UseReducer

UseReducer is a hook that manages state using reducer function. An alternative to UseState .

Tip

UseReducer is usually preferable over UseState when you have complex state logic that involves multiple sub-values or when the next state depends on the previous one.

Syntax

UseReducer<T>(
  T reducer(T state, RtAction action),
  T initialValue,
  { String? debugLabel },
);

UseReducer accepts these parameters:

• reducer : A function that takes the current state and an action, and returns a new state.
• initialState: Initial value of T type that it will hold.
• debugLabel: (optional) A label to identify the hook in the DevTools extension.

Properties & Methods

UseReducer provides the following properties and methods:

• value: A getter that allows to read its state.
• dispatch : A method that allows to dispatch an action to update the state.
  • Syntax:

    void dispatch(RtAction action);

Properties and methods inherited from RtState

• debugLabel: A string that represents the label of the state object for debugging purposes.
• debugInfo: A map that contains debug information about the state object.

• update : Executes a callback function and notify its observers that the state has changed. When it is invoked, it emits two lifecycle events to signal the state transition:
  • Lifecycle.willUpdate is emitted first, indicating the impending update.
  • Lifecycle.didUpdate is emitted once the update process is complete.
• notify : Forces the state to notify its observers. Unlike update , it emits only the Lifecycle.didUpdate event, as it doesn’t involve any preparatory steps before the notification.
• bind : Establishes a connection between the state and a specific instance. This connection allows the instance to reactively update based on changes to the state. By binding the state, the instance becomes aware of changes to the state and can appropriately reflect those changes in its behavior.
• unbind : Releases the connection between the state and the instance. When unbinding, the instance will no longer receive updates from the state. This can be useful when an instance is no longer actively using the state or when it needs to detach from the state temporarily or permanently.
• dispose : Is responsible for cleaning up the state and any associated observers or resources. Disposing of the state ensures that it is properly released and no longer consumes memory or processing resources unnecessarily.

Note

While the bind , unbind and dispose methods are normally used to manage the lifecycle of a state, Reactter handles this automatically, when the state is declared inside an instance that exists within Reactter’s context via the dependecy injection, so you don’t need to worry about it(Learn more about Binding State to Dependency).

Usage

Declaration

UseReducer can be initialized using the constructor class:

import 'package:reactter/reactter.dart';

class User {
  final String name;
  final int age;

  const User({required this.name, required this.age});

  @override
  String toString() => "User(name: $name, age: $age)";
}

class UserAction extends ReactterAction {
  UserAction.incrementedAge() : super(type: 'incremented_age', payload: null);
  UserAction.changedName(String name)
      : super(type: 'changed_name', payload: name);
}

User reducer(User state, ReactterAction action) {
  switch (action.type) {
    case "incremented_age":
      return User(state.name, state.age + 1);
    case "changed_name":
      return User(action.payload as String, state.age);
    default:
      return state;
  }
}

final uUser = UseReducer<User>(
  reducer,
  const User(
    name: "John Doe",
    age: 17,
  ),
);

void main() {
  print("${uUser.value}") // User(name: "John Doe", age: 17)
  uUser.dispatch(UserAction.incrementedAge());
  print("${uUser.value}"); // User(name: "John Doe", age: 18)
  uUser.dispatch(UserAction.changedName("Jane Doe"));
  print("${uUser.value}"); // User(name: "Jane Done", age: 18)
}

Note

In some cases, you may need to declare a state variable that will be initialized later. For this, you can use the late keyword. When using UseReducer as late within a dependency(class registered via the dependency injection), use Rt.lazyState (Learn more about Lazy state binding). e.g:

user_controller.dart

class UserController {
  final String userId;

  late final uUser = Rt.lazyState(
    () => UseReducer<User?>(this.reducer, null),
    this,
  );

  UserController(this.userId);

  User? reducer(User? state, RtAction action) {
    if (action is IncrementedEgeUserAction) {
      return User(name: state!.name, age: state.age + 1);
    } else if (action is ChangedNameUserAction) {
      return User(name: action.name, age: state!.age);
    }
    return state;
  }
}

Writting the action

The action is a class that inherits from the RtAction class.

