无法从 Firestore 获取数据

Question

I tried to fetch docs from the firestore, it returns an empty array but when I run console.log(docs); outside the declared function, it returns the actual array. I know this error occurs because my useEffect function runs first before getting the docs from the firestore. I want to know how to fix this issue.

const Component = (props) => {
    const { docs } = useFirestore('books');
    const id = props.match.params.id;
    
    const loadContent = () => {
        const book = docs && docs.filter(doc => doc.id === id);
        console.log(book); //not getting book from the docs because docs is empty
    }
    
    useEffect(() => {
        async function getContent(){
            await loadContent();
        }
        getContent()
    },[]);
};

useFirestore.js

import { useState, useEffect } from 'react';
import { firestore } from '../config/fbConfig';

const useFirestore = (collection) => {
    const [docs, setDocs] = useState([]);
    const [loading, setLoading] = useState(true);

    // getting realtime data from the firebase for books
    useEffect(() => {
        let unsubscribe = firestore.collection(collection)
        // .orderBy('createdAt', 'desc')
        .onSnapshot((querySnapshot) => {
          const items = [];
          querySnapshot.forEach((doc) => {
            items.push({...doc.data(), id: doc.id});
          });
          setDocs(items);
          setLoading(false);
        });
        return unsubscribe;
      }, []); // eslint-disable-line react-hooks/exhaustive-deps
  
    return { docs, loading };
}

export default useFirestore;

Answer 1

I need more informations on the 'useFirestore' code but you should at least write your code like that.

• do not list every document on firestore to just get one (you pay for each read request)

• load the document in useEffect, not outside

• useEffect must depend on the id

   const Component = (props) => { const id = props.match.params.id; const firestore = //; const [book, bookSet] = useState(false); useEffect(() => { //Depending on useFirestore code firestore.collections('books').doc(id) .then( snapshot => { if ( !snapshot.exists ) { bookSet(null); } else { bookSet(snapshot.data()); } }); }, [id]); if( book === false) return <p>Loading</p> if (!book) return <p>Not exists</p> return <p>Display it</p>; };

Edition

Here is my guess with your 'useFirestore' hooks

  const Component = (props) => {
      const id = props.match.params.id;
      const { docs, loading } = useFirestore('books');
      
      useEffect(() => {
          if( loading) console.log('Loading');
          return;

          const book = docs && docs.filter(doc => doc.id === id);
          console.log({book});
      },[loading, docs, id]);
  };

Answer 2

I think you are pretty close to getting the expected behaviour. Here's how I'd approach it:

const Component = props => {
  const { docs } = useFirestore("books");

  const id = props.match.params.id;

  // useMemo - whenever `docs` change, recalculate the book variable.
  // If `docs` don't change, `book` will also not change.
  // Your `docs` will probably change on every snapshot.
  const book = useMemo(() => docs && docs.filter(doc => doc.id === id), [docs]);

  console.log(book);

  useEffect(() => {
    if (book) {
      // Do something with the book, e.g. loadContent(book).
      // Keep in mind that this will run on every snapshot.
      // If you only want to run this once, you'll need an
      // extra state variable to store that the effect was
      // already run, and check it here.
    }
  }, [book]); // The effect will run whenever book changes
};

The useFirestore hook looks almost fine, only one remark: right now even if you change the collection parameter, the snapshot listener won't change. You might want to do this:

useEffect(() => {
  const unsubscribe = firestore.collection(collection).onSnapshot(snapshot => {
    const items = snapshot.map(doc => ({ ...doc.data(), id: doc.id }));
    setDocs(items);
    setLoading(false);
  });

  return unsubscribe;

  // Whenever `collection` changes, `unsubscribe` will be called, and then this hook
  // will subscribe to the new collection.
}, [collection]);

Upd.

If you want the useFirestore hook to only query for a particular book, you need to change the hook to accept and use a document ID, something like this:

const getDoc = doc => ({ ...doc.data(), id: doc.id });

const useFirestore = (collection, docId) => {
  const [docs, setDocs] = useState([]);
  const [loading, setLoading] = useState(true);

  useEffect(() => {
    let subject = firestore.collection(collection);

    if (docId) {
      // If docId is available, listen for changes to a
      // particular document
      subject = subject.doc(docId);
    }

    const unsubscribe = subject.onSnapshot(snapshot => {
      // Notice here that if listening for a particular docId,
      // the snapshot will be that document, not an array.
      // To maintain the interface of the hook, I convert that
      // document to an array with a single item.
      const items = docId ? [getDoc(doc)] : snapshot.map(getDoc);
      setDocs(items);
      setLoading(false);
    });

    return unsubscribe;
  }, [collection, docId]);

  return { docs, loading };
};