Easiest way to scale your management of your Flutter page/widget state without any package.

page_state_notifier.dart

import 'dart:async';

import 'package:flutter/foundation.dart';
import 'package:flutter/widgets.dart';

/// Defines the possible states of a UI page or widget.
///
/// This enum helps to clearly represent different stages of a UI,
/// such as waiting for data (`loading`), displaying content (`success`),
/// or indicating an issue (`error`).
enum PageState {
  /// The initial or default state.
  /// No operation is currently in progress, and no error has occurred.
  idle,

  /// Indicates that an asynchronous operation is in progress,
  /// such as fetching data from an API.
  loading,

  /// Indicates that an error has occurred during an operation.
  /// An associated error object can provide more details.
  error,

  /// Indicates that an operation has completed successfully.
  success;

  /// Returns `true` if the current state is [PageState.loading].
  bool get isLoading => this == loading;

  /// Returns `true` if the current state is [PageState.error].
  bool get isError => this == error;

  /// Returns `true` if the current state is [PageState.success].
  bool get isSuccess => this == success;

  /// Returns `true` if the current state is [PageState.idle].
  bool get isIdle => this == idle;
}

/// A `ChangeNotifier` that helps manage and communicate the state
/// of a UI page or widget throughout its lifecycle.
///
/// This class simplifies state management by providing a clear way
/// to set and react to different `PageState` values. It's ideal for
/// scenarios where a UI needs to respond to asynchronous operations
/// (e.g., data loading, form submissions) by displaying appropriate
/// visual feedback (loading indicators, error messages, success views).
///
/// Type parameter `Error`: The type of the error object that can be
/// stored when the state is [PageState.error].
///
/// Example Usage:
/// ```dart
/// class MyPage extends StatefulWidget {
///   @override
///   _MyPageState createState() => _MyPageState();
/// }
///
/// class _MyPageState extends State<MyPage> {
///   final pageStateNotifier = PageStateNotifier<String>(); // Error type is String
///
///   @override
///   void initState() {
///     super.initState();
///     // Listen for state changes to perform side effects
///     pageStateNotifier.onSuccess(() {
///       ScaffoldMessenger.of(context).showSnackBar(
///         SnackBar(content: Text('Operation successful!')),
///       );
///     });
///     pageStateNotifier.onError((error) {
///       ScaffoldMessenger.of(context).showSnackBar(
///         SnackBar(content: Text('Error: $error')),
///       );
///     });
///   }
///
///   Future<void> _fetchData() async {
///     return Future.value();
///   }
///
///   @override
///   Widget build(BuildContext context) {
///     return Scaffold(
///       appBar: AppBar(title: Text('State Notifier Example')),
///       body: Center(
///         child: Column(
///           mainAxisAlignment: MainAxisAlignment.center,
///           children: [
///             // Use stateBuilder to render different UI based on the PageState
///             pageStateNotifier.stateBuilder(
///               idle: () => Text('Press button to load data'),
///               loading: () => CircularProgressIndicator(),
///               success: () => Text('Data loaded!'),
///               error: (error) => Text('Failed: $error'),
///             ),
///             SizedBox(height: 20),
///             _pageDataNotifier.builder((state) {
///               return ElevatedButton(
///                 onPressed: state.isLoading ? null : _fetchData,
///                 child: state.isLoading ? Text('Loading...') : Text('Load Data'),
///               );
///            });
///           ],
///         ),
///       ),
///     );
///   }
///
///   @override
///   void dispose() {
///     pageStateNotifier.dispose();
///     super.dispose();
///   }
/// }
/// ```
class PageStateNotifier<Error extends Object?> extends ChangeNotifier {
  PageState _state = PageState.idle;

  /// The current [PageState] of the notifier.
  ///
  /// This getter provides the current state, which should be used
  /// to control the UI's appearance and behavior.
  PageState get state => _state;

  void _setState(final PageState state) {
    _state = state;
    notifyListeners();
  }

  /// Sets the current state to [PageState.success] and notifies
  /// all registered listeners, triggering UI updates.
  void setSuccess() => _setState(PageState.success);

