Is there a way to refresh data?

[nsxz1234]

when I use riverpod, I can use ref.invalidate()

[jinyus]

ref.invalidate just re executes the future that's used to compute the initial state. I don't think it's possible with the current design, because signalFromFuture returns a ReadOnlySignal which can't be modified from the outside, so it would need to expose a function that can re-execute the future that was passed in.

I modified the code to get this working but it's very hacky and changes the public API so lets wait to hear what the maintainer has to say about this.

[rodydavis]

I am totally open for it. I also used this a lot in Riverpod.

I think how the fix for the widget effect subscribing was fixed with a set, we could do the same here.

But I like returning a read only by default and introducing a mutable signal as another callback.

It is open for discussion though!

[rodydavis]

@jinyus can you share the hack?

[jinyus]

@rodydavis Since the signal cannot be updated from the outside, I returned a record with the signal along with a function to invalidate the state. I also removed an unnecessary subscription and the start variable.

https://github.com/rodydavis/preact_signals.dart/blob/main/packages/preact_signals/lib/src/extensions/future.dart#L16

({ReadonlySignal<SignalState> signal, VoidCallback refresh})
    signalFromFuture<T>(Future<T> Function() getFuture) {
  final s = signal<SignalState>(SignalLoading());

  Future<SignalState> execute() async {
    try {
      return SignalValue(await getFuture());
    } catch (e) {
      return SignalError(e);
    }
  }

  init() {
    if (s.peek() is! SignalLoading) {
      s.value = SignalLoading();
    }
    execute().then((value) => s.value = value);
  }

  init();

  return (signal: s, refresh: init);
}

Another limitation I discovered is that once a future completes, it cannot be reset so no loading state would appear when refreshing, it would just return the old value instantly...so signalFromFuture now takes a function that should return a unique future, downside is that the onus is on the caller to make sure the future is unique. ie: not const:

Good: Future.delayed(Duration(seconds: 1), () => 'abc')
Bad: Future.delayed(const Duration(seconds: 1), () => 'abc')

edit: It looks like this isn't a problem with real world scenarios. as Future.delayed(const Duration(seconds: 1), () => getStr()) works fine.

In flutter:

final futureSignal = signalFromFuture<String>(
  () => Future.delayed(Duration(seconds: 1), () => 'abc'),
);

class MyHomePage extends StatelessWidget {
  const MyHomePage({super.key, required this.title});

  final String title;

  void _reloadData() {
    futureSignal.refresh();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Builder(builder: (context) {
              final state = futureSignal.signal.watch(context);
              final text = switch (state) {
                SignalError(value: var v) => 'Error: $v',
                SignalValue(value: var v) => 'Success: $v',
                _ => 'Loading...',
              };
              return Text(
                text,
                style: Theme.of(context).textTheme.headlineMedium!,
              );
            }),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _reloadData,
        child: const Icon(Icons.refresh),
      ),
    );
  }
}

flutter_signal_demo.mp4

[jinyus]

@rodydavis I solved the problem in a cleaner way by creating a FutureSignal class that extends Signal, just created a PR.

[rodydavis]

Can we do the same for Streams?

[rodydavis]

Fixed with: #7