form field validator: allow async function #56414
Description
Use case
Using a TextFormField inside a form with a validation function.
Example: TextFormField for an email address.
String validateEmail(String value) {
Pattern pattern =
r'^(([^<>()[\]\\.,;:\s@\"]+(\.[^<>()[\]\\.,;:\s@\"]+)*)|(\".+\"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$';
RegExp regex = new RegExp(pattern);
if (!regex.hasMatch(value))
return 'Enter a Valid Email';
else
return null;
}
But also I need to check if the email is not already registered on the database, but the database function that queries the email is async.
Future<String> validateEmail(String value) async {
Pattern pattern =
r'^(([^<>()[\]\\.,;:\s@\"]+(\.[^<>()[\]\\.,;:\s@\"]+)*)|(\".+\"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$';
RegExp regex = new RegExp(pattern);
if (!regex.hasMatch(value)) return 'Enter Valid Email';
bool isAvailable =
await DataService.isEmailAvailable(value);
if(isAvailable == false) return 'Email already in use';
return null;
}
I could put the dup email validation logic on the submit/save function, but it would be a much better experience for the user to be able to see the error for the specific input.
Proposal
Change the FormFieldValidator type to async, also implement the logic to await the function return
/// Signature for validating a form field.
///
/// Returns an error string to display if the input is invalid, or null
/// otherwise.
///
/// Used by [FormField.validator].
typedef FormFieldValidator<T> = String Function(T value);
typedef FormFieldValidatorAsync<T> = Future<String> Function(T value);