pizza provider demo (provider state management)

Reading a value 

The easiest way to read a value is by using the extension methods on [BuildContext]:

• context.watch<T>(), which makes the widget listen to changes on T
• context.read<T>(), which returns T without listening to it
• context.select<T, R>(R cb(T value)), which allows a widget to listen to only a small part of T.

One can also use the static method Provider.of<T>(context), which will behave similarly to watch. When the listen parameter is set to false (as in Provider.of<T>(context, listen: false)), then it will behave similarly to read.

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

void main() {
  runApp(const MyApp());
}



//Step 1: create the model=======================
class PizzaProvider extends ChangeNotifier {
  late int numPizza;

  PizzaProvider({
    this.numPizza = 0
  });

  void set_numPizza(int new_value){
    numPizza = new_value;
    notifyListeners();
  }
}
//Step 1 end=====================================



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


  @override
  Widget build(BuildContext context) {

    
    
    
    //Step 2: Setup/Activate the provider================
    return MultiProvider(
      providers: [
        ChangeNotifierProvider(create: (context)=>PizzaProvider())
      ],
      child: MaterialApp(
        //Step 2 end=====================================
        
        
        
        title: 'Flutter Demo',
        theme: ThemeData(

          primarySwatch: Colors.blue,
        ),
        home: const MyHomePage(title: 'Flutter Demo Home Page'),
      ),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key, required this.title}) : super(key: key);


  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {


  @override
  Widget build(BuildContext context) {

    
    //Step 3: get value of Provider when build==========================
    int numPizza = Provider.of<PizzaProvider>(context).numPizza;
    //==================================================================

    
    
    return Scaffold(
      appBar: AppBar(

        title: Text(widget.title),
      ),
      body: Center(

        child: Column(

          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'Pizza:'+numPizza.toString(),
            ),

          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          
          
          //Step 4: set the value==========================================================
          Provider.of<PizzaProvider>(context,listen: false).set_numPizza(numPizza+1);
          //===============================================================================
          
          
          
        },
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ),
    );
  }
}