如何等待observable.merge过程完成

Question

我有两个集合。 home和device。我需要调用home集合并从home_id获取home，然后将条件添加到device集合查询中。

在完成代码之前，_home$.add(_homeListData)正在触发，并且homeModel.device数据也是重复的

第一次运行应用程序时，The getter 'devices' was called on null.

String id;
List<Device> _d = [];


var stream1 = databaseReference
    .collection('home-management')
    .where("homeAdmin", isEqualTo: userID);

await stream1.getDocuments().then((data) {
  data.documents.forEach((doc) {
    id = doc.documentID;
  });
});

Stream<QuerySnapshot> stream2 = databaseReference
    .collection('devices')
    .where("homeId", isEqualTo: id)
    .snapshots();

Observable.merge([stream1.snapshots(), stream2]).listen((data) {
   data.documents.forEach((doc) {
     TempModel.fromFirestore(doc);
    if (doc.data['homeName'] != null) {
      homeModel.home = Home.fromDocument(doc);
    } else {
      _d.add(Device.fromDocument(doc));
        homeModel.devices = _d;
    }
     _homeListData.add(homeModel);
     _home$.add(_homeListData); // before complete above code, this one is firing, and also homeModel.device data duppplicate
  });
});

完整的单例课程

class HomeAndDeviceService {
  static final databaseReference = Firestore.instance;
  static List<HomeModel> _homeListData = new List<HomeModel>();
  static final HomeAndDeviceService _singleton =
      HomeAndDeviceService._internal();
  static String userID;
//  static ReplaySubject<List<HomeModel>> _home$ =
//      ReplaySubject<List<HomeModel>>();
  static BehaviorSubject<List<HomeModel>> _home$ =
  BehaviorSubject<List<HomeModel>>();

  factory HomeAndDeviceService(String _userID) {
    userID = _userID;
    return _singleton;
  }

  HomeAndDeviceService._internal() {
    getHomeData();
  }

  BehaviorSubject<List<HomeModel>> getHomes() {
    return _home$;
  }

  static getHomeData() async {
     HomeModel homeModel = new HomeModel();
//add here to above code