firebase return onSnapshot promise

Question

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 })
    })
})

Answer 1

A Promise in JavaScript can resolve (or reject) exactly once. A onSnapshot on the other hand can give results multiple times. That's why onSnapshot doesn't return a promise.

In your current code, you're left with a dangling listener to status_database_ref. Since you don't do anything with the data, it is wasteful to keep listening for it.

Instead of using onSnapshot, use get:

onlineUsers(callback){
    this.status_database_ref.where('state','==','online').get((querySnapshot)=>{
        callback(querySnapshot.size)
    }) 
}

Or in your original approach:

onlineUsers(){
    return this.status_database_ref.where('state','==','online').get();
}

Answer 2

I know it's too late but here is my solution using TypeScript & Javascript.

TYPESCRIPT

const _db=firebase.firestore;
const _collectionName="users";

    onDocumentChange = (
    document: string,
    callbackSuccess: (currentData: firebase.firestore.DocumentData, source?: string | 'Local' | 'Server') => void,
    callbackError?: (e: Error) => void,
    callbackCompletion?: () => void
) => {
    this._db.collection(this._collectionName).doc(document).onSnapshot(
        {
            // Listen for document metadata changes
            includeMetadataChanges: true
        },
        (doc) => {
            const source = doc.metadata.hasPendingWrites ? 'Local' : 'Server';
            callbackSuccess(doc.data(), source);
        },
        (error) => callbackError(error),
        () => callbackCompletion()
    );
};

JAVASCRIPT (ES5)

var _this = this;
onDocumentChange = function (document, callbackSuccess, callbackError, callbackCompletion) {
    _this._db.collection(_this._collectionName).doc(document).onSnapshot({
        // Listen for document metadata changes
        includeMetadataChanges: true
    }, function (doc) {
        var source = doc.metadata.hasPendingWrites ? 'Local' : 'Server';
        callbackSuccess(doc.data(), source);
    }, function (error) { return callbackError(error); }, function () { return callbackCompletion(); });
};