flutter_stasis

Flutter layer of the Stasis ecosystem. Provides StasisViewModel, reactive widgets, and ephemeral UI events.

For the pure-Dart contracts (StateObject, Command, CommandPolicy) see flutter_stasis_core. For dartz Either integration see flutter_stasis_dartz.

---

Installation

dependencies:
  flutter_stasis: ^1.0.0

---

StasisViewModel

Base class for all ViewModels. Requires an initial StateObject and exposes helpers to transition lifecycle and execute async commands.

class ProjectsViewModel
    extends StasisViewModel<AppFailure, List<Project>, ProjectsState> {

  ProjectsViewModel(this._getProjects)
      : super(ProjectsState.initial());

  final GetProjectsUseCase _getProjects;

  Future<void> load() => execute(
    command: _getProjects,
    onLoading: setLoading,
    onError: setError,
    onSuccess: setSuccess,
  );

  void setFilter(ProjectFilter f) =>
      update((s) => s.copyWith(filter: f));
}

State helpers

setLoading()                         // → LoadingState
setSuccess(data)                     // → SuccessState(data)
setError(failure)                    // → ErrorState(failure)
setInitialState()                    // → InitialState
update((s) => s.copyWith(...))       // update fields, keep lifecycle

Combine lifecycle transition + field update in one call with withUpdate:

setSuccess(
  audioFile,
  withUpdate: (s) => s.copyWith(waveform: audioFile.waveform),
);

setError is the automatic fallback for execute when onError is not provided.

execute

Single entry point for all async actions.

onSuccess is required — you must decide what to do with the result.
onError is optional — when omitted, setError is called automatically.
onLoading is optional — pass setLoading when you want a loading indicator.

// Full form
Future<void> load() => execute(
  command: _getProjects,
  onSuccess: setSuccess,
  onError: setError,    // optional, this is the default behaviour
  onLoading: setLoading,
);

// Minimal — onError falls back to setError automatically
Future<void> load() => execute(
  command: _getProjects,
  onSuccess: setSuccess,
);

// Custom error handling — override the default
Future<void> load() => execute(
  command: _getProjects,
  onSuccess: setSuccess,
  onError: (f) => emit(ShowSnackBarEvent(f.message)),
);

// With concurrency policy
Future<void> search(String query) => execute(
  command: TaskCommand(() => _searchUseCase(query)),
  onSuccess: setSuccess,
  onLoading: setLoading,
  policy: CommandPolicy.restartable,
);

See flutter_stasis_core for Command, CommandPolicy and result composition.

Lifecycle

class _ScreenState extends State<MyScreen> {
  late final vm = MyViewModel(getIt());

  @override
  void dispose() {
    vm.dispose(); // clears state, events, and concurrency scope
    super.dispose();
  }
}

---

StasisBuilder

Rebuilds whenever the StateObject changes:

StasisBuilder(
  listenable: vm.stateListenable,
  builder: (context, state, _) {
    if (state.isLoading) return const Loader();
    if (state.isError)   return ErrorBanner(state.errorMessage!);
    return ProjectList(state.projects ?? []);
  },
);

---

StasisSelector

Rebuilds only when the selected value changes. Use this inside StasisBuilder to avoid rebuilding heavy subtrees:

// Rebuilds only when isDragging changes
StasisSelector<AudioState, bool>(
  listenable: vm.stateListenable,
  selector: (state) => state.isDragging,
  builder: (context, isDragging, _) => DropZone(active: isDragging),
);

Custom equality for complex types:

StasisSelector<AudioState, List<double>>(
  listenable: vm.stateListenable,
  selector: (state) => state.waveform,
  equals: (a, b) => a.length == b.length,
  builder: (context, waveform, _) => WaveformWidget(points: waveform),
);

---

StasisEventListener

One-shot events that should never be stored in state — navigation, snackbars, dialogs.

The ViewModel dispatches; the View reacts:

// Define your events
final class ShowSnackBarEvent extends UiEvent {
  const ShowSnackBarEvent(this.message);
  final String message;
}

final class PopEvent extends UiEvent {
  const PopEvent();
}

// ViewModel dispatches
void onSaved() {
  emit(const ShowSnackBarEvent('Saved successfully'));
  emit(const PopEvent());
}

// View reacts
StasisEventListener(
  stream: vm.events,
  onEvent: (context, event) async {
    switch (event) {
      case ShowSnackBarEvent(:final message):
        ScaffoldMessenger.of(context)
            .showSnackBar(SnackBar(content: Text(message)));
      case PopEvent():
        Navigator.of(context).pop();
    }
  },
  child: ...,
);

---

Full example

// state
class CounterState extends StateObject<String, int, CounterState> {
  const CounterState({required super.state});

  int get count => dataOrNull ?? 0;

  @override
  CounterState withState(ViewModelState<String, int> state) =>
      CounterState(state: state);

