I'm using firebase/firestore and I'm looking a way to return promise of snapshot.

onlineUsers(){

     // i want to return onSnapshot

    return this.status_database_ref.where('state','==','online').onSnapshot();

}

in other file I did

  componentDidMount(){

    // this.unsubscribe = this.ref.where('state','==','online').onSnapshot(this.onCollectionUpdate) 

    firebaseService.onlineUsers().then(e=>{

        console.log(e)

    })

}

I get the errors

Error: Query.onSnapshot failed: Called with invalid arguments.

TypeError: _firebaseService2.default.unsubscribe is not a function

if i do this way

onlineUsers(){

   return  this.status_database_ref.where('state','==','online').onSnapshot((querySnapshot)=>{

        return querySnapshot

    }) 

}

I get

TypeError: _firebaseService2.default.onlineUsers(...).then is not a function

in addition,
when I do this way

   this.unsubscribe = firebaseService.onlineUsers().then((querySnapshot)=>{

        console.log(querySnapshot.size)

        this.setState({count:querySnapshot.size})

    })

// other file

 onlineUsers(callback) {

    return this.status_database_ref.where('state', '==', 'online').get()

}

it not listen to change into firebase, means if I change in firebase it's not update or change the size..

---- firestore function ---
I tried to make firestore function that trigger each time the UserStatus node updated but this take some seconds and it slow for me.

module.exports.onUserStatusChanged = functions.database

.ref('/UserStatus/{uid}').onUpdate((change, context) => {

    // Get the data written to Realtime Database

    const eventStatus = change.after.val();



    // Then use other event data to create a reference to the

    // corresponding Firestore document.

    const userStatusFirestoreRef = firestore.doc(`UserStatus/${context.params.uid}`);





    // It is likely that the Realtime Database change that triggered

    // this event has already been overwritten by a fast change in

    // online / offline status, so we'll re-read the current data

    // and compare the timestamps.

    return change.after.ref.once("value").then((statusSnapshot) => {

        return statusSnapshot.val();

    }).then((status) => {

        console.log(status, eventStatus);

        // If the current timestamp for this data is newer than

        // the data that triggered this event, we exit this function.

        if (status.last_changed > eventStatus.last_changed) return status;



        // Otherwise, we convert the last_changed field to a Date

        eventStatus.last_changed = new Date(eventStatus.last_changed);



        // ... and write it to Firestore.

        //return userStatusFirestoreRef.set(eventStatus);

        return userStatusFirestoreRef.update(eventStatus);

    });

});

function to calculate and update count of online users

module.exports.countOnlineUsers = functions.firestore.document('/UserStatus/{uid}').onWrite((change, context) => {



    const userOnlineCounterRef = firestore.doc('Counters/onlineUsersCounter');



    const docRef = firestore.collection('UserStatus').where('state', '==', 'online').get().then(e => {

        let count = e.size;

        return userOnlineCounterRef.update({ count })

    })

})