Flutter & Firebase - 从文档中获取特定字段

Question

我一直在尝试从特定文档中获取特定字段。我需要toWho的令牌。但我总是为空。我该如何解决这个问题？

主代码是

Future<String> getUserToken(String toWho) async {
    DocumentSnapshot _doc = await FirebaseFirestore.instance.doc("tokens/" + toWho).get();
    if (_doc != null) {
      Map<String, dynamic> _data = _doc.data();
      return _data["token"];
    } else {
      return null;
    }
}

在存储库中

Future<bool> sendMessage(
      MessageModel sendingMessage, UserModel currentUser) async {
    if (appMode == AppMode.DEBUG) {
      return true;
    } else {
      var _writePrcs = await _firestoreDBService.saveMessage(sendingMessage);
      if (_writePrcs) {
        var _token = "";
        if (_userToken.containsKey(sendingMessage.toWho)) {
          _token = _userToken[sendingMessage.toWho];
          print("Token lokalden geldi.");
        } else {
          _token = await _firestoreDBService.getUserToken(sendingMessage.toWho);
          _userToken[sendingMessage.toWho] = _token;

          print("Token  veritabanından geldi.");
        }

从现在开始感谢您的帮助

Answer 1

Try ........... 


 Future<String> getUserToken(String toWho) async {
    DocumentSnapshot _doc = await 
    FirebaseFirestore.instance.collection("tokens/groupChatId/message").doc(toWho).get();
      if (_doc != null) {
         Map<String, dynamic> _data = _doc.data();
         return _data["token"];
      } else {
        return null;
   }
}