Styjun

I have an asynchronous function in Flutter which takes the value of the validator as an argument:

validatePhone(number) 
 bool _isValid;

 Requests.get("http://apilayer.net/api/validate?value=$number", json: true)
 .then((val) 
 if (val['valid']) 
 // setState(() <- also tried setting state here
 _isValid = true;
 // );
 else 
 // setState(() 
 _isValid = false;
 // );
 
 );

 return _isValid;

and

TextFormField(
 validator: (value) 
 if (value.isEmpty) 
 return 'Please your phone number';
 else 
 if (validatePhone(value)) 
 return 'Your phone number is not valid';
 
 
 ,
),

but it does not work it always returns null or the initial value set in validatePhone. Any idea how I can make this work?

As it was said in the comments, it is not possible to have async validators as validator is expected to return a String and not a `Future'.

However, there are a number of things that's wrong in your code. First of all, validatePhone returns before _isValid is set, which is why you're getting a null value, because it was never set to anything. Your request completes after validatePhone returns and setting _isValid is useless at that point.

Let's try to fix validatePhone:

Future<bool> validatePhone(number) async 
 bool _isValid;

 final val = await Requests.get(
 "http://apilayer.net/api/validate?value=$number",
 json: true);

 if (val['valid']) 
 // setState(() 
 _isValid = true;
 // );
 else 
 // setState(() 
 _isValid = false;
 // );
 

 return _isValid;

as you see, it's return value had to become Future<bool>, not bool. There is no way to fix this. If validator was allowed to return Future, then it could work.

You're going to have to implement your validation logic in a custom painful way.

Edit: here comes a custom painful way :)

String lastValidatedNumber;
String lastRejectedNumber;

// this will be called upon user interaction or re-initiation as commented below
String validatePhone(String number) 
 if (lastValidatedNumber == number) 
 return null;
 else if (lastRejectedNumber == number) 
 return "Phone number is invalid";
 else 
 initiateAsyncPhoneValidation(number);
 return "Validation in progress";
 


Future<void> initiateAsyncPhoneValidation(String number) async 
 final val = await Requests.get(
 "http://apilayer.net/api/validate?value=$number",
 json: true);

 if (val['valid']) 
 lastValidatedNumber = number;
 else 
 lastRejectedNumber = number;
 
 _formKey.currentState.validate(); // this will re-initiate the validation

You need to have a form key:

final _formKey = GlobalKey<FormState>();

And your form should auto validate:

 child: Form(
 key: _formKey,
 autovalidate: true,
 child: TextFormField(
 validator: validatePhone
 )
 )

I'm not 100% sure if this would work, but it's worth a shot.