firebase - 在 Dart/Flutter 中，如果 Firestore 数据库中没有 Collection，如何查看？

Question

我想检查一个集合(users 集合)是否存在于 Firestore 数据库中。但是我找不到任何方法来做到这一点。如果该集合存在，我想流式传输其文档，否则如您在以下方法中看到的那样为空流

- 有什么方法可以在不获取快照的情况下找到一个集合？
- 为什么 break; 或 yield* Stream.empty() 挂流，源源不断!

Stream<userInfo> getCurrentUserInfos() async* {
  final String usersCollectionPath = "users";
  Stream<QuerySnapshot> snapshots = Firestore.instance.collection(usersCollectionPath).snapshots();
  snapshots.isEmpty.then((hasNoUserCollection) {
    // Although I don't have 'users' collection
    // in my database but I never reach this point!
    // i.e. hasNoUserCollection is always FALSE! 
    if(hasNoUserCollection) {
      print("users collection doesn't exist, create it!");
      // next line (i.e. `break;`) hangs the tool!
      // And sometimes hangs the VSCode's IDE as well, if I have a breakpoint on it!!!
      break;
      // I tried yielding an empty stream instead of break, again another hang!
      // yield* Stream<userInfo>.empty();
    } else {
      // previous stream is dead, so create it again!
      snapshots = Firestore.instance.collection(usersCollectionPath ).snapshots();
      await for (QuerySnapshot snap in snapshots) {
      for (DocumentSnapshot userDoc in snap.documents) {
        yield (new userInfo.fromQuerySnapshot(userDoc));
      }
    }
  });
}

现在，当流为空时，即使是 try-catch block 也无法捕捉到错误!

try{
  getCurrentUserInfos().last.then((userInfolastOne) {
    print("last one: $lastOne.name");
  });

 // the following line (i.e. await ...) at least doesn't hang and
 // `catch` block is able to catch the error `Bad state: No element`,
 // when the stream is empty
 //
 // userInfo lastOne = await stream.last;
} catch (ex) {
  print ("ex: $ex");
}

Answer 1

没有检测集合是否存在的 API。事实上:Firestore 中的集合只有在其中有文档时才存在。

我能想到的最便宜的检查是对单个文档进行查询，然后检查该查询是否有任何结果。