  /// Returns `true` if the current [state] is [PageState.success].
  bool get isSuccess => _state.isSuccess;

  Error? _error;

  /// Sets an error object and transitions the state to [PageState.error].
  ///
  /// This method notifies all registered listeners, enabling the UI
  /// to display error messages or appropriate feedback.
  ///
  /// `error`: An optional error object. If not provided, the existing
  /// error (if any) will remain, or it will be set to `null`.
  void setError([final Error? error]) {
    _error = error;
    _setState(PageState.error);
  }

  /// The error object associated with the [PageState.error] state.
  ///
  /// If the current `state` is [PageState.error], this getter returns
  /// the error object that was set. Otherwise, it returns `null`.
  Error? get error => _error;

  /// Returns `true` if the current [state] is [PageState.error].
  bool get isError => _state.isError;

  /// Returns `true` if the current [state] is [PageState.idle].
  bool get isIdle => _state.isIdle;

  /// Returns `true` if the current [state] is [PageState.loading].
  bool get isLoading => _state.isLoading;

  /// Sets the current state to [PageState.idle] and notifies
  /// all registered listeners.
  ///
  /// Use this to reset the state to its initial or inactive status.
  void setIdle() => _setState(PageState.idle);

  /// Sets the current state to [PageState.loading] and notifies
  /// all registered listeners.
  ///
  /// Use this when an asynchronous operation begins.
  void setLoading() => _setState(PageState.loading);

  /// Registers a callback to be executed specifically when the
  /// `state` changes to [PageState.error].
  ///
  /// The provided `callback` will receive the current `error` object.
  /// This is useful for displaying error messages or performing error-specific
  /// actions.
  ///
  /// `callback`: A function that takes an `Error?` object.
  void onError(final void Function([Error? error]) callback) => addListener(() {
    if (isError) callback(error);
  });

  /// Registers a callback to be executed specifically when the
  /// `state` changes to [PageState.success].
  ///
  /// This is useful for navigating, showing success messages, or
  /// performing other actions upon successful completion.
  ///
  /// `callback`: A function that takes no arguments.
  void onSuccess(final VoidCallback callback) => addListener(() {
    if (isSuccess) callback();
  });

  /// Registers a callback to be executed specifically when the
  /// `state` changes to [PageState.loading].
  ///
  /// This is useful for displaying loading indicators or preventing
  /// multiple concurrent operations.
  ///
  /// `callback`: A function that takes no arguments.
  void onLoading(final VoidCallback callback) => addListener(() {
    if (isLoading) callback();
  });

  /// Registers a callback to be executed specifically when the
  /// `state` changes to [PageState.idle].
  ///
  /// This can be used to reset UI elements or perform actions
  /// when the page returns to an inactive state.
  ///
  /// `callback`: A function that takes no arguments.
  void onIdle(final VoidCallback callback) => addListener(() {
    if (isIdle) callback();
  });

  /// Creates a [ListenableBuilder] that rebuilds its UI whenever
  /// the `PageStateNotifier`'s `state` changes.
  ///
  /// This is a generic builder that provides the current `PageState`
  /// to its `builder` function, allowing for flexible UI construction.
  ///
  /// `builder`: A function that receives the current [PageState]
  /// and returns a [Widget].
  Widget builder(Widget Function(PageState state) builder) => ListenableBuilder(
    listenable: this,
    builder: (final context, final _) => builder(state),
  );

