Flutter SteamBuilder snapshot.data为空

我是新来的扑扑。我正在尝试使用SteamBuilder检索快照数据。但是snapshot.data返回null。snapshot.connectionState == ConnectionState.done为false。snapshot.hasData为false。

这是我试图在其中检索快照数据的dart文件

return StreamBuilder<UserData>(
        stream: DatabaseService(uid: user.uid).userData,
        builder: (context, snapshot) {
          //print(snapshot.connectionState == ConnectionState.done);
          //print(snapshot.data);
          if(snapshot.hasData){
            print('snapshot has data');
            UserData userData = snapshot.data;
            return Form(
              key: _formKey,
              child: Column(
                children: <Widget>[
                  Text(
                    'Update your brew settings.',
                    style: TextStyle(fontSize: 18.0),
                  ),
                  SizedBox(height: 20.0),
                  TextFormField(
                    initialValue: userData.name,
                    decoration: textInputDecorator,
                    validator: (val) => val.isEmpty ? 'Please enter a name' : null,
                    onChanged: (val) => setState(() => _currentName = val),
                  ),
                  SizedBox(height: 10.0),
                  DropdownButtonFormField(
                    value: _currentSugars ?? userData.sugars,
                    decoration: textInputDecorator,
                    items: sugars.map((sugar) {
                      return DropdownMenuItem(
                        value: sugar,
                        child: Text('$sugar sugars'),
                      );
                    }).toList(),
                    onChanged: (val) => setState(() => _currentSugars = val ),
                  ),
                  SizedBox(height: 10.0),
                  Slider(
                    value: _currentStrength.toDouble() ?? userData.strength,
                    activeColor: Colors.brown[_currentStrength ?? userData.strength],
                    inactiveColor: Colors.brown[_currentStrength ?? userData.strength],
                    min: 100.0,
                    max: 900.0,
                    divisions: 8,
                    onChanged: (val) => setState(() => _currentStrength = val.round()),
                  ),
                  RaisedButton(
                      color: Colors.pink[400],
                      child: Text(
                        'Update',
                        style: TextStyle(color: Colors.white),
                      ),
                      onPressed: () async {
                        print(_currentName);
                        print(_currentSugars);
                        print(_currentStrength);
                      }
                  ),
                ],
              ),
            );
          } else {
            print('snapshot doesnt have data');
            return Loading();
          }
        }
    );

这是流

Stream<UserData> get userData {
    return brewCollection.document(uid).snapshots().map(_userDataFromSnapshot);
  }

brewCollection-

  final CollectionReference brewCollection = Firestore.instance.collection('brews');

_userDataFromSnapshot

UserData _userDataFromSnapshot(DocumentSnapshot snapshot) {
    return UserData(
      uid: uid,
      name: snapshot.data['name'],
      sugars: snapshot.data['sugars'],
      strength: snapshot.data['strength']
    );
  }

用户数据

class UserData {

  final String uid;
  final String name;
  final String strength;
  final int sugars;

  UserData({ this.uid, this.sugars, this.strength, this.name });
}

由于我是Flutter的新手，所以我不知道如何解决此问题。请帮帮我。谢谢。