Flutter: State Management using BloC Pattern

Deepak Kumar

27 April 2023

Why do we use BloC Pattern ?

It is a Design Pattern and It stands for Business Logic Component. So we can use the Bloc Pattern whenever we need to manage the state of the UI and have to update it automatically when data changes.

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There should be a bloc in between the data layer and UI because this Bloc creates a connection between the data layer and UI which consequently manages the state of the UI accordingly.

Below all three images will change when we, clicking on the Change Image button as the input event changes.

How to use this pattern

Let’s create a program that will change the UI image as we click on the Change Image button.

Bloc does the process of mapping events to the state.
In this project, we initially provide the 0 value as input to the event to show a default image on the screen.

On pressing the Change Image button, we calculate the value which is increased by one in the block, and use check on button press and then send event according to the condition.

We need to create three files in our project:

1 . Main. dart

2 . HomeScreen

3 . ChangeImage

Now first of all add the package to pubspec.yaml file

  bloc: ^7.0.0
  flutter_bloc: ^7.0.1

1 2	bloc: ^7.0.0 flutter_bloc: ^7.0.1

Let’s Check Implementation in Main.dart

From the root of the application, we provide the initial value to the change image class. And make Ui class as a child of BlocProvider

import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'bloc_file.dart';
import 'homescreen.dart';
void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      // Wrapping UI class with BlocProvider
      home: BlocProvider(
        // Initial input
          create: (_)=> ChangeImage(0),
          child: HomeScreen()),
    );
  }
}

import 'package:flutter/material.dart';

import 'package:flutter_bloc/flutter_bloc.dart';

import 'bloc_file.dart';

import 'homescreen.dart';

void main() {

runApp(MyApp());

}

class MyApp extends StatelessWidget {

// This widget is the root of your application.

@override

Widget build(BuildContext context) {

return MaterialApp(

debugShowCheckedModeBanner: false,

// Wrapping UI class with BlocProvider

home: BlocProvider(

// Initial input

create: (_)=> ChangeImage(0),

child: HomeScreen()),

);

}

Now Check Implementation in HomeScreen

In the below code, we are doing the UI part in which we are providing the automatically updated input on the button click which eventually results in updating the UI automatically.

And BlocBuilder is used to show changes that will counter from bloc file along with this it also manages state as events changed.

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

class HomeScreen extends StatefulWidget {
  @override
  _HomeScreenState createState() => _HomeScreenState();
}

class _HomeScreenState extends State<HomeScreen> {
  int num = 1;
  @override
  Widget build(BuildContext context) {
    final _imageVar = BlocProvider.of<ChangeImage>(context);

    return Scaffold(
      appBar: AppBar(
        title: Text("BLoC State Management"),
      ),
      body: SingleChildScrollView(
        child: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            crossAxisAlignment: CrossAxisAlignment.center,
            children: [
              SizedBox(
                height: 20.0,
              ),
              //Wrap with BlocBuilder where we want to show update in UI
              BlocBuilder<ChangeImage, int>(
                  builder: (BuildContext context, int state) {
                if (state % 3 == 1) {
                  return Icon(
                    Icons.add_shopping_cart,
                    size: 180.0,
                    color: Colors.green,
                  );
                } else if (state % 3 == 2) {
                  return Icon(
                    Icons.build,
                    size: 180.0,
                    color: Colors.red,
                  );
                } else if (state % 3 == 0) {
                  return Icon(
                    Icons.add_call,
                    size: 180.0,
                    color: Colors.blue,
                  );
                } else {
                  return Container();
                }
              }),
              SizedBox(
                height: 80,
              ),

