大枠
・StreamBuilder・・・リアムタイムに更新する値を再描画なしで反映する
・FutureBuilder・・・一度のみ値を取得し、リアムタイムの変更を反映しない

[FutureBuilder]

class HomeScreen extends StatelessWidget {
  final Future<QuerySnapshot> _wordStream =
    FirebaseFirestore.instance.collection('cities').get();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Memo Pad'),
      ),
      body: FutureBuilder(
        future: _wordStream,
        builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
          if (snapshot.hasData) {
            return ListView(
              children: snapshot.data!.docs.map((DocumentSnapshot document) {
                return ListTile(
                  title: Text(document['name']),
                );
              }).toList(),
            );
          } else {
            return Container(
              child: CircularProgressIndicator(),
            );
          }
        },
      ),
    );

[StreamBuilder]

class HomeScreen extends StatelessWidget {
  final Stream<QuerySnapshot> _wordStream =
     FirebaseFirestore.instance.collection('cities').snapshots();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Memo Pad'),
      ),
      body: StreamBuilder(
        stream: _wordStream,
        builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
          if (snapshot.hasData) {
            return ListView(
              children: snapshot.data!.docs.map((DocumentSnapshot document) {
                return ListTile(
                  title: Text(document['name']),
                );
              }).toList(),
            );
          } else {
            return Container(
              child: CircularProgressIndicator(),
            );
          }
        },
      ),
    );
  }
}