How to use transaction with int data - Firestore Flutter - firebase

Question

Upon creating an account the user's firestore creates a field that displays the current amount of plastics the user has. So far, I have a button that updates that amount using the user's text field input. I've heard of something called a transaction which apparently allows one to intsead add the input amount to the overall data for it to be displayed? How would I accomplish this in my case when the use inputs a new amount?
Code:
database.dart
Future<void> userSetup(String displayName) async {
int plastics = 0;
final CollectionReference users =
FirebaseFirestore.instance.collection('UserNames');
FirebaseAuth auth = FirebaseAuth.instance;
String uid = auth.currentUser.uid.toString();
users.doc(uid).set({'displayName': displayName, 'uid': uid});
//This is the field for plastics amount starting at 0.
users.doc(uid).update({'plastics': plastics});
return;
}
How I retrieve the amount data:
final firestore = FirebaseFirestore.instance;
FirebaseAuth auth = FirebaseAuth.instance;
Future<String> getPlasticNum() async {
final CollectionReference users = firestore.collection('UserNames');
final String uid = auth.currentUser.uid;
final result = await users.doc(uid).get();
return result.data()['plastics'].toString();
}
How I display it:
FutureBuilder(
future: getPlasticNum(),
builder: (_, AsyncSnapshot snapshot) {
if (snapshot.connectionState == ConnectionState.waiting) {
return Center(child: CircularProgressIndicator());
}
return Text(snapshot.data,
style: TextStyle(color: Colors.amber[400], fontSize: 20));
},
),
Currently how the user can replace the amount, but not add to it(The problem)
OnPressed: () async {
try {
final String uid = auth.currentUser.uid;
FirebaseFirestore.instance
.collection('UserNames')
.doc(uid)
.update({
"plastics": int.parse(_plasticController.text),
});
Navigator.of(context).pop();
} catch (e) {}
},

Answer 1

I made a separate future to take care of adding:
final firestore = FirebaseFirestore.instance; //
FirebaseAuth auth = FirebaseAuth.instance;
Future<bool> addPlastic(String amount) async {
try {
String uid = auth.currentUser.uid;
var value = double.parse(amount);
DocumentReference documentReference =
FirebaseFirestore.instance.collection('UserNames').doc(uid);
FirebaseFirestore.instance.runTransaction((transaction) async {
DocumentSnapshot snapshot = await transaction.get(documentReference);
if (!snapshot.exists) {
documentReference.set({'plastics': value});
return true;
}
double newAmount = snapshot.data()['plastics'] + value;
transaction.update(documentReference, {'plastics': newAmount});
return true;
});
} catch (e) {
return false;
}
}
Then I just called it when the button was pressed
onPressed(){
addPlastics(_plasticController.text);
}