example_future_builder.dart

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

Future<String> apiCall() async {
  print("apiCall is executed...");
  await Future.delayed(const Duration(seconds: 3));
  return Future.value("${Random().nextInt(100)} update");
}

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  final String title;

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

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

class _MyHomePageState extends State<MyHomePage> {
  late Future<String> localFuture;

  @override
  void initState() {
    localFuture = apiCall();// if you define as local state, it will not executed
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
          child: Column(mainAxisAlignment: MainAxisAlignment.center, children: [
        ElevatedButton(
            onPressed: () {
              print("update btn pressed");
              setState(() {});
            },
            child: const Text("update")),
        FutureBuilder(
            future: apiCall(),
            builder: (context, snapshot) {
              if (snapshot.connectionState == ConnectionState.waiting) {
                return const Center(child: CircularProgressIndicator());
              } else {
                return Text("apiCall : ${snapshot.data}");
              }
            }),
            
             FutureBuilder(
            future: localFuture,
            builder: (context, snapshot) {
              if (snapshot.connectionState == ConnectionState.waiting) {
                return const Center(child: CircularProgressIndicator());
              } else {
                return Text("Local Future: ${snapshot.data}");
              }
            }),
      ])),
    );
  }
}