Can Cloud Firestore onSnapshot() only trigger on changes, and not get the initial state?

Question

The documentation says:

You can listen to a document with the onSnapshot() method. An initial call using the callback you provide creates a document snapshot immediately with the current contents of the single document. Then, each time the contents change, another call updates the document snapshot.

I just want my listener to fire when the data changes. I don't want it to fire when the app loads, to get the initial state of the data. Any suggestions?

Answer 1

Firestore listeners don't work that way. You will always be delivered the document(s) relevant to the fetch or query, then updates after that for as long as the listener remains added. There is no mode to receive deltas only.

If you want to receive only certain data, you might want to figure out how to query for it, for example, by adding a timestamp field and having the client only query for documents that have changed since some prior time.

Answer 2

After reading the first and second solution, I have made a solution that seems to work. First you initialize a variable to true and inside the onSnapshot() method you check if this variable is true, if it is, you change it to false, and on else you write your retrieving data algorithm. Something like this:

var initState = true;

let observer = records.onSnapshot(docSnapshot => {

    console.log(`Received doc snapshot`);

    if (initState) {
        initState = false;
    } else {
        if (!docSnapshot.docChanges().empty) {     
            docSnapshot.docChanges().forEach(function (change) {
               //Write here wahtever you want

            });
        }
    }

}, err => {
    console.log(`Encountered error: ${err}`);
});