How to listen for document changes in Cloud Firest

问题:

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?

回答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.