使用firestore的streambuilder中的嵌套查询

StreamBuilder<QuerySnapshot>(
  stream: Firestore.instance.collection('users').snapshots(),
  builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
    if (snapshot.hasError) {
      return Text('Error: ${snapshot.error}');
    }

    if (snapshot.connectionState == ConnectionState.waiting) {
      return Text('Loading...');
    }

    // 获取所有用户的快照
    final usersSnapshot = snapshot.data.documents;

    return ListView.builder(
      itemCount: usersSnapshot.length,
      itemBuilder: (BuildContext context, int index) {
        final user = usersSnapshot[index];

        // 获取用户所在的国家
        final country = user['country'];

        // 执行嵌套查询，获取该国家的所有用户信息
        final countryUsersQuery = Firestore.instance.collection('users')
            .where('country', isEqualTo: country)
            .snapshots();

        return StreamBuilder<QuerySnapshot>(
          stream: countryUsersQuery,
          builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
            if (snapshot.hasError) {
              return Text('Error: ${snapshot.error}');
            }

            if (snapshot.connectionState == ConnectionState.waiting) {
              return Text('Loading...');
            }

            // 获取该国家的所有用户的快照
            final countryUsersSnapshot = snapshot.data.documents;

            // 构建UI，显示该国家的所有用户信息
            return ListView.builder(
              itemCount: countryUsersSnapshot.length,
              itemBuilder: (BuildContext context, int index) {
                final countryUser = countryUsersSnapshot[index];
                return ListTile(
                  title: Text(countryUser['name']),
                  subtitle: Text(countryUser['email']),
                );
              },
            );
          },
        );
      },
    );
  },
);