future_provider_example.dart

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


class Dog with ChangeNotifier {
  final String name;
  final String breed;
  int age;
  Dog({
    required this.name,
    required this.breed,
    this.age = 1,
  });

  void grow() {
    age++;
    notifyListeners();
  }
}

class Babies {
  final int age;
  Babies({
    required this.age,
  });

  Future<int> getBabies() async {
    await Future.delayed(Duration(seconds: 3));

    if (age > 1 && age < 5) {
      return 4;
    } else if (age <= 1) {
      return 0;
    } else {
      return 2;
    }
  }
}

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

// The `FutureProvider` widget is used when the provider needs time to fetch data, so it requires an initial value for the required parameter.
// Widgets that reference FutureProvider are built twice: once for the initial value and once for the return value.
// in most cases, the `FutureBuilder` widget is used instead.
// If a widget needs multiple providers, it requires multiple levels of parents. However, `MultiProvider` makes it easy to read and is essentially syntax sugar.
// In this case, the FureProvider can access the DogChangeNotifierProvider because they are parent and child. Although they may appear to be siblings due to the syntax sugar provided by MultiProvider.
// The type of provider corresponds to the return type of the future method.
class FutureProviderExample extends StatelessWidget {
  const FutureProviderExample({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MultiProvider(
      providers: [
        ChangeNotifierProvider<Dog>(
          create: (context) => Dog(name: 'dog06', breed: 'breed06', age: 3),
        ),
        FutureProvider<int>(
          initialData: 0,
          create: (context) {
            final int dogAge = context.read<Dog>().age;
            final babies = Babies(age: dogAge);
            return babies.getBabies();
          },
        ),
      ],
      child: MaterialApp(
        title: 'Provider 06',
        debugShowCheckedModeBanner: false,
        theme: ThemeData(
          primarySwatch: Colors.blue,
        ),
        home: const MyHomePage(),
      ),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key}) : super(key: key);

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

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Provider 05'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          mainAxisSize: MainAxisSize.min,
          children: [
            Text(
              '- name: ${context.watch<Dog>().name}',
              style: TextStyle(fontSize: 20.0),
            ),
            SizedBox(height: 10.0),
            BreedAndAge(),
          ],
        ),
      ),
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        Text(
          '- breed: ${context.select<Dog, String>((Dog dog) => dog.breed)}',
          style: TextStyle(fontSize: 20.0),
        ),
        SizedBox(height: 10.0),
        Age(),
      ],
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        Text(
          '- age: ${context.select<Dog, int>((Dog dog) => dog.age)}',
          style: TextStyle(fontSize: 20.0),
        ),
        SizedBox(height: 10.0),
        Text(
          '- number of babies: ${context.watch<int>()}',
          style: TextStyle(fontSize: 20.0),
        ),
        SizedBox(height: 20.0),
        ElevatedButton(
          onPressed: () => context.read<Dog>().grow(),
          child: Text(
            'Grow',
            style: TextStyle(fontSize: 20.0),
          ),
        ),
      ],
    );
  }
}