Version: 8.5.0

StateNotifier

If you have a more complex state consider using the StateNotifier class instead of SimpleNotifier.

A StateNotifier stores a single immutable state.

An immutable state is an instance of one Class that overrides == and hashCode. For example you could use equatable or freezed to create an immutable Class.

With equatable#

Add equatable as a dependency in your pubspec.yaml file

equatable: latest_version

Now you can create a Class to manage your state

import 'package:equatable/equatable.dart';

class LoginState extends Equatable {
  final String email, password;
  LoginState({
    required this.email,
    required this.password,
  });

  static LoginState get initialState => LoginState(email: '', password: '');

  LoginState copyWith({
    String? email,
    String? password,
  }) {
    return LoginState(
      email: email ?? this.email,
      password: password ?? this.password,
    );
  }

  @override
  List<Object?> get props => [email, password];
}

With freezed#

To use freezed you need build_runner and freezed_annotation

in your pubspec.yaml file (replace latest_version with the latest version of each dependency)

dependencies:
  flutter:
    sdk: flutter
  freezed_annotation: latest_version

dev_dependencies:
  flutter_test:
    sdk: flutter

  build_runner: latest_version
  freezed: latest_version

If you have conflicts when you try to install freezed check the oficial documentation https://pub.dev/packages/freezed

import 'package:freezed_annotation/freezed_annotation.dart';
part 'login_state.freezed.dart';
@freezed
class LoginState with _$LoginState {
  const LoginState._();
  const factory LoginState({
    @Default('') String email,
    @Default('') String password,
  }) = _LoginState;

  static LoginState get initialState => LoginState();
}

next run the next command to generate the .frezeed.dart files

flutter pub run build_runner build --delete-conflicting-outputs

Now you can use the LoginState class to create a StateNotifier

import 'package:flutter_meedu/meedu.dart';

class LoginController extends StateNotifier<LoginState> {
  // you need pass an inital state using super
  LoginController():super(LoginState.initialState);

  void onEmailChanged(String email) {
    state = state.copyWith(email: email);
  }

  void onPasswordChanged(String password) {
    state = state.copyWith(password: password);
  }
}

info

If you only want to update the state of your StateNotifier but you don't want to notify to the listeners (don't rebuild the Consumer widgets and don't listen the changes in the ProviderListener widget) you can use the onlyUpdate method in your StateNotifier.

class LoginController extends StateNotifier<LoginState> {
  // you need pass an inital state using super
  LoginController():super(LoginState.initialState);

  void onEmailChanged(String email) {
    // only update the state
    onlyUpdate(state.copyWith(email: email));
  }
  .
  .
  .

}

Next you need to create a StateProvider and use the Consumer widget to listen the changes in your state

import 'package:flutter/material.dart';
import 'package:flutter_meedu/ui.dart';
import 'package:flutter_meedu/.dart';


final loginProvider = StateProvider<LoginController, LoginState>(
  (_) => LoginController(),
);


class LoginPage extends StatelessWidget {
  const LoginPage({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SafeArea(
        child: Padding(
          padding: const EdgeInsets.all(8.0),
          child: Column(
            children: [
              TextField(
                onChanged: loginProvider.read.onEmailChanged,
                decoration: InputDecoration(
                  labelText: "Email",
                ),
              ),
              TextField(
                onChanged: loginProvider.read.onPasswordChanged,
                decoration: InputDecoration(
                  labelText: "Password",
                ),
              ),
              SizedBox(height: 30),
              Consumer(
                builder: (_, ref, __) {
                  final controller = ref.watch(loginProvider);
                  final state = controller.state;

                  final email = state.email;
                  final password = state.password;

                  final enabled = email.isNotEmpty && password.isNotEmpty;

                  return ElevatedButton(
                    onPressed: enabled
                        ? () {
                            // YOUR CODE HERE
                          }
                        : null,
                    child: Text("SEND"),
                  );
                },
              )
            ],
          ),
        ),
      ),
    );
  }
}

Also you can use the .select filter to only rebuild your Consumer when is need it.

For example the next code only rebuilds the Consumer widget when the email in our LoginState has changed.

here we use ref.select to direct access to the value returned by loginProvider.select but if you want get the notifier linked to loginProvider you can use ref.watch with loginProvider.select

Consumer(
  builder: (_, ref, __) {
    final email = ref.select(
      loginProvider.select((_)=>_.email),
    );
    return Text(email);
  },
)

note

Also you can use the when method for a more complex condition.

Consumer(
  builder: (_, ref, __) {
    final email = ref.watch(
      loginProvider.when((prev, current) => prev.email != current.email),
    ).state.email;
    return Text(email);
  },
)

info

In the same way you can listen the changes of your StateNofier using the ProviderListener widget or using a StreamSubscription

Also the StateNotifier class allows you to listen when the state is going to change and listen when the state has changed.

class LoginController extends StateNotifier<LoginState> {
  // you need pass an inital state using super
  LoginController() : super(LoginState.initialState);

  void onEmailChanged(String email) {
    state = state.copyWith(email: email);
  }

  void onPasswordChanged(String password) {
    state = state.copyWith(password: password);
  }

  @override
  void onStateChanged(LoginState oldState, LoginState currentState) {
    // YOUR CODE HERE
  }

  @override
  bool onStateWillChange(LoginState oldState, LoginState newState) {
    // YOUR CODE HERE
    return true; // you need to return a boolean to decide if the update of the state is allowed
  }
}