我如何防止这种无休止的重新渲染反应？

Question

I'm building a React app, but I haven't quite got my head around the use of hooks yet.

My data is stored in the following format in Firestore:

userlists > key1 > users: [user1, user2, user3]
                 > date_created: date
                 > listname: "Work Colleagues"
          > key2 > users: [user1, user4, user5]
                 > date_created: date
                 > listname: "Badminton Friends"

(where user1, user2 etc are objects holding user data)

So, in EditUserlistPage I want to retrieve the userlist with key key1 , and render a UserList component to show it. The UserList component then renders an individual UserItem component for each user

EditUserlistPage is as follows ( key is passed in via props):

const EditUserlistPage = (props) => {

  // Get list key
  const listKey = props.key

  // Set up state variables
  const [userlist, setUserlist] = useState({})

  // Load list from db once component has mounted
  useEffect(() => {
    props.firebase.getListByKey(listKey).get().then(doc => {
        let userlist = doc.data()
        setUserList(userList)
    })
  }, [listKey, props.firebase]) 

  return (
    <div>
      <h1>Edit User List</h1>
      <UserList
        userlist={userList}
      />
    </div>
  )
}

export default withFirebase(EditUserlistPage)

The UserList component is:

const UserList = (props) => {

  // Get list
  const { userlist } = props
  
  // Set up state variable - users
  const [ users,  setUsers] = useState([])

  // Now get all users as objects
  let usersTemp = []

  for(let ii=0; ii<userlist.users.count; ii++) {

      const user = userlist.users[ii]
      
      const userItem = {
        id: user.index,
        name: user.firstname + user.surname
        ... // More things go here, but I don't think they're relevant
      }
      usersTemp.push(userItem)
    }
  }

  setUsers(usersTemp)

  return (
    <div className="userList">
      { // This will render a UserItem component}
    </div>
  )

}

export default UserList

Finally, props.firebase.getListByKey is:

getListByKey = (key) => this.firestore.collection('userlists').doc(key)

I'm getting an error and a warning.

Firstly, displayed on screen is: Error: Too many re-renders. React limits the number of renders to prevent an infinite loop. Error: Too many re-renders. React limits the number of renders to prevent an infinite loop. In the console I can also see this error, and it says:

The above error occurred in the <UserList> component:
    in UserList (at EditUserlistPage/index.js:59)
    in div (at EditUserlistPage/index.js:54)
    in EditUserlistPage (at context.js:7)

This error goes away if I comment out the line in EditUserlistPage which renders the UserList component.

Secondly, I'm getting a warning in the console:

Warning: Can't perform a React state update on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in a useEffect cleanup function.
    in EditUserlistPage (at context.js:7)

context.js is the Firebase Context, and is:

const FirebaseContext = React.createContext(null)

export const withFirebase = Component => props => (
  <FirebaseContext.Consumer>
    {firebase => <Component {...props} firebase={firebase} />} // Line 7
  </FirebaseContext.Consumer>
)
 
export default FirebaseContext

I've tried to read the React documentation on Hooks, and I've gathered that useEffect can caused infinite re-rendering if not implemented properly, but I can't figure out how to do it correctly.

Answer 1

The main problem is with setUsers(usersTemp) in the code of the UserList .

Whenever some local state is changed, the component re-renders. So, since you always re-set the users during the render, you trigger another render.

You could use a useEffect and only update the users when the userList changes

const UserList = (props) => {

  // Get list
  const {
    userlist
  } = props

  // Set up state variable - users
  const [users, setUsers] = useState([])

  useEffect(() => {
      // Now get all users as objects
      let usersTemp = []

      for (let ii = 0; ii < userlist.users.count; ii++) {
        const user = userlist.users[ii];
        const userItem = {
          id: user.index,
          name: user.firstname + user.surname
            ... // More things go here, but I don't think they're relevant
        }
        usersTemp.push(userItem)
      }
    }

    setUsers(usersTemp)
  }, [userlist])

return (
    <div className="userList">
      { // This will render a UserItem component}
    </div>
  )
}

export default UserList

Answer 2

When you're using functional components, your entire function will run again when your state updates. So in your UserList component, you're updating the state every time it renders, therefore triggering a new render.

To prevent that, use useEffect with no dependencies. That will cause the useEffect to only run once when the component mounts, very much like a componentDidMount.

const UserList = (props) => {

  // Get list
  const { userlist } = props
  
  // Set up state variable - users
  const [ users,  setUsers] = useState([])

  useEffect(() => {
    // Now get all users as objects
    let usersTemp = []

    for(let ii=0; ii<userlist.users.count; ii++) {

        const user = userlist.users[ii]
      
        const userItem = {
          id: user.index,
          name: user.firstname + user.surname
          ... // More things go here, but I don't think they're relevant
        }
        usersTemp.push(userItem)
      }
    }

    setUsers(usersTemp)
  },[])



  return (
    <div className="userList">
      { // This will render a UserItem component}
    </div>
  )

}

export default UserList