  @override
  List<Object?> get props => [state];
}

// view model
class CounterViewModel extends StasisViewModel<String, int, CounterState> {
  CounterViewModel()
      : super(const CounterState(state: SuccessState(0)));

  Future<void> increment() => execute(
    command: TaskCommand(() async {
      await Future.delayed(const Duration(milliseconds: 200));
      return CommandSuccess((state.count) + 1);
    }),
    onSuccess: setSuccess,
    onLoading: setLoading,
    policy: CommandPolicy.droppable,
  );
}

// screen
class CounterScreen extends StatefulWidget {
  const CounterScreen({super.key});

  @override
  State<CounterScreen> createState() => _CounterScreenState();
}

class _CounterScreenState extends State<CounterScreen> {
  final vm = CounterViewModel();

  @override
  void dispose() {
    vm.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) => Scaffold(
    body: Center(
      child: StasisBuilder(
        listenable: vm.stateListenable,
        builder: (_, state, __) => state.isLoading
            ? const CircularProgressIndicator()
            : Text('${state.count}', style: const TextStyle(fontSize: 48)),
      ),
    ),
    floatingActionButton: FloatingActionButton(
      onPressed: vm.increment,
      child: const Icon(Icons.add),
    ),
  );
}

Usage patterns

Stasis works regardless of how your use cases return results. Pick the pattern that fits your project.

---

Pattern 1 — Use case with dartz + adapter installed

The most ergonomic path if you already use dartz. Import flutter_stasis_dartz and call executeEither directly with your use case.

// use case (your domain layer — untouched)
class GetProjectsUseCase {
  GetProjectsUseCase(this._repository);
  final ProjectRepository _repository;

  Future<Either<AppFailure, List<Project>>> call() =>
      _repository.getAll();
}

// view model
import 'package:flutter_stasis_dartz/flutter_stasis_dartz.dart';

class ProjectsViewModel
    extends StasisViewModel<AppFailure, List<Project>, ProjectsState> {

  ProjectsViewModel(this._getProjects) : super(ProjectsState.initial());

  final GetProjectsUseCase _getProjects;

  Future<void> load() => executeEither(
    command: _getProjects.call,
    onSuccess: setSuccess,         // onError omitted → setError called automatically
    onLoading: setLoading,
  );
}

---

Pattern 2 — Use case with dartz, no adapter

If you use dartz but don't want to add flutter_stasis_dartz, convert the Either manually inside a TaskCommand. One extra line, no extra dependency.

// use case (same as above)
class GetProjectsUseCase {
  Future<Either<AppFailure, List<Project>>> call() =>
      _repository.getAll();
}

// view model — no adapter import needed
class ProjectsViewModel
    extends StasisViewModel<AppFailure, List<Project>, ProjectsState> {

  ProjectsViewModel(this._getProjects) : super(ProjectsState.initial());

  final GetProjectsUseCase _getProjects;

  Future<void> load() => execute(
    command: TaskCommand(() async {
      final result = await _getProjects();
      return result.fold(
        (failure) => CommandFailure(failure),
        (data)    => CommandSuccess(data),
      );
    }),
    onSuccess: setSuccess,         // onError omitted → setError called automatically
    onLoading: setLoading,
  );
}

---

Pattern 3 — No dartz, no adapter

Use cases return plain Future and throw on failure, or you catch errors yourself. Wrap everything in a TaskCommand and return CommandSuccess / CommandFailure directly.

// use case — plain Future, throws on error
class GetProjectsUseCase {
  Future<List<Project>> call() => _repository.getAll();
}

// view model
class ProjectsViewModel
    extends StasisViewModel<AppFailure, List<Project>, ProjectsState> {

  ProjectsViewModel(this._getProjects) : super(ProjectsState.initial());

  final GetProjectsUseCase _getProjects;

  Future<void> load() => execute(
    command: TaskCommand(() async {
      try {
        final data = await _getProjects();
        return CommandSuccess(data);
      } on NetworkException catch (e) {
        return CommandFailure(AppFailure(e.message));
      } catch (e) {
        return CommandFailure(AppFailure('Unexpected error'));
      }
    }),
    onSuccess: setSuccess,         // onError omitted → setError called automatically
    onLoading: setLoading,
  );
}

Alternatively, if you want the use case to already speak CommandResult, implement Command<F, R> directly — then pass it straight to execute with no wrapper:

// use case implements Command directly
class GetProjectsUseCase implements Command<AppFailure, List<Project>> {
  @override
  Future<CommandResult<AppFailure, List<Project>>> call() async {
    try {
      final data = await _repository.getAll();
      return CommandSuccess(data);
    } on NetworkException catch (e) {
      return CommandFailure(AppFailure(e.message));
    }
  }
}

// view model — cleanest form, no wrapper, no onError needed
Future<void> load() => execute(
  command: _getProjects,    // use case IS the command
  onSuccess: setSuccess,
);

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License

MIT