[Fluth]タブ(ページ)の切り替え時のリフレッシュ(再ロード)の防止

class HelloWorld extends StatefulWidget {
  @override
  _HelloWorldState createState() => _HelloWorldState();
}

class _HelloWorldState extends State<HelloWorld> with AutomaticKeepAliveClientMixin // (1) 추가 {

  // (2) 추가
  @override
  bool get wantKeepAlive => true;
  
              .
              .
              .
}