How to get Firestore Data by method in Flutter - firebase

Question

I am trying to get users name but Flutter gives this error:
The body might complete normally, causing 'null' to be returned, but the return type is a potentially non-nullable type.
Try adding either a return or a throw statement at the end.
Method:
String getUserNameFromUID(String uid) {
FirebaseFirestore.instance
.collection('users')
.where('uid', isEqualTo: uid)
.get()
.then((QuerySnapshot querySnapshot) {
querySnapshot.docs.forEach((doc) {
return doc["name"];
});
});
}
How can I solve my problem? if I add return 0 to end of the method it always gives 0.
It always gives 0.(I do not want 0, I want get user name from uid)
String getUserNameFromUID(String uid) {
FirebaseFirestore.instance
.collection('users')
.where('uid', isEqualTo: uid)
.get()
.then((QuerySnapshot querySnapshot) {
querySnapshot.docs.forEach((doc) {
return doc["name"];
});
});
return "0";
}
EDIT: I need a String solution, not Future. The method should return String...
Because my UI is not future builder. Isn't there any way to return one data as String in Firestore database?

Answer 1

First your function should return a Future<String> since it relies on firestore's get wich also returns a future. Also docs is a list, you have to return just one. The first one i guess. In the UI just use a FutureBuilder
Future<String> getUserNameFromUID(String uid) async {
final snapshot = await FirebaseFirestore.instance
.collection('users')
.where('uid', isEqualTo: uid)
.get();
return snapshot.docs.first['name'];
}
Since you can't use FutureBuilder. An ugly alternative is to pass a callback to getUserNameFromUID and call setState from there.
void getUserNameFromUID(String uid, Function (String name) onData) {
final snapshot = await FirebaseFirestore.instance
.collection('users')
.where('uid', isEqualTo: uid)
.get().then((s) => onData(s.docs.first['name']));
}
On your UI
...
getUserNameFromUID(uid, (String name){
setState(()=> name = name);
});
From your last comment just inherit from StatefulWidget. And call the function from inside.
#override
void initState() {
getUserNameFromUID(uid);
}
If you had special requirements about not being able to modify the UI, you should mention that as it conditions the way to use the backend services.