Flutter async form validation

问题

I am having the same problem as the poster in this question: flutter validate form asynchronously.

However, I'm having trouble implementing this and understand how the validation is working.

First, what does this code return? Does this return a boolean?

Future checkUser() async {
var user = await Firestore.instance
    .collection('users')
    .document(userNameController.text)
    .get();
return user.exists;

Second, what is the userNameValidator in this case? Is this a boolean?

new TextFormField(
              validator: (value) {
                return usernameValidator;
              },

Thanks, for the help.

回答1:

The method checkUser will begin a call to Firebase. The await keyword is telling the compiler that we need the output of that call, but there is no need to block everything else from running. After the call has been made, the function will return a bool indicating if the given user exists.

On the other question checkUser is called via the onPressed call which waits for a return value and assigns it to the usernameValidator variable - therefore making it a bool.

来源：https://stackoverflow.com/questions/53104710/flutter-async-form-validation