Flutter How to validate uniqe username? - firebase

Question

I am trying to validate uniqe username for SignUp Page.
Here is my TextFormField code:
TextFormField(
onSaved: (deger) => _username = deger!,
textInputAction: TextInputAction.done,
keyboardType: TextInputType.name,
controller: usernameController,
decoration: const InputDecoration(
suffixIcon: Icon(Icons.person),
label: Text("username"),
),
),
Here is my Button:
ElevatedButton(
onPressed: () async {
final valid = await _checkUserName(
usernameController.text);
if (valid!) {
Get.snackbar(
"hata", "username exist");
} else if (formKey.currentState!.validate()) {
formKey.currentState!.save();
//myAuthController codes here it doesnt metter.
} else {
debugPrint("error");
}
},
child: const Text("SIGNUP"),
),
My function for validate existed username in Firestore:
Future<bool?> _checkKullaniciAdi(String username) async {
final result = await FirebaseFirestore.instance
.collection("customer")
.where('username', isEqualTo: username)
.get();
return result.docs.isEmpty;
}
This codes are always returning
Get.snackbar("hata", "username exist");
What can I do ?

Answer 1

if (valid!) {
Get.snackbar("hata", "username exist");
}
if the username is valid, show hata username exists
That's what the above code says. This is more likely what you wanted:
if (!valid!) {
Get.snackbar("hata", "username exist");
}
The first ! means not, so if it is not valid, the second ! means that valid can't be null, if valid is null, it will throw an error.
Finally, the reason why we have to add the second ! is because your _checkKullaniciAdi method returns bool? so it can return true, false or null. But it doesn't return null, you marked it as a possibility, but it never happens, you could change the return type to Future<bool> and that way you would be able to remove the second !
Future<bool> _checkKullaniciAdi(String username) {
}
if (!valid) {
}