main.dart

abstract class _State {
  T use<T>(
      T Function(_ChooseProviderState) useChooseProviderState,
      T Function(_LoadingState) useLoadingState,
      T Function(_ChooseAccountsState) useChooseAccountState,
      T Function(_ImportingState) useImportingState) {
    if (this is _ChooseProviderState) {
      return useChooseProviderState(this);
    }
    if (this is _LoadingState) {
      return useLoadingState(this);
    }
    if (this is _ChooseAccountsState) {
      return useChooseAccountState(this);
    }
    if (this is _ImportingState) {
      return useImportingState(this);
    }
    throw Exception('Invalid state');
  }
}

class _ChooseProviderState extends _State {}

class _LoadingState extends _State {}

class _ChooseAccountsState extends _State {
  final List<String> accounts;
  final Set<String> selected;

  _ChooseAccountsState(this.accounts) : selected = Set();
}

class _ImportingState extends _State {
  final List<String> imported;
  final List<String> failed;
  final List<String> processing;
  final List<String> queue;

  _ImportingState(this.imported, this.failed, this.processing, this.queue);
}

// just to compile
class Widget {
  final String something;
  Widget(this.something);
}

class BuildContext {}

class MyWidget {
  _State _state = _LoadingState();
  @override
  Widget build(BuildContext context) {
    return _state.use(
        (chooseProvider) => _buildChooseProvider(context, chooseProvider),
        (loading) => _buildLoading(context, loading),
        (chooseAccounts) => _buildChooseAccounts(context, chooseAccounts),
        (importing) => _buildImporting(context, importing));
  }

  _buildChooseProvider(BuildContext context, _ChooseProviderState state) =>
      Widget("choose provider");
  _buildLoading(BuildContext context, _LoadingState state) => Widget("loading");
  _buildChooseAccounts(BuildContext context, _ChooseAccountsState state) =>
      Widget("choose accounts");
  _buildImporting(BuildContext context, _ImportingState state) =>
      Widget("importing");
}

main() {
  print(MyWidget().build(BuildContext()).something);
}