Clarify usage of injected autoDisposal signals inside modals

[JohnCido]

Let's say we have this structure:

class ViewModel {
  ...

  static final provider = Provider(
    (_) => ViewModel(),
  );

  final signal = Signal<int>(0);
}

...
  showBottomSheet(
    ...
    builder: (_) => ProviderScopePortal(
      mainContext: context,
      child: SignalBuilder(
        builder: (innerContext, _) {
          final viewModel = ViewModel.provider.of(innerContext);

          return Row(
            children: [
              Text(viewModel.signal().toString()),
              FilledButton(
                onPressed: () => viewModel.signal.updateValue((s) => s + 1),
              ...

The button works as expected when you open the sheet for the first time.

But once you dismiss it and reopen the sheet again. The Signal was already disposed since everything inside the sheet that used to be the subscriber are all disposed. And Signal auto dispose itself by default, the UI lost its reactiveness at this point. If you click the button, nothing changes on UI. Only after you close the sheet and reopen again, the UI get updated.

For new developers in Flutter land like myself, this could be really confuisng. I only figured it out by adding the logger to the config. Before that, I've spent several hours trying to identify the issue.

For use cases like this, should we have a section in the document to indicate how to correctly handle ViewModel based injected signal usage? Or is it possible to handle this inside the solidart library itself?

[nank1ro]

Can you share the code for the widget that triggers the bottom sheet too?
I want to understand when the ViewModel.provider is created for the first time.

[JohnCido]

class SomeWidget extends StatelessWidget {
  ...
  @override
  Widget build(BuildContext context) {
    return ProviderScope(
      providers: [ViewModel.provider],
      child: SignalBuilder(
        builder: (innerContext, _) {
          final viewModel = ViewModel.provider.of(innerContext);

          return Card.filled(
            ...
            FilledButton(
              onPressed: () => showModalBottomSheet(
...

This SomeWidget was placed on the home screen and I can confirm the viewModel was not disposed since the choice was preserved when sheet open state change.

[nank1ro]

Ok, the behaviour is correct.
The ViewModel.provider stays alive as long as the home page is present.
The signal has auto-dispose, which means it will be automatically disposed when there are no observers.
When you open the bottom sheet, the signal is observed, when you close it you are removing the only observer available, so it is disposed.
When you reopen the bottom sheet, you are still using the old signal which is displaced; even if you put the signal inside the ViewModel class, it is not attached to the provider lifecycle, it works independently.

You have two options:

1. Disable the auto-dispose of the signal and add something like

class ViewModel {
 static final provider = Provider(
    (_) => ViewModel(),
+    dispose: (model) => model.dispose(),
  );

+  final signal = Signal<int>(0, autoDispose: false);
+
+  void dispose() {
+    signal.dispose();
+  }
}

2. Create the signal inside the state of a StatefulWidget you use in the bottom sheet. This way the signal only lives when it is rendered, and auto-disposes when the bottom sheet is removed from the widget tree.

[JohnCido]

Should I disable autoDisposal at the app level or should I just disable it for certain signals?

[nank1ro]

Depends on how you intend to use the signals.
I made many apps where I almost never changed the default auto disposal behavior and all is working fine.
All depends on how you architect it.

For example in my apps I put some signals inside "blocs" (business logic) and others directly in the widgets (StatefulWidgets) to handle pure UI stuff.

If you want to have full control, I suggest disabling it.
If you want to easily debug them you can do that by using SolidartConfig.devToolsEnabled = true and by launching the Flutter DevTools.

The green ones are the active signals, while the red ones are disposed.

[JohnCido]

Thanks for your suggestion! I'll try the dev tool to see if disable at app level is necessary.