Flutter check if username already exist in Firestore database - firebase

Question

In my App I have to implement a SignUpScreen.
Everything works fine but one thing not. When someone wants to sign up he cannot use the same email like a other users, this works, but I want also that he cannot use the same username. I have a collection which is called 'SerX' and a document field which is called 'Username:' in cloud firestore. If anyone knows how to do that please comment your solution.
There should be a return like this return 'This username already exists!';
Widget _buildUserNameTF() {
return Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: <Widget>[
Text(
'Username',
style: kLabelStyle,
),
SizedBox(height: 10.0),
Container(
alignment: Alignment.centerLeft,
decoration: kBoxDecorationStyle,
height: 60.0,
child: TextFormField(
keyboardType: TextInputType.text,
validator: (input) {
if (input.isEmpty) {
return 'Please enter a username';
}
if(input.length < 6){
return 'Your username needs to be at least 6 characters';
}else if(input.length > 12){
return 'Your username needs to be at most 12 characters';
}
if (!RegExp(
r'^[a-zA-Z0-9]+$')
.hasMatch(input)) {
return 'Please enter a valid username.';
}
},
onSaved: (input) => _Username = input,
style: TextStyle(
color: Colors.black,
fontFamily: 'Orbitron',
),
decoration: InputDecoration(
border: InputBorder.none,
contentPadding: EdgeInsets.only(top: 14.0),
prefixIcon: Icon(
Icons.supervised_user_circle,
color: Colors.black,
),
hintText: 'Enter your Username',
hintStyle: kHintTextStyle,
),
),
),
],
);
}

Answer 1

Here is a simple function for checking if the username is already exists on client side.
var _instance = Firestore.instance;
Future<bool> userExists(String username) async =>
(await _instance.collection("users").where("username", isEqualTo: username).getDocuments()).documents.length > 0;
Client side check is not a secure solution. You should use Cloud Functions for Firebase for it.

Answer 2

I don't know if it is the perfect solution but it works for me.
doesUserExist(currentUserName) async {
try {
// if the size of value is greater then 0 then that doc exist.
await FirebaseFirestore.instance
.collection('Users')
.where('email', isEqualTo: currentEmail)
.get()
.then((value) => value.size > 0 ? true : false);
} catch (e) {
debugPrint(e.toString());
}
}

Answer 3

var formKey = GlobalKey<FormState>();
var username = TextEditingController();
var password = TextEditingController();
Future<bool> userExists(username , password) async {
return await FirebaseFirestore.instance.collection('users')
.where('username', isEqualTo: username)
.where('password', isEqualTo: password)
.get()
.then((value) => value.size > 0 ? true : false);
}
login() async {
bool result = await userExists(username.text.toString(),
password.text.toString());
// ignore: unrelated_type_equality_checks
if(result == true){
Navigator.push(
context, MaterialPageRoute(builder: (context) => Home()));
}else{
password.text = '';
Fluttertoast.showToast(msg: "Incorrect Username or Password", toastLength:
Toast.LENGTH_LONG);
}
}