  /// Creates a [ListenableBuilder] that renders different widgets
  /// based on the current [PageState].
  ///
  /// This method provides a convenient way to define separate UI components
  /// for each state ([idle], [loading], [success], [error]), making it
  /// easy to manage complex UI flows.
  ///
  /// `idle`: A widget builder for the [PageState.idle] state. Defaults
  /// to `SizedBox.shrink()`.
  /// `loading`: An optional widget builder for the [PageState.loading] state.
  /// `success`: An optional widget builder for the [PageState.success] state.
  /// `error`: An optional widget builder for the [PageState.error] state.
  ///   It receives the current `error` object.
  /// `fallbackState`: The state to fall back to if a specific state's
  ///   widget builder is not provided and that state becomes active.
  ///   Assertions ensure that a widget builder is provided if `fallbackState`
  ///   is set to `loading`, `error`, or `success`.
  ///
  /// Example:
  /// ```dart
  /// pageStateNotifier.stateBuilder(
  ///   idle: () => Text('Ready to start'),
  ///   loading: () => CircularProgressIndicator(),
  ///   success: () => Icon(Icons.check_circle),
  ///   error: (e) => Text('Error: ${e ?? 'Unknown error'}'),
  /// );
  /// ```
  Widget stateBuilder({
    final Widget Function() idle = SizedBox.shrink,
    final Widget Function()? loading,
    final Widget Function()? success,
    final Widget Function(Error? error)? error,
    final PageState fallbackState = PageState.idle,
  }) {
    assert(
      !fallbackState.isLoading || loading != null,
      'Your fallbackState is loading, but you did not provide a loading widget',
    );
    assert(
      !fallbackState.isError || error != null,
      'Your fallbackState is error, but you did not provide an error widget',
    );
    assert(
      !fallbackState.isSuccess || success != null,
      'Your fallbackState is success, but you did not provide a success widget',
    );

    return ListenableBuilder(
      listenable: this,
      builder: (final context, final _) {
        final widgetBuilder = switch (_state) {
          PageState.idle => idle(),
          PageState.loading => loading?.call(),
          PageState.success => success?.call(),
          PageState.error => error?.call(_error),
        };

        if (widgetBuilder == null) {
          return switch (fallbackState) {
            PageState.idle => idle(),
            PageState.loading => loading!.call(),
            PageState.error => error!.call(_error),
            PageState.success => success!.call(),
          };
        }

        return widgetBuilder;
      },
    );
  }
}

a mixin aggregation example:

you can reuse UserMixin in any class that extends PageStateNotifier,
and any controllers can have many mixins it needs

class AuthController extends PageStateNotifier with UserMixin {
  void createNewUser({
    required String email, 
    required String password,
  }) {
    setEmail(email); // or `this.name = name;` by using setter
    setPassword(password);
  }
}

mixin UserMixin on PageStateNotifier {
  String? _email;
  String? get email => _email;
  bool get isEmailValid => email != null && email!.isNotEmpty;

  String? _password;
  String? get password => _password;
  bool get isPasswordValid => password != null && password!.isNotEmpty;

  bool get isUserValid => isEmailValid && isPasswordValid;

  // same as `setEmail` but using setter 
  set email(String? email) {
    _email = email;
    notifyListeners();
  }

  void setEmail(String email) {
    _email = email;
    notifyListeners();
  }

  void setPassword(String password) {
    _password = password;
    notifyListeners();
  }
}

with multiples mixins:

class AuthController extends PageStateNotifier with UserMixin, AccountMixin, ExampleMixin {}

other utils

ComputedNotifier:

ComputedNotifier lets you combine the values of multiple listenables into a new ValueListenable that can be reacted to, or even used by additional ComputedNotifier.

import 'dart:async';

import 'package:flutter/foundation.dart';
import 'package:flutter/widgets.dart';

/// Provides utility methods for any object that implements [Listenable].
///
/// This extension simplifies common operations with listenable objects,
/// especially in the context of Flutter widgets.
extension ListenableUtils on Listenable {
  /// Creates a [ListenableBuilder] that rebuilds its UI whenever this
  /// [Listenable] notifies its listeners.
  ///
  /// This is a convenient way to react to changes from any [Listenable]
  /// (e.g., `ChangeNotifier`, `AnimationController`) and update a part
  /// of the widget tree.
  ///
  /// Example:
  /// ```dart
  /// final myNotifier = ChangeNotifier();
  /// // In your build method:
  /// myNotifier.builder(() {
  ///   return Text('Notifier updated!');
  /// });
  /// ```
  ///
  /// `builder`: A function that returns the [Widget] to be built
  /// when the listenable changes.
  Widget builder(final Widget Function() builder) {
    return ListenableBuilder(
      listenable: this,
      builder: (context, _) {
        return builder();
      },
    );
  }

