Async method return null in flutter

Question

I am trying to fetch the verificationid from APIClient class to login screen.

**In Login.dart**
       FetchVerificationID() {
              ApiProviderObj.FetchFirebaseVeriID().then((value) {
                print(value); // This always get null
              });
            }

In APIPROVIDER.dart class

Future<String> FetchFirebaseVeriID() async {
    final PhoneCodeSent smsOTPSent = (String verId, [int forceCodeResend]) {
      verificationId = verId;
      return verId; // This is what i am expecting the return value. It take some time to reach here but the method return value before reaching here
    };
    try {
      await _auth.verifyPhoneNumber(
          phoneNumber: '+91 93287 46333', // PHONE NUMBER TO SEND OTP
          codeAutoRetrievalTimeout: (String verId) {
            //Starts the phone number verification process for the given phone number.
            //Either sends an SMS with a 6 digit code to the phone number specified, or sign's the user in and [verificationCompleted] is called.
            verificationId = verId;
          },
          codeSent:
          smsOTPSent, // WHEN CODE SENT THEN WE OPEN DIALOG TO ENTER OTP.
          timeout: const Duration(seconds: 20),
          verificationCompleted: (AuthCredential phoneAuthCredential) {
            print('phoneAuthCredential => ${phoneAuthCredential}');
            return verId;
          },
          verificationFailed: (AuthException exceptio) {
           return "Error";
          });
    } catch (e) {
      return "Error";
    }
  }

@AlexMyznikov thank you for the comment. I wanted to return Verificationid but in try block i dont have that yet. Its like i have sync menthod inside other sync method. This is reason i am stuck on this — ios developer
– ios developer, Commented Feb 26, 2020 at 19:57

Alexandre · Accepted Answer · 2020-02-26 19:59:23Z

2

You should have the return statement at the end of the method.

I don't really know what is the cause of this, but I had an issue like you with a FutureBuilder.

In the async method I have the return values, but the return value was null.

Future<String> FetchFirebaseVeriID() async {

    String returnVar; //Create return variable

    final PhoneCodeSent smsOTPSent = (String verId, [int forceCodeResend]) {
      verificationId = verId;
      return verId;
    };
    try {
      await _auth.verifyPhoneNumber(
          phoneNumber: '+91 93287 46333', // PHONE NUMBER TO SEND OTP
          codeAutoRetrievalTimeout: (String verId) {
            //Starts the phone number verification process for the given phone number.
            //Either sends an SMS with a 6 digit code to the phone number specified, or sign's the user in and [verificationCompleted] is called.
            verificationId = verId;
          },
          codeSent:
          smsOTPSent, // WHEN CODE SENT THEN WE OPEN DIALOG TO ENTER OTP.
          timeout: const Duration(seconds: 20),
          verificationCompleted: (AuthCredential phoneAuthCredential) {
            print('phoneAuthCredential => ${phoneAuthCredential}');
            returnVar = verId; //Set the return value
          },
          verificationFailed: (AuthException exceptio) {
           returnVar = "Error"; //Set the return value
          });
    } catch (e) {
      returnVar = "Error"; //Set the return value
    }

    return returnVar; //And return the value

  }

answered Feb 26, 2020 at 19:59

Alexandre

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4 Comments

ios developer Over a year ago

Thank you for the answer. I did tried that but its not working . becase returnVar = null Where as it take some time to reach to final PhoneCodeSent smsOTPSent = (String verId, [int forceCodeResend]) { verificationId = verId; return verId; }

Alexandre Over a year ago

Verify that you have await or .then in all your async calls. If the value set to the smsOTPSent is async do PhoneCodeSent smsOTPSent = await (String verId, [int forceCodeResend]) { verificationId = verId; return verId; }

ios developer Over a year ago

I added debugger. Problem is it does not take time to go inside the _auth.verifyphonenumber or smsOTPSent method. it just moves outside and return the value and after sometime by breakpoint move inside those method. so it is taking time to goes inside but by the time it just return the value

Alexandre Over a year ago

You can set a timeout to the future to wait more time.. api.dart.dev/stable/2.7.1/dart-async/Future/timeout.html

Alex Myznikov · Accepted Answer · 2020-02-26 21:30:43Z

I think you can try to use the Completer class to complete your Future when verificationCompleted gets called and verId is available.

Like so:

Future<String> FetchFirebaseVeriID() async {

    // ...

    final completer = new Completer();

    try {
      await _auth.verifyPhoneNumber(

          // ...

          verificationCompleted: (AuthCredential phoneAuthCredential) {
            print('phoneAuthCredential => ${phoneAuthCredential}');
            completer.complete(verId);
          },
          verificationFailed: (AuthException exceptio) {
           completer.completeError("Error");
          });
    } catch (e) {
      completer.completeError("Error");
    }

    return completer.future;
}

https://api.dart.dev/stable/2.7.1/dart-async/Completer-class.html

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