How to listen for document changes in Cloud Firestore using Flutter?

Question

I would like to have a listener method that checks for changes to a collection of documents if changes occur.

Something like:

import 'package:cloud_firestore/cloud_firestore.dart';     Future<Null> checkFocChanges() async {     Firestore.instance.runTransaction((Transaction tx) async {       CollectionReference reference = Firestore.instance.collection('planets');       reference.onSnapshot.listen((querySnapshot) {         querySnapshot.docChanges.forEach((change) {           // Do something with change         });       });     });   } 

The error here is that onSnapshot isn't defined on CollectionReference.

Any ideas?

Answer 1

Reading through cloud_firestore's documentation you can see that a Stream from a Query can be obtained via snapshots().

For you to understand, I will transform your code just a tiny bit:

CollectionReference reference = Firestore.instance.collection('planets'); reference.snapshots().listen((querySnapshot) {   querySnapshot.documentChanges.forEach((change) {     // Do something with change   }); }); 

You should also not run this in a transaction. The Flutter way of doing this is using a StreamBuilder, straight from the cloud_firestore Dart pub page:

StreamBuilder<QuerySnapshot>(   stream: Firestore.instance.collection('books').snapshots(),   builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {     if (!snapshot.hasData) return new Text('Loading...');     return new ListView(       children: snapshot.data.documents.map((DocumentSnapshot document) {         return new ListTile(           title: new Text(document['title']),           subtitle: new Text(document['author']),         );       }).toList(),     );   }, ); 

If you want to know any more, you can take a look at the source, it is well documented, where it is not self-explanatory.

Also take note that I changed docChanges to documentChanges. You can see that in the query_snapshot file. If you are using an IDE like IntelliJ or Android Studio, it is also pretty easy to click through the files in it.