How to update an inventory with offline users in Firebase? - firebase

Question

Firebase: How to update an inventory with offline users?
I have an application for Agro built with Flutter and Firebase. The application works as follows:
You have a Food Inventory in the Warehouse
Users