Base state that can be used in Flutter apps using BLoC (inspired by Riverpod).

base.state.dart

import 'package:equatable/equatable.dart';
import 'package:meta/meta.dart';

sealed class BaseState<T> extends Equatable {
  bool get isInitial;
  bool get isLoading;
  bool get isSuccessful;
  bool get hasFailed;
  T? get data;
  Exception? get exception;

  const BaseState._();

  const factory BaseState.initial() = InitialState<T>;
  const factory BaseState.loading() = LoadingState<T>;
  const factory BaseState.successful(T data) = SuccessfulState<T>;
  const factory BaseState.failed(Exception exception) = FailedState<T>;
}

typedef OnInitialOrLoading<R> = R Function();
typedef OnData<T, R> = R Function(T data);
typedef OnException<R> = R Function(Exception exception);

extension BaseStateExt<T> on BaseState<T> {
  T get _safeData {
    if (isSuccessful && data != null) {
      return data as T;
    }

    throw StateError('Cannot have null data.');
  }

  R when<R>({
    required OnInitialOrLoading<R> onInitial,
    required OnInitialOrLoading<R> onLoading,
    required OnData<T, R> onData,
    required OnException<R> onException,
  }) {
    if (isInitial &&
        !isLoading &&
        !isSuccessful &&
        !hasFailed &&
        data == null &&
        exception == null) {
      return onInitial();
    }

    if (isLoading) {
      return onLoading();
    }

    if (hasFailed || exception != null) {
      return onException(exception!);
    }

    return onData(_safeData);
  }
}

@immutable
final class InitialState<T> extends BaseState<T> {
  @override
  bool get isInitial => true;

  @override
  bool get isLoading => false;

  @override
  bool get isSuccessful => false;

  @override
  bool get hasFailed => false;

  @override
  final T? data;

  @override
  final Exception? exception;

  const InitialState()
      : data = null,
        exception = null,
        super._();

  @override
  List<Object?> get props => [
        isLoading,
        isSuccessful,
        hasFailed,
        data,
        exception,
      ];
}

@immutable
final class LoadingState<T> extends BaseState<T> {
  @override
  bool get isInitial => false;

  @override
  bool get isLoading => true;

  @override
  bool get isSuccessful => false;

  @override
  bool get hasFailed => false;

  @override
  final T? data;

  @override
  final Exception? exception;

  const LoadingState()
      : data = null,
        exception = null,
        super._();

  @override
  List<Object?> get props => [
        isLoading,
        isSuccessful,
        hasFailed,
        data,
        exception,
      ];
}

@immutable
final class SuccessfulState<T> extends BaseState<T> {
  @override
  bool get isInitial => false;

  @override
  bool get isLoading => false;

  @override
  bool get isSuccessful => true;

  @override
  bool get hasFailed => false;

  @override
  final T data;

  @override
  final Exception? exception;

  const SuccessfulState(this.data)
      : exception = null,
        super._();

  @override
  List<Object?> get props => [
        isLoading,
        isSuccessful,
        hasFailed,
        data,
        exception,
      ];
}

@immutable
final class FailedState<T> extends BaseState<T> {
  @override
  bool get isInitial => false;

  @override
  bool get isLoading => false;

  @override
  bool get isSuccessful => false;

  @override
  bool get hasFailed => true;

  @override
  final T? data;

  @override
  final Exception exception;

  const FailedState(this.exception)
      : data = null,
        super._();

  @override
  List<Object?> get props => [
        isLoading,
        isSuccessful,
        hasFailed,
        data,
        exception,
      ];
}