websocket.onerror = (error) => {
    console.error('WebSocket Error:', error);
    // Handle reconnect logic or error reporting
  };

  websocket.onclose = () => {
    console.log('WebSocket Disconnected');
    // Attempt to reconnect after a delay
    setTimeout(() => connectWebSocket(url), 5000); // Reconnect after 5 seconds
  };
}

export function subscribeToWebSocket(callback) {
  listeners.add(callback);
  // If not connected, try to connect
  if (!websocket || websocket.readyState !== WebSocket.OPEN) {
    connectWebSocket('wss://your-websocket-server.com'); // Replace with your WebSocket URL
  }

  // Return the unsubscribe function
  return () => {
    listeners.delete(callback);
    // Optionally, close the WebSocket if no listeners remain, depending on desired behavior
    // if (listeners.size === 0) {
    //   websocket.close();
    // }
  };
}

export function getLatestMessage() {
  // In a real scenario, you'd store the last message received globally or in a state manager.
  // For this example, let's assume we have a variable holding the last message.
  // This needs to be updated by the onmessage handler.
  // For simplicity, returning a placeholder. You'd need state to hold this.
  return 'No message received yet'; // Placeholder
}

// A more robust implementation would store the last message:
let lastMessage = null;

export function subscribeToWebSocketWithState(callback) {
  listeners.add(callback);
  if (!websocket || websocket.readyState !== WebSocket.OPEN) {
    connectWebSocket('wss://your-websocket-server.com');
  }
  // Important: Immediately call callback with the last known message if available
  if (lastMessage) {
    callback(lastMessage);
  }
  return () => {
    listeners.delete(callback);
  };
}

export function getLatestMessageWithState() {
  return lastMessage;
}

// Modify the onmessage handler to update lastMessage:
// websocket.onmessage = (event) => {
//   const data = JSON.parse(event.data);
//   lastMessage = data;
//   listeners.forEach(listener => listener(data));
// };

            

              
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            import React from 'react';
import { experimental_useSubscription } from 'react-experimental';
import { subscribeToWebSocketWithState, getLatestMessageWithState } from './websocketService';

function RealTimeFeed() {
  // Using the stateful version of the service
  const message = experimental_useSubscription(
    (callback) => subscribeToWebSocketWithState(callback),
    getLatestMessageWithState
  );

  return (
    

      
Real-time Feed:
      
{message ? JSON.stringify(message) : 'Waiting for messages...'}
    
  );
}

export default RealTimeFeed;

            

              
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            const listeners = new Set();

function initializeOnlineStatusListener() {
  const handleOnlineChange = () => {
    const isOnline = navigator.onLine;
    listeners.forEach(listener => listener(isOnline));
  };

  window.addEventListener('online', handleOnlineChange);
  window.addEventListener('offline', handleOnlineChange);

  // Return a cleanup function
  return () => {
    window.removeEventListener('online', handleOnlineChange);
    window.removeEventListener('offline', handleOnlineChange);
  };
}

export function subscribeToOnlineStatus(callback) {
  listeners.add(callback);
  // If this is the first listener, set up the event listeners
  if (listeners.size === 1) {
    initializeOnlineStatusListener();
  }
  // Immediately call callback with the current status
  callback(navigator.onLine);

  return () => {
    listeners.delete(callback);
    // If this was the last listener, remove event listeners to prevent memory leaks
    if (listeners.size === 0) {
      // This cleanup logic needs to be managed carefully. A better approach might be to have a singleton service that manages listeners and only removes global listeners when truly no one is listening.
      // For simplicity here, we rely on the component's unmount to remove its specific listener.
      // A global cleanup function might be needed at app shutdown.
    }
  };
}

export function getOnlineStatus() {
  return navigator.onLine;
}

            

              
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            import React from 'react';
import { experimental_useSubscription } from 'react-experimental';
import { subscribeToOnlineStatus, getOnlineStatus } from './onlineStatusService';

function NetworkStatusIndicator() {
  const isOnline = experimental_useSubscription(
    (callback) => subscribeToOnlineStatus(callback),
    getOnlineStatus
  );

  return (
    

      Network Status: {isOnline ? 'Online' : 'Offline'}
    
  );
}

export default NetworkStatusIndicator;

            

              
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            // chatService.js
import { experimental_useSubscription } from 'react-experimental';
import { subscribeToChatMessages, getMessages, sendMessage } from './chatApi';

// This hook encapsulates the chat subscription logic
export function useChat() {
  const messages = experimental_useSubscription(subscribeToChatMessages, getMessages);
  return { messages, sendMessage };
}

// ChatComponent.js
import React from 'react';
import { useChat } from './chatService';

function ChatComponent() {
  const { messages, sendMessage } = useChat();
  // ... render messages and send input
}

            

              
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            // Assuming subscribe function takes an ID
function subscribeToUserData(userId, callback) {
  // ... setup subscription for userId ...
  return () => { /* ... unsubscribe logic ... */ };
}

function UserProfile({ userId }) {
  const userData = experimental_useSubscription(
    (callback) => subscribeToUserData(userId, callback),
    () => getUserData(userId) // getSnapshot might also need userId
  );
  // ...
}

            

              
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            // SubscriptionContext.js
import React, { createContext, useContext } from 'react';
import { experimental_useSubscription } from 'react-experimental';
import { subscribeToService, getServiceData } from './service';

const SubscriptionContext = createContext();

export function SubscriptionProvider({ children }) {
  const data = experimental_useSubscription(subscribeToService, getServiceData);

  return (
    
      {children}
    
  );
}

export function useSubscriptionData() {
  return useContext(SubscriptionContext);
}

// App.js
// 
//   
// 

// MyComponent.js
// const data = useSubscriptionData();

            

              
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