This class contains the action type and payload that will be dispatched to the reducer method.

import 'package:reactter/reactter.dart';

class User {
  final String name;
  final int age;

  const User({required this.name, required this.age});

  @override
  String toString() => "User(name: $name, age: $age)";
}

class UserAction extends ReactterAction {
  UserAction.incrementedAge() : super(type: 'incremented_age', payload: null);
  UserAction.changedName(String name)
      : super(type: 'changed_name', payload: name);
}

User reducer(User state, ReactterAction action) {
  switch (action.type) {
    case "incremented_age":
      return User(state.name, state.age + 1);
    case "changed_name":
      return User(action.payload as String, state.age);
    default:
      return state;
  }
}

final uUser = UseReducer<User>(
  reducer,
  const User(
    name: "John Doe",
    age: 17,
  ),
);

void main() {
  print("${uUser.value}") // User(name: "John Doe", age: 17)
  uUser.dispatch(UserAction.incrementedAge());
  print("${uUser.value}"); // User(name: "John Doe", age: 18)
  uUser.dispatch(UserAction.changedName("Jane Doe"));
  print("${uUser.value}"); // User(name: "Jane Done", age: 18)
}

Writting the reducer method

The reducer method is a function that takes the current state and an action, and returns a new state. This method is responsible for updating the state based on the action dispatched. The state is immutable, so it should return a new state object.

import 'package:reactter/reactter.dart';

class User {
  final String name;
  final int age;

  const User({required this.name, required this.age});

  @override
  String toString() => "User(name: $name, age: $age)";
}

class UserAction extends ReactterAction {
  UserAction.incrementedAge() : super(type: 'incremented_age', payload: null);
  UserAction.changedName(String name)
      : super(type: 'changed_name', payload: name);
}

User reducer(User state, ReactterAction action) {
  switch (action.type) {
    case "incremented_age":
      return User(state.name, state.age + 1);
    case "changed_name":
      return User(action.payload as String, state.age);
    default:
      return state;
  }
}

final uUser = UseReducer<User>(
  reducer,
  const User(
    name: "John Doe",
    age: 17,
  ),
);

void main() {
  print("${uUser.value}") // User(name: "John Doe", age: 17)
  uUser.dispatch(UserAction.incrementedAge());
  print("${uUser.value}"); // User(name: "John Doe", age: 18)
  uUser.dispatch(UserAction.changedName("Jane Doe"));
  print("${uUser.value}"); // User(name: "Jane Done", age: 18)
}

Dispatching an action and reading the state

The dispatch method is responsible for dispatching an action to the reducer method. After dispatching an action, you can read the state using the value property.

import 'package:reactter/reactter.dart';

class User {
  final String name;
  final int age;

  const User({required this.name, required this.age});

  @override
  String toString() => "User(name: $name, age: $age)";
}

class UserAction extends ReactterAction {
  UserAction.incrementedAge() : super(type: 'incremented_age', payload: null);
  UserAction.changedName(String name)
      : super(type: 'changed_name', payload: name);
}

User reducer(User state, ReactterAction action) {
  switch (action.type) {
    case "incremented_age":
      return User(state.name, state.age + 1);
    case "changed_name":
      return User(action.payload as String, state.age);
    default:
      return state;
  }
}

final uUser = UseReducer<User>(
  reducer,
  const User(
    name: "John Doe",
    age: 17,
  ),
);

void main() {
  print("${uUser.value}") // User(name: "John Doe", age: 17)
  uUser.dispatch(UserAction.incrementedAge());
  print("${uUser.value}"); // User(name: "John Doe", age: 18)
  uUser.dispatch(UserAction.changedName("Jane Doe"));
  print("${uUser.value}"); // User(name: "Jane Done", age: 18)
}

Note

UseReducer notifies that its value has changed when the previous value is different from the current value. If its value is an Object , it does not detect internal changes, only when value is setted to another Object . Learn more about updating objects in state.

Using the action callable

The actions can be created as a callable class, extending from RtActionCallable e.g.:

class IncrementedEgeUserAction extends RtActionCallable<User, void> {
  const IncrementedEgeUserAction()
    : super(type: 'incremented_ege', payload: null);

  @override
  User call(User state) => User(name: state.name, age: state.age + 1);
}

class ChangedNameUserAction extends RtActionCallable<User, String> {
  final String name;

  const ChangedNameUserAction(this.name)
    : super(type: 'changed_name', payload: name);

  @override
  User call(User state) => User(name: payload, age: state.age);
}

RtActionCallable has a call method that returns the new state based on the state and payload.

The action callable can be applied in reducer method:

User reducer(User state, RtAction action) {
  if (action is RtActionCallable) return action(state);
  return state;
}

And dispatched as:

userState.dispatch(IncrementedEgeUserAction());
userState.dispatch(ChangedNameUserAction('Jane Doe'));

Updating the state

Use update method to notify changes after run a set of instructions:

uState.update((value) {
  uState.value = "New value";
});

Use notify method to force to notify changes.

userState.notify();

Listening to changes

When value has changed, the UseReducer will emit the following events(learn about it here):

• Lifecycle.willUpdate event is triggered before the change in value or update method have been invoked.
• Lifecycle.didUpdate event is triggered after the change in value or after update or notify methods have been invoked.

Example of listening to changes:

Rt.on(
  uUser,
  Lifecycle.didUpdate,
  (_, state) => print("State value has changed to: ${state.value}"),
);