              Container(
                padding: EdgeInsets.all(6.0),
                width: MediaQuery.of(context).size.width,
                child: ElevatedButton(
                    onPressed: () {
                      if (num % 3 == 1) {
                        _imageVar.add(ImageUpdater.zero);
                      } else if (num % 3 == 2) {
                        _imageVar.add(ImageUpdater.one);
                      } else if (num % 3 == 0) {
                        _imageVar.add(ImageUpdater.two);
                      }
                      setState(() {
                        num = num + 1;
                      });
                    },
                    child: Text("Change Image")),
              ),
              Row(
                mainAxisAlignment: MainAxisAlignment.center,
                  children: [
                    Text("Note -",style: TextStyle(fontWeight: FontWeight.bold),),
                    Text(" You can change state of image by pressing button"),
                  ],
              )
            ],
          ),
        ),
      ),
    );
  }
}

import 'package:flutter/cupertino.dart';

import 'package:flutter/material.dart';

import 'package:flutter_bloc/flutter_bloc.dart';

import 'bloc_file.dart';

class HomeScreen extends StatefulWidget {

@override

_HomeScreenState createState() => _HomeScreenState();

}

class _HomeScreenState extends State<HomeScreen> {

int num = 1;

@override

Widget build(BuildContext context) {

final _imageVar = BlocProvider.of<ChangeImage>(context);

return Scaffold(

appBar: AppBar(

title: Text("BLoC State Management"),

body: SingleChildScrollView(

child: Center(

child: Column(

mainAxisAlignment: MainAxisAlignment.center,

crossAxisAlignment: CrossAxisAlignment.center,

children: [

SizedBox(

height: 20.0,

//Wrap with BlocBuilder where we want to show update in UI

BlocBuilder<ChangeImage, int>(

builder: (BuildContext context, int state) {

if (state % 3 == 1) {

return Icon(

Icons.add_shopping_cart,

size: 180.0,

color: Colors.green,

);

} else if (state % 3 == 2) {

return Icon(

Icons.build,

size: 180.0,

color: Colors.red,

);

} else if (state % 3 == 0) {

return Icon(

Icons.add_call,

size: 180.0,

color: Colors.blue,

);

} else {

return Container();

}

}),

SizedBox(

height: 80,

Container(

padding: EdgeInsets.all(6.0),

width: MediaQuery.of(context).size.width,

child: ElevatedButton(

onPressed: () {

if (num % 3 == 1) {

_imageVar.add(ImageUpdater.zero);

} else if (num % 3 == 2) {

_imageVar.add(ImageUpdater.one);

} else if (num % 3 == 0) {

_imageVar.add(ImageUpdater.two);

}

setState(() {

num = num + 1;

});

child: Text("Change Image")),

Row(

mainAxisAlignment: MainAxisAlignment.center,

children: [

Text("Note -",style: TextStyle(fontWeight: FontWeight.bold),),

Text(" You can change state of image by pressing button"),

)

);

}

Let’s Check Implementation in ChangeImage

And this is the bloc part of the project where events are taken as input and update the state accordingly and we will be using the switch to check events from the input.

import 'dart:async';
import 'package:bloc/bloc.dart';

enum ImageUpdater {zero,one,two,}

class ChangeImage extends Bloc<ImageUpdater, int>{
  ChangeImage(int initialState) : super(initialState);

  @override
  //managing state according to input event
  Stream<int> mapEventToState(ImageUpdater event)async* {

    switch(event){
      case ImageUpdater.zero:
        yield state+1;
        break;
      case ImageUpdater.one:
        yield state +1;
        break;
      case ImageUpdater.two:
        yield state +1;
        break;
    }
  }
}

import 'dart:async';

import 'package:bloc/bloc.dart';

enum ImageUpdater {zero,one,two,}

class ChangeImage extends Bloc<ImageUpdater, int>{

ChangeImage(int initialState) : super(initialState);

@override

//managing state according to input event

Stream<int> mapEventToState(ImageUpdater event)async* {

switch(event){

case ImageUpdater.zero:

yield state+1;

break;

case ImageUpdater.one:

yield state +1;

break;

case ImageUpdater.two:

yield state +1;

break;

}

I hope this blog will help you to learn about Bloc Pattern and you will be able to implement it.

Happy Learning ✍️

Reference Links:
https://pub.dev/packages/flutter_bloc