Flutter phone number authentication null verificationId? - firebase

Question

I am trying to conduct a phone number login by sending sms code to my phone device using firebase auth. But the await keyword does not seem to work here. Why? my assumption is when I assign await to a function, Flutter prints the null value of the verificationId variable, verificationId gets its value within the _sendCodeToPhoneNumber function. Hence this could mean the function _sendCodeToPhoneNumber hasn't finished yet but the its next line of code (print(verificationId) in this case) is already executed. But if I am mistaken, then what is the problem? and more important, please teach me how to resolve problem? Because I have to get verificationId's value and pass it to the next class.
String verificationId;
Future<void> _sendCodeToPhoneNumber() async {
...
final PhoneCodeSent codeSent = (String verificationId, [int forceResendingToken]) async {
this.verificationId = verificationId; //this.verificationId here is not null because it has got a value
print('Code sent to ${_phoneNumberController.text} with $verificationId');
};
...
await FirebaseAuth.instance.verifyPhoneNumber(
codeAutoRetrievalTimeout: codeAutoRetrievalTimeout,
phoneNumber: _phoneNumberController.text,
codeSent: codeSent,
timeout: const Duration(seconds: 120),
verificationCompleted: verificationCompleted,
verificationFailed: vericationFailed
);
}
...
FlatButton(onPressed: (){
await _sendCodeToPhoneNumber();
print(verificationId); // printed null despite having assigned `await` to the _sendCodeToPhoneNumber function
...
})

Answer 1

Change the function to the following and return the verificationId:
Future<String> _sendCodeToPhoneNumber() async {
final PhoneCodeSent codeSent = (String verificationId, [int forceResendingToken]) async {
this.verificationId = verificationId; //this.verificationId here is not null because it has got a value
print('Code sent to ${_phoneNumberController.text} with $verificationId');
};
await FirebaseAuth.instance.verifyPhoneNumber(
codeAutoRetrievalTimeout: codeAutoRetrievalTimeout,
phoneNumber: _phoneNumberController.text,
codeSent: codeSent,
timeout: const Duration(seconds: 120),
verificationCompleted: verificationCompleted,
verificationFailed: vericationFailed
);
return this.verificationId;
}
Also change the onPressed, when using await you need to use async:
FlatButton(onPressed: (()async{
var result = await _sendCodeToPhoneNumber();
print(result);
}))

Answer 2

final PhoneCodeSent codeSent = (String verificationId, [int forceResendingToken]) async {
this.verificationId = verificationId; //this.verificationId here is not null because it has got a value
print('Code sent to ${_phoneNumberController.text} with $verificationId');
};
i think you dont need async here