Listen only to additions to a cloud firestore collection?

Question

I've noticed, when I try to use a realtime listener on a Collection in firestore, each time a new Document is added to the collection, the logic will be rerun, and I will download everything already in the collection

right now:

firebase.firestore().collection("Tweets").onSnapshot(function(querySnapshot) {
      querySnapshot.forEach(function(doc) {
        console.log("snapshot added ", doc)
      });
    });

Is there a way to track only ADDITIONS to the collection? I guess I could do that device side, but theres no need to transfer all the additional data I have already queryed for..

Output of that log would print out every single "tweet" in the collection, regardless of only one being added

EX: Initial Query

-Tweet 1

-Tweet 2

-Tweet 3

New Tweet, Tweet 4 is added

Output:

Tweet 1

Tweet 2

Tweet 3

Tweet 4

If that makes sense

Answer 1

It seems that the onSnapshotis only for listening an entire document,which means it would return all of the fields in a document that you're listening. So you are not able to only retrieve the changed field. But you can do different things according to different types of change:

xxxx.onSnapshot(function(querySnapshot){
     querySnapshot.docChanges.forEach(function(change){
                    if(change.type=="added"){//first time it will be triggered

                    }else if(change.type == "modified"){//modified

                    }else if(change.type == "removed"){//removed


                    }
    })
})

Hope this helps

Answer 2

When you use onSnapshot() on a collection, you're not actually downloading the entire collection on each invocation. The documents are cached and will be reused when the collection changes again.

For each change that causes your callback to be invoked, you can find out which documents are new seen since the first invocation by checking the changes within the snapshot. An example of how to do this is in the documentation. With a query snapshot in hand, you can use this logic to determine which documents are new:

snapshot.docChanges.forEach(function(change) {
    if (change.type === "added") {
        // change.doc here is new a new document
    }
});