Why is My SignalR `HubConnection` State Not Properly Updated in React?

If you’ve been working with React and SignalR, you might have come across an issue where it seems like your SignalR HubConnection state isn’t being updated correctly. Specifically, you may encounter a situation where you set up your connection state with useState, start the connection, and still find that your state is null. Let’s dissect a similar scenario to understand what’s happening.

Here’s an example of how you might set up your SignalR connection in a React application using hooks:

function App() {
  const [connection, setConnection] = useState();

  const createConn = async (navigateFunc) => {
    const conn = new HubConnectionBuilder()
      .withUrl('https://localhost:7028/chat')
      .configureLogging(LogLevel.Information)
      .build();
    await conn.start().then(function () {
      setConnection(conn);
      if (connection == null) console.log("connection is null")
    })
    .catch(err => console.log(err))
  }

  return (
    <div className="App">
      <Routes>
        <Route path='/' element={<Login createConn={createConn}/>}></Route>
      </Routes>
    </div>
  );
}

And here is a shortened version of a Login component that uses the createConn function:

const Login = ({ createConn }) => {
  const handleSubmit = (e) => {
    e.preventDefault();
    createConn();
  };

  return (
    <Form onSubmit={handleSubmit} className='mt-5'>
      <Form.Group>
        <Form.Control placeholder='Email...' type='text' className='d-flex p-2' />
        <Button variant='success' type='submit'>Join</Button>
      </Form.Group>
    </Form>
  );
}

In this setup, you might observe that the message “connection is null” is printed to the console even though you expected the connection to be established and the state to be updated. So, what is going on here?

The Issue: Understanding State Updates in React

First, it’s crucial to understand how state updates work in React, especially in functional components utilizing hooks. When you call setConnection(conn), React schedules an update to your component’s state. However, this update does not happen immediately. The state is updated after the current function execution context completes, which means that the connection variable will not reflect the updated state value immediately within the same function scope.

Here’s the critical line of code:

if (connection == null) console.log("connection is null")

At this point, right after setConnection(conn), the connection variable still holds its previous value, which is null. React hasn’t processed the state update yet. Hence, the condition evaluates to true, and “connection is null” gets printed.

Correctly Handling Asynchronous State Updates

To correctly handle asynchronous state updates, you need to understand that React state updates are batched and applied after the function completes. If you want to immediately use the value of the state after setting it, you’ll need to rely on different lifecycle hooks or effects.

Let’s revise the code to correctly reflect our intention. One way to address this is to use the updated state in a useEffect to perform actions based on the new state value.

Here’s how you can modify your App component:

function App() {
  const [connection, setConnection] = useState();

  const createConn = async () => {
    const conn = new HubConnectionBuilder()
      .withUrl('https://localhost:7028/chat')
      .configureLogging(LogLevel.Information)
      .build();
    
    try {
      await conn.start();
      setConnection(conn);
    } catch (err) {
      console.log(err);
    }
  };

  useEffect(() => {
    if (connection) {
      console.log("Connection established:", connection);
    }
  }, [connection]);

  return (
    <div className="App">
      <Routes>
        <Route path='/' element={<Login createConn={createConn}/>}></Route>
      </Routes>
    </div>
  );
}

const Login = ({ createConn }) => {
  const handleSubmit = (e) => {
    e.preventDefault();
    createConn();
  };

  return (
    <Form onSubmit={handleSubmit} className='mt-5'>
      <Form.Group>
        <Form.Control placeholder='Email...' type='text' className='d-flex p-2' />
        <Button variant='success' type='submit'>Join</Button>
      </Form.Group>
    </Form>
  );
}

Explanation

1. State Initialization: We initialize connection with useState(), starting it as undefined.

1. createConn Function: This function creates and starts the SignalR connection asynchronously. Upon successful start, it sets the connection state using setConnection(conn).

1. useEffect Hook: This hook listens to changes in the connection state. Whenever connection is updated, the useEffect callback runs, so you can perform actions depending on the updated state here. This is now where we log the successful connection establishment.

With this revised approach, you don’t rely on the state value immediately after setting it within the same scope. Instead, you make use of useEffect to handle the state-dependent logic effectively after the state has been updated.

By understanding the asynchronous nature of state updates and utilizing hooks like useEffect, you can handle state changes more reliably in your React applications.