/// Minimal async state container so screens can pattern-match over
/// loading / data / error instead of juggling three nullable fields.
sealed class AsyncValue<T> {
  const AsyncValue();

  const factory AsyncValue.loading() = AsyncLoading<T>;
  const factory AsyncValue.data(T value) = AsyncData<T>;
  const factory AsyncValue.error(String message) = AsyncError<T>;

  T? get valueOrNull => this is AsyncData<T> ? (this as AsyncData<T>).value : null;

  bool get isLoading => this is AsyncLoading<T>;

  R map<R>({
    required R Function() loading,
    required R Function(T value) data,
    required R Function(String message) error,
  }) {
    return switch (this) {
      AsyncLoading<T>() => loading(),
      AsyncData<T>(value: final v) => data(v),
      AsyncError<T>(message: final m) => error(m),
    };
  }
}

final class AsyncLoading<T> extends AsyncValue<T> {
  const AsyncLoading();
}

final class AsyncData<T> extends AsyncValue<T> {
  const AsyncData(this.value);

  final T value;
}

final class AsyncError<T> extends AsyncValue<T> {
  const AsyncError(this.message);

  final String message;
}