Firestore 快照侦听器：“null 不是对象”错误

Question

Here is what I am trying to do with my React Native app using Firestore:

1. If the user is signed in, activate snapshot listener and save the value as a state.
2. If the user is signed out, set the state as null.

When the user is signed in, the code works just fine. However, when the user signs out, the error prompt 'null is not an object (evaluating doc.exists)' shows up.

What might be wrong with the below code? Any advice? Thanks!

useEffect(()=>{
  if (user) {
    firestore().collection('points').doc(user.uid).onSnapshot(
      (doc)=>{
        if (doc.exists){
            setPoints(doc.data());
        }
        else {
            setPoints(null);
        }
      }
    )
  }
  else {
    setPoints(null);
  }
},[user]);

Answer 1

I see 2 problems here.

1. if (user) {} is obviously evaluating to true . I'd clarify by changing it to if (user !== null (or undefined or whatever it is when a user signs out).
2. Your snapshot is probably still listening and throwing said error because you haven't unsubscribed from it. The React documentation has a good section on cleanup from subscribe functions .

Basically you need to wrap your onSnapshot() inside a named function, then return that function from the useEffect() .