flutter validate form asynchronously - firebase

Question

new TextFormField(
validator: (value) async{
if (value.isEmpty) {
return 'Username is required.';
}
if (await checkUser()) {
return 'Username is already taken.';
}
},
controller: userNameController,
decoration: InputDecoration(hintText: 'Username'),
),
I have a form for user, and I want to check if the user already exists in the firestore datebase.
Future checkUser() async {
var user = await Firestore.instance
.collection('users')
.document(userNameController.text)
.get();
return user.exists;
}
This is my function to check if the user document already exists in the database.
But validator gives me this error.
[dart] The argument type '(String) → Future' can't be assigned to the parameter type '(String) → String'.
How should I fix this issue?

Answer 1

At this time I think that you can't associate a Future to a validator.
What you can do is this verifying the data on a button click or in another way and set the state on the validator response var.
#override
Widget build(BuildContext context) {
// TODO: implement build
return Scaffold(
body: Form(
key: _formKey,
child: Column(children: [
new TextFormField(
validator: (value) {
return usernameValidator;
},
decoration: InputDecoration(hintText: 'Username')),
RaisedButton(
onPressed: () async {
var response = await checkUser();
setState(() {
this.usernameValidator = response;
});
if (_formKey.currentState.validate()) {}
},
child: Text('Submit'),
)
])));
}

Answer 2

I needed to do this for username validation recently (to check if a username already exists in firebase) and this is how I achieved async validation on a TextFormField ( without installation of any additional packages). I have a "users" collection where the document name is the unique username ( Firebase can't have duplicate document names in a collection but watch out for case sensitivity)
//In my state class
class _MyFormState extends State<MyForm> {
final _usernameFormFieldKey = GlobalKey<FormFieldState>();
//Create a focus node
FocusNode _usernameFocusNode;
//Create a controller
final TextEditingController _usernameController = new TextEditingController();
bool _isUsernameTaken = false;
String _usernameErrorString;
#override
void initState() {
super.initState();
_usernameFocusNode = FocusNode();
//set up focus node listeners
_usernameFocusNode.addListener(_onUsernameFocusChange);
}
#override
void dispose() {
_usernameFocusNode.dispose();
_usernameController.dispose();
super.dispose();
}
}
Then in my TextFormField widget
TextFormField(
keyboardType: TextInputType.text,
focusNode: _usernameFocusNode,
textInputAction: TextInputAction.next,
controller: _usernameController,
key: _usernameFormFieldKey,
onEditingComplete: _usernameEditingComplete,
validator: (value) => _isUsernameTaken ? "Username already taken" : _usernameErrorString,)
Listen for focus changes on the widget i.e when it loses focus. You can also do something similar for "onEditingComplete" method
void _onUsernameFocusChange() {
if (!_usernameFocusNode.hasFocus) {
String message = UsernameValidator.validate(_usernameController.text.trim());
//First make sure username is in valid format, if it is then check firebase
if (message == null) {
Firestore.instance.collection("my_users").document(_usernameController.text.trim()).get().then((doc) {
if (doc.exists) {
setState(() {
_isUsernameTaken = true;
_usernameErrorString = null;
});
} else {
setState(() {
_isUsernameTaken = false;
_usernameErrorString = null;
});
}
_usernameFormFieldKey.currentState.validate();
}).catchError((onError) {
setState(() {
_isUsernameTaken = false;
_usernameErrorString = "Having trouble verifying username. Please try again";
});
_usernameFormFieldKey.currentState.validate();
});
} else {
setState(() {
_usernameErrorString = message;
});
_usernameFormFieldKey.currentState.validate();
}
}
}
For completeness, this is my username validator class
class UsernameValidator {
static String validate(String value) {
final regexUsername = RegExp(r"^[a-zA-Z0-9_]{3,20}$");
String trimmedValue = value.trim();
if (trimmedValue.isEmpty) {
return "Username can't be empty";
}
if (trimmedValue.length < 3) {
return "Username min is 3 characters";
}
if (!regexUsername.hasMatch(trimmedValue)) {
return "Usernames should be a maximum of 20 characters with letters, numbers or underscores only. Thanks!";
}
return null;
}
}

Answer 3

I had the same problem while using Firebase's Realtime Database but I found a pretty good solution similar to Zroq's solution. This function creates a simple popup form to have the user input a name. Essentially, I was trying to see if a particular name for a specific user was already in the database and show a validation error if true. I created a local variable called 'duplicate' that is changed anytime the user clicks the ok button to finish. Then I can call the validator again if there is an error, and the validator will display it.
void add(BuildContext context, String email) {
String _name;
bool duplicate = false;
showDialog(
context: context,
builder: (_) {
final key = GlobalKey<FormState>();
return GestureDetector(
onTap: () => FocusScope.of(context).requestFocus(new FocusNode()),
child: AlertDialog(
title: Text("Add a Workspace"),
content: Form(
key: key,
child: TextFormField(
autocorrect: true,
autofocus: false,
decoration: const InputDecoration(
labelText: 'Title',
),
enableInteractiveSelection: true,
textCapitalization: TextCapitalization.sentences,
onSaved: (value) => _name = value.trim(),
validator: (value) {
final validCharacters =
RegExp(r'^[a-zA-Z0-9]+( [a-zA-Z0-9]+)*$');
if (!validCharacters.hasMatch(value.trim())) {
return 'Alphanumeric characters only.';
} else if (duplicate) {
return 'Workspace already exists for this user';
}
return null;
},
),
),
actions: <Widget>[
FlatButton(
child: const Text("Ok"),
onPressed: () async {
duplicate = false;
if (key.currentState.validate()) {
key.currentState.save();
if (await addToDatabase(_name, email) == false) {
duplicate = true;
key.currentState.validate();
} else {
Navigator.of(context).pop(true);
}
}
},
),
FlatButton(
child: const Text('Cancel'),
onPressed: () {
Navigator.of(context).pop(false);
},
),
],
),
);
});
}