Flutter Firebase：使用流生成器时如何删除Firebase侦听器？

Question

Every example about StreamBuilder starts with a StatelessWidget even in flutter example but how do you cancel the subscription in a StatelessWidget widget? For example, I was going through firestore example .

class MessageList extends StatelessWidget {
 MessageList({this.firestore});

 final Firestore firestore;

 @override
 Widget build(BuildContext context) {
 return StreamBuilder<QuerySnapshot>(
  stream: firestore.collection('messages').snapshots(),
  builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
    if (!snapshot.hasData) return const Text('Loading...');
    final int messageCount = snapshot.data.documents.length;
    return ListView.builder(
      itemCount: messageCount,
      itemBuilder: (_, int index) {
        final DocumentSnapshot document = snapshot.data.documents[index];
        return ListTile(
          title: Text(document['message'] ?? '<No message retrieved>'),
          subtitle: Text('Message ${index + 1} of $messageCount'),
        );
      },
    );
  },
 );
 }
}

Now how do I cancel listening to firestore.collection('messages').snapshots() stream?

I use realtime database in my app and this is how I do it

class MessgaeView extends StatefulWidget {
 final String _chatId;
 MessgaeView(this._chatId);

 @override
 _MessgaeViewState createState() => _MessgaeViewState();
}

class _MessgaeViewState extends State<MessgaeView> {
 Stream<Event> _messageStream;
 @override
 void initState() {
  _messageStream = _database
    .reference()
    .child("message/${widget._chatId}")
    .limitToLast(1)
    .onChildAdded;
  super.initState();
 }

 @override
 void dispose() {
  _messageStream.drain();
  super.dispose();
 }
@override
Widget build(BuildContext context) {
return StreamBuilder(
    stream: _messageStream,
    builder: (context, snapshot) {
      if (!snapshot.hasData) return CupertinoActivityIndicator();
      final message =
          (snapshot.data.snapshot as DataSnapshot).value['message'];
      return Text(message);
    });
 }
}

Answer 1

Simply replace the previous stream instance with null .

This will require a code similar to the following somewhere:

setState(() {
  _messageStream = null;
});

---

Doing so will stop listening to the stream. But StreamBuilder will still hold the previous value.

Answer 2

I hava same problem and solved by StreamSubscription

For example define StreamSubscription as global

StreamSubscription<Event> _counterSubscription;

then in your place you want to listen to data register your Subscription like this

_counterSubscription = _counterRef.onValue.listen((Event event) {
      setState(() {
        _counter = event.snapshot.value ?? 0;
      });
 });

and when you want to remove listener just make this code

if(_counterSubscription !=null)
     _counterSubscription .cancel();