Listen to changes without caring about state in Flutter Bloc-CodePudding

I am currently learning how to use the Bloc state management library in Flutter, and I have a scenario where I would like to listen to an action performed (e.g. clicking a button) and react to that action in a child widget down the widget tree using a Cubit. From the documentation, it seems like the only way to rebuild child widgets is by storing a state object and reacting to changes in the state. However in my case, I don't need to store any state, I just want to know if an action has been done and react every single time it has been invoked.

Given this example below, I want WidgetB to rebuild whenever the button in WidgetA has been pressed, but I can't figure out how to construct my Cubit to allow that without storing some kind of state object.

import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';

// What should the code be here if I don't want to store any state?
class WidgetACubit extends Cubit<void> {
  WidgetACubit({initialState}) : super(initialState);

  void doSomething() => emit(null);
}

class App extends StatelessWidget {
  App({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: BlocProvider(
          create: (_) => WidgetACubit(),
          child: WidgetA(),
        ),
      ),
    );
  }
}

class WidgetA extends StatelessWidget {
  WidgetA({super.key});

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        ElevatedButton(
          onPressed: () {
            BlocProvider.of<WidgetACubit>(context).doSomething();
            print("Something has been done");
            WidgetB.count  ;
          },
          child: const Text("Press me to do something"),
        ),
        WidgetB(),
      ],
    );
  }
}

class WidgetB extends StatelessWidget {
  static int count = 0;

  WidgetB({super.key});

  @override
  Widget build(BuildContext context) {
    return BlocBuilder<WidgetACubit, void>(
      builder: (context, state) {
        return Text("I've been rebuilt $count times");
      },
    );
  }
}

CodePudding user response：

The BlocBuilder widget works by subscribing to the Cubit's state stream. So, you will not be able to rebuild your widget using a BlocBuilder while completely circumventing the usage of any state object. You could either just use a dummy state class without any methods and properties for your Cubit's state, and emit a new instance of that whenever you call cubit.doSomething. Or you can create a custom Widget for rebuilding which you use instead of BlocBuilder.

CodePudding user response：

It is always emitting the same instance(null) that's why it wont rebuild. if you insist to rebuilt it like this, you can emit different value like

class WidgetACubit extends Cubit<void> {
  WidgetACubit({initialState}) : super(initialState);
  int count = 0;
  void doSomething() => emit(count  );
}

Btw I cannot get why do you need to use void cubit which can an event on bloc.

You can check the flutter bloccore concepts for more.