  /// Creates a [ComputedNotifier] that observes changes in this [Listenable]
  /// and recomputes a derived value.
  ///
  /// This is useful for creating "computed" or "derived" states that
  /// automatically update when their dependencies change. The `value`
  /// of the returned `ComputedNotifier` will be re-evaluated whenever
  /// this `Listenable` notifies.
  ///
  /// Example:
  /// ```dart
  /// final counter = ValueNotifier<int>(0);
  /// final isEven = counter.computed(() => counter.value % 2 == 0);
  ///
  /// // In your build method:
  /// isEven.builder((value) => Text('Is counter even? ${value.toString()}'));
  /// // When counter changes, isEven will automatically recompute and update.
  /// ```
  ///
  /// `value`: A function that computes the derived value `T`. This function
  /// will be re-executed when this `Listenable` changes.
  ///
  /// Returns a [ComputedNotifier] of type `T`.
  ValueListenable computed<T>(T Function() value) {
    return ComputedNotifier<T>([this], value);
  }
}

/// A [ValueListenable] that recomputes its value based on changes
/// in a list of other [Listenable] dependencies.
///
/// `ComputedNotifier` acts like a reactive variable. When any of its
/// registered `listenableList` dependencies notify a change, it re-evaluates
/// its `_compute` function. If the recomputed value is different from
/// the previous one, it then notifies its own listeners.
/// This is a powerful pattern for creating derived state that automatically
/// stays in sync with its sources.
///
/// Type parameter `T`: The type of the value that this `ComputedNotifier` holds.
class ComputedNotifier<T> extends ChangeNotifier implements ValueListenable<T> {
  /// Creates a `ComputedNotifier`.
  ///
  /// `listenableList`: A list of [Listenable] objects that this
  /// `ComputedNotifier` will observe. Any change in these listenables
  /// will trigger a re-computation of this notifier's value.
  /// `_compute`: A function that defines how the value of this notifier
  /// is derived. This function is called initially and then whenever
  /// any of the `listenableList` dependencies change.
  ComputedNotifier(List<Listenable> listenableList, this._compute)
      : _currentValue = _compute() {
    _listenable = Listenable.merge(listenableList);
    _listenable.addListener(_onDependenciesChanged);

    // Enables Flutter DevTools memory allocation tracking if available.
    if (kFlutterMemoryAllocationsEnabled) {
      ChangeNotifier.maybeDispatchObjectCreation(this);
    }
  }

  /// The function used to compute the value of this notifier.
  /// This function will be called whenever a dependency changes.
  final T Function() _compute;

  /// A merged [Listenable] that aggregates all the individual listenables
  /// provided in the constructor.
  late final Listenable _listenable;

  /// The current value held by this notifier.
  /// This value is updated when `_compute` is re-evaluated and its
  /// result differs from the previous value.
  late T _currentValue;

  /// Determines if the notifier should notify its listeners after
  /// a dependency change.
  ///
  /// This check prevents unnecessary rebuilds by only notifying if the
  /// `_compute` function returns a new value.
  bool get _shouldNotify {
    final T newValue = _compute();
    return newValue != _currentValue;
  }

  /// Callback function executed when any of the observed dependencies change.
  ///
  /// It recomputes the value and notifies listeners only if the value
  /// has actually changed.
  void _onDependenciesChanged() {
    if (_shouldNotify) {
      _currentValue = _compute();
      notifyListeners();
    }
  }

  @override
  /// The current value of this computed notifier.
  ///
  /// This value is derived from the `_compute` function and is updated
  /// automatically when dependencies change.
  T get value => _currentValue;

  @override
  /// Disposes the `ComputedNotifier` by removing its listener from
  /// the aggregated [Listenable].
  ///
  /// It's crucial to call this method when the `ComputedNotifier` is no
  /// longer needed to prevent memory leaks.
  void dispose() {
    _listenable.removeListener(_onDependenciesChanged);
    super.dispose();
  }

  @override
  /// A string representation of the `ComputedNotifier`, including its
  /// current value for debugging purposes.
  String toString() => '${describeIdentity(this)}($value)';
}