room_detail_view.dart

body: SafeArea(
  child: usersStream.when(
    data: (model) {
      return Stack(
        children: [
          Column(
            children: [
              /* 略 */,
              Expanded(
                child: FutureBuilder<List<User>>(
                  future: model.participants(),
                  builder: (BuildContext context, AsyncSnapshot<List<User>> snapshot) {
                    if (snapshot.connectionState == ConnectionState.done) {
                      List<User> users = snapshot.data;
                      return Padding(
                        padding: EdgeInsets.only(top: 16.0),
                        child: GridView.count(
                          crossAxisCount: numTilesHorizontal,
                          children: List.generate(users.length, (index) {
                            final user = users[index];
                            return SpeakerTile(
                              model: SpeakerTileViewModel(speaker: user)
                            );
                          }),
                        ),
                      );
                    } else {
                      return Center(child: CircularProgressIndicator());
                    }
                  }
                )
              ),
            ]
            /*
            後略
            */