main.dart

import 'package:flutter/material.dart';

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

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

class ListScreen extends StatelessWidget {
  Future<bool> isLoggedIn() async {
    // can do all login check logic here safely
    await Future<void>.delayed(const Duration(seconds: 3));
    return false;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Center(
        child: FutureBuilder(
          future: isLoggedIn(),
          builder: (context, snapshot) {
            if (!snapshot.hasData) {
              return CircularProgressIndicator();
            }

            final loggedIn = snapshot.data;
            if (!loggedIn) {
              // do not push/pop in build method (i.e. while building a frame)
              // schedule it for the next frame
              WidgetsBinding.instance.addPostFrameCallback((_) {
                Navigator.of(context).pushReplacement(
                  MaterialPageRoute(
                    builder: (BuildContext _) => LoginScreen(),
                  ),
                );
              });

              // return an empty screen while we transition
              return Container();
            }

            return ListItemView();
          },
        ),
      ),
    );
  }
}

class ListItemView extends StatelessWidget {
  Future<List<String>> _loadItems() async {
    // could return anything here/ call as many async functions as needed
    await Future<void>.delayed(const Duration(seconds: 3));
    return ['a', 'b', 'c', 'd'];
  }

  @override
  Widget build(BuildContext context) {
    return FutureBuilder(
      future: _loadItems(),
      builder: (context, snapshot) {
        if (!snapshot.hasData) {
          return CircularProgressIndicator();
        }

        final items = snapshot.data;
        return Padding(
          padding: const EdgeInsets.all(8.0),
          child: ListView(
            children: [
              for (var item in items) Text(item),
            ],
          ),
        );
      },
    );
  }
}

class LoginScreen extends StatelessWidget {
  final emailController = TextEditingController();
  final passwordController = TextEditingController();

  Future<String> login(String email, String password) async {
    // do login stuff here, then return session id
    return '';
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Center(
        child: Padding(
          padding: const EdgeInsets.all(20),
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            crossAxisAlignment: CrossAxisAlignment.center,
            children: [
              Text('Login'),
              TextFormField(
                controller: emailController,
              ),
              TextFormField(
                controller: passwordController,
              ),
              IconButton(
                onPressed: () async {
                  print(emailController.text);
                  print(passwordController.text);
                  String sessionID = await login(
                      emailController.text, passwordController.text);
                  print("button pressed, sessionID: $sessionID");
                  Navigator.pushReplacement(
                    context,
                    MaterialPageRoute(builder: (context) => ListScreen()),
                  );
                },
                icon: Icon(Icons.login),
              ),
            ],
          ),
        ),
      ),
    );
  }
}