dart - Flutter firebase validation of form field inputs

Question

I'm new to Flutter and Firebase and I am trying to validate user input from a form and run it up against a the firebase collection.

I ask users to create a username and want to validate that there is not an existing username in my Firebase directory.

I got this validation method from the discussion on Check if Field Already exists in Flutter Firestore. I want to feed the method a name, build a List of documents with that name, and confirm that none exist.

  Future<bool> doesNameAlreadyExist(String name) async {
    final QuerySnapshot result = await Firestore.instance
        .collection('user')
        .where('user_id', isEqualTo: name)
        .limit(1)
        .getDocuments();
    final List<DocumentSnapshot> documents = result.documents;
    return documents.length == 1;
  }

I hope to be able to call this method within the validator of the text form field as I have done below.

new TextFormField(
          decoration: new InputDecoration(labelText: 'Username'),
          validator: (value) => doesNameAlreadyExist(value) ? "Username already taken" : null,
          onSaved: (value) => _username = value,
        ),

However, this throws an error that I must have a static type of bool for the validator.

My question: how can I (1) create a static boolean method that would be able to search Firebase or (2) how can I get the same functionality with the async call (ideally, without having to build another widget)?

Answer 1

Waitting for answers