Wait for a future function to complete its execution - firebase

Question

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);
}
}

Answer 1

If you want a future to complete inside a build/builder method then using FutureBuilder is a good way to go. This way you can set up a placeholder widget whilst the future is completing, and then once its done output the desired results. In the example below I've used a circular progress indicator as the placeholder but you could use any widget:
return StreamBuilder(
stream: FirebaseAuth.instance.onAuthStateChanged,
builder: (BuildContext, snapshot) {
if(snapshot.hasData) {
return FutureBuilder(
future: readData(),
builder: (context, futureSnapshot) {
if (!futureSnapshot.hasData) return CircularProgressIndicator();
print(flag);
if(flag ==true)
return DashboardPage();
else
return UserProfile();
}
);
}
else {
return SignIn();
}
},
);

Answer 2

Since readData() is An AsyncFunction ,
Adding the keyword await before readData() , will force readData() to finish,and after that it will print , However you can't do it since StreamBuilder is waiting for Widget return type.
So what you can do is .
maybe modify your readData to return a Future boolean readData() ;
Future<bool> 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)
{
print(data.key);
print(data.value);
return true;
}
else{
print('User not found');
return false;
}
});
}
and you call it :
readData().then((value)
{
if (value ==true)
return Widget
else
return Widget;
}
);
Hope that helps.