using didUpdateWidget to replace a stream causes "bad state: Stream has already been listened to"

Question

I have a StreamBuilder that I want to be able to pause and unpause and, since StreamBuilder doesn't expose it's underlying StreamSubscription, I have to create a proxy stream to forward the source stream's elements while exposing a StreamSubscription that I can pause and unpause. This forces me to convert my stateless widget into a stateful widget so I have access to dispose and can close it's StreamController.

I'm following the didUpdateWidget documentation's instructions:

In initState, subscribe to the object. In didUpdateWidget unsubscribe from the old object and subscribe to the new one if the updated widget configuration requires replacing the object. In dispose, unsubscribe from the object.

I need to replace the Stream whenever the user types in a new search query. My stateful widget is re-constructed within a ChangeNotifier whenever a new stream is received.

Here's the relevant part of my stateful widget:

  final Stream<List<Entry>> _entryStream;
  StreamController<List<Entry>> _entryStreamController;
  StreamSubscription<List<Entry>> _entryStreamSubscription;

  _EntryListState(this._entryStream);

  void initStream() {
    _entryStreamController?.close();
    _entryStreamController = StreamController();

    _entryStreamSubscription = _entryStream.listen((event) {
      _entryStreamController.add(event);
    }
    )..onDone(() => _entryStreamController.close());
  }

  @override
  void initState() {
    initStream();
    super.initState();
  }

  @override
  void dispose() {
    _entryStreamController?.close();
    super.dispose();
  }

  @override
  void didUpdateWidget(EntryList oldEntryList) {
    // Initialize the stream only if it has changed
    if (oldEntryList._entryStream != _entryStream) initStream();

    super.didUpdateWidget(oldEntryList);
  }

This is the line throwing the BadState error:

    _entryStreamSubscription = _entryStream.listen((event) {
      _entryStreamController.add(event);
    }

And here is where my widget is being constructed:

              child: Consumer<SearchStringModel>(
                builder: (context, searchStringModel, child) {
                  print('rebuilding with: ${searchStringModel.searchString}');
                  var entryStream = _getEntries(searchStringModel.searchString);
                  return EntryList(entryStream);
                }
              ),

I don't think I understand didUpdateWidget in particular and suspect that's where the two issues (bad state, and not updating) are coming from? I'm also re-constructed the stateful widget instead of just modifying its state which is a little confusing, but the widget is intended to act as a stateless widget for all intents and purposes so it'd be a pain to update it to update state instead of reconstructing. Any advice?

score 2 · Accepted Answer · answered Nov 18 '20 at 20:20

I had not read the "didUpdateWidget" documentation closely enough. This is the key paragraph:

  /// If the parent widget rebuilds and request that this location in the tree
  /// update to display a new widget with the same [runtimeType] and
  /// [Widget.key], the framework will update the [widget] property of this
  /// [State] object to refer to the new widget and then call this method
  /// with the previous widget as an argument.

In my consumer, I'm rebuilding with a new EntryList. As the above paragraph describes, the framework will then call didUpdateWidget on the same state object (as opposed to a new one). In order to check whether or not the configuration has changed and, if it has, to access the new configuration, I need to access the widget property of my state object. With this in mind, here is my new implementation of didUpdateWidget:

  @override
  void didUpdateWidget(EntryList oldEntryList) {
    super.didUpdateWidget(oldEntryList);
    // Initialize the stream only if it has changed
    if (widget._entryStream != _entryStream) {
      _entryStream = widget._entryStream;
      initStream();
    }
  }

Note that I am now checking widet._entryStream and then setting the state's _entryStream to it if it's changed.

Hopefully someone else running into a similar problem will be helped by this!

using didUpdateWidget to replace a stream causes "bad state: Stream has already been listened to"

1 Answers1