Why does StreamController not work properly when initialized inside function

Question

Hoping someone(s) can help explain to me why this happens cause it's not making sense to me and has been annoyingly taken days to figure out what was causing this issue.

The basic situation is when I define the StreamController globally it works fine, but once I try to define it within function the StreamBuilder never gets notified of new data/messages:

class _MyHomePageState extends State<MyHomePage> {
  static StreamController<bool?>? streamController = StreamController();   // Works
  // static StreamController<bool?>? streamController;                     // Doesn't work


  static void start() async {
    // streamController = StreamController();                              // Doesn't work

    streamController!.sink.add(true);
  }

  @override
  Widget build(BuildContext context) {
   ...
            StreamBuilder<bool?>(
                stream: streamController?.stream,
                builder: (context, snapshot) {
                  switch (snapshot.connectionState) {
                    case ConnectionState.active:
                      _counter++;
                      continue keepGoing;
                    keepGoing:
                    case ConnectionState.done:
                    case ConnectionState.waiting:
                      return Text(
                        '$_counter',
                        style: Theme.of(context).textTheme.headlineMedium,
                      );
                    case ConnectionState.none:
                      return const CircularProgressIndicator();
                      return const Placeholder();
                    default:
                      return const Text("Unknown State of Stream.");
                  }
                }),
            FloatingActionButton(
              onPressed: () => start(),
              tooltip: 'Increment',
              child: const Icon(Icons.play_arrow_outlined),
            ),
  ...
  }
}

My current guess is that the reference to the StreamController in the StreamBuilder never gets the updated reference but I'm doubtful of that since I believe the Widget's state should get updated with the button press. Any other suggestions and explanations would be great.

Thanks in advance.

score 2 · Accepted Answer · answered May 23 '23 at 08:11

As you have already realised, the reference is not updated. A button press doesn't trigger a rebuild (that wouldn't make sense either, because the button can't know if you cause a state change). You assign streamController?.stream to your StreamBuilder, which only does a rebuild at the beginning before you assign a value to streamController. If you want the UI to update itself when your State changes, you should use setState:

setState(() => streamController = StreamController())

Is it intended that the streamController is static and reassigned every time a button is pressed without being closed first? Even if you use setState here, no value will reach the StreamBuilder because you call streamController.add(true) before the actual rebuild has been performed and the StreamBuilder has received the new stream reference.

Or are you just trying out streams?

Thanks, yeah I ended up trying out setState soon after posting this and that did it . But yeah I'm reassigning the streamcontroller in my actual app. This was a simplified version that was focused on the unexpected streamcontroller behavior. The equivalent behavior in this simplified example would be to only have the button available went the counter is less than 1. — Bernardin Dezius, May 23 '23 at 16:43

Why does StreamController not work properly when initialized inside function

1 Answers1