Wait for a future function to complete its executiom

I am new to flutter and in my below code there is read data function is called at line 18(it performs Firebase Database operations) I want that the readData() function must complete its execution before going to the print statement(line 19) and further execution.

import 'package:firebase_auth/firebase_auth.dart';
import 'package:firebase_database/firebase_database.dart';
import 'package:flutter/cupertino.dart';
import 'package:udharibook/Screens/SignInPage.dart';
import 'package:udharibook/Screens/UserProfile.dart';
import 'package:udharibook/Screens/dashboard.dart';

class AuthService  {
  bool flag = false;
  final FirebaseAuth _auth = FirebaseAuth.instance;
  final DBRef = FirebaseDatabase.instance.reference().child('Users');

    handleAuth(){
    return StreamBuilder(
      stream: FirebaseAuth.instance.onAuthStateChanged,
      builder: (BuildContext, snapshot) {
        if(snapshot.hasData) {
         readData();
          print(flag);
          if(flag ==true)
            return DashboardPage();
          else
            return UserProfile();
        }
        else {
          return SignIn();
        }
      },
    );
  }

  Future<void> readData() async {
    final FirebaseUser user = await _auth.currentUser();
    final userid = user.uid;
    DBRef.child(userid).once().then((DataSnapshot data){
      print(userid);
      if(data.value!=null)
        {
          flag =  true;
          print(data.key);
          print(data.value);
        }
      else{
        print('User not found');
        flag = false;
      }
    });
  }

  signOut(){
    FirebaseAuth.instance.signOut();
  }

  signIn(AuthCredential authCreds){
    FirebaseAuth.instance.signInWithCredential(authCreds);
  }

  signInWithOTP(smsCode,verId){
    AuthCredential authCreds = PhoneAuthProvider.getCredential(
        verificationId: verId,
        smsCode: smsCode
    );
    signIn(authCreds);
  }
}<