import 'package:flutter/material.dart';

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

// リスト 5.10
class TestPage extends StatelessWidget {
  const TestPage({Key? key}) : super(key: key);

  static const String _title = '状態に応じて表示切り替え';

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: _title,
      home: TestWidget(),
    );
  }
}

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

  @override
  State<TestWidget> createState() => _TestWidgetState();
}

class _TestWidgetState extends State<TestWidget> {
  Future<String> _getFuture() async {
    return await Future<String>.delayed(Duration(seconds: 5), 
        () => '読み込み完了');
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('状態に応じて表示切り替え')),
      body: Center(
        child: FutureBuilder<String>(
          future: _getFuture(),
          builder: (BuildContext context, AsyncSnapshot<String> snapshot) {
            if (snapshot.connectionState != ConnectionState.done) {
              return CircularProgressIndicator();
            }
            if (snapshot.hasData) {
              return Text(snapshot.data!);
            }
            return Text('データがない');
          }
        )
      )
    );
  }
}