flutter-bloc-call-event-just-once.dart

main.dart

/*
 * Goal:
 * Call event only on first time by bloc state
 * */

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

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) => MultiBlocProvider(
        providers: [
          BlocProvider(create: (_) => PageBloc<Page1>()),
          BlocProvider(create: (_) => PageBloc<Page2>()),
        ],
        child: MaterialApp(
          title: 'Example',
          home: MyHomePage(),
        ),
      );
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) => Scaffold(
        body: Center(
          child: Column(
            children: [
              const SizedBox(height: 20),
              ElevatedButton(
                onPressed: () => Navigator.push(
                    context, MaterialPageRoute(builder: (context) => Page1())),
                child: const Text('Page 1: load just once'),
              ),
              const SizedBox(height: 20),
              ElevatedButton(
                onPressed: () => Navigator.push(
                    context, MaterialPageRoute(builder: (context) => Page2())),
                child: const Text('Page 2: load every time'),
              )
            ],
          ),
        ),
      );
}

class Page1 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    var pageBloc = context.read<PageBloc<Page1>>();

    // This make data being load just once, even if widget is re-build
    if (pageBloc.state is PageInitialState) {
      pageBloc.add(LoadPageEvent(pageNumber: 1));
    }

    return Scaffold(
      appBar: AppBar(title: const Text('Page 1: Load Just Once')),
      body: Center(child:
          BlocBuilder<PageBloc<Page1>, PageState>(builder: (context, state) {
        if (state is PageLoadingState) {
          return const CircularProgressIndicator();
        }

        return Text(state is PageLoadedState ? state.pageData : "");
      })),
    );
  }
}

class Page2 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    context.read<PageBloc<Page2>>().add(LoadPageEvent(pageNumber: 1));

    return Scaffold(
      appBar: AppBar(title: const Text('Page 2: Load Every Time')),
      body: Center(child:
          BlocBuilder<PageBloc<Page2>, PageState>(builder: (context, state) {
        if (state is PageLoadingState) {
          return const CircularProgressIndicator();
        }

        return Text(state is PageLoadedState ? state.pageData : "");
      })),
    );
  }
}

/******************************
 * Bloc Page Events
 * ****************************/

abstract class PageEvent {}

class LoadPageEvent extends PageEvent {
  final int pageNumber;

  LoadPageEvent({required this.pageNumber});
}

/******************************
 * Bloc Page States
 * ****************************/

abstract class PageState {}

class PageInitialState extends PageState {}

class PageLoadingState extends PageState {}

class PageLoadingFailedState extends PageState {
  final String error;

  PageLoadingFailedState({required this.error});
}

class PageLoadedState extends PageState {
  final String pageData;

  PageLoadedState({required this.pageData});
}

/******************************
 * Page Bloc
 * ****************************/

class PageBloc<T> extends Bloc<PageEvent, PageState> {
  PageBloc() : super(PageInitialState()) {
    on<LoadPageEvent>((event, emit) async {
      emit(PageLoadingState());

      try {
        String pageData = await Future.delayed(Duration(seconds: 2))
            .then((_) => "...Page Data...");
        emit(PageLoadedState(pageData: pageData));
      } catch (e) {
        emit(PageLoadingFailedState(error: e.toString()));
      }
    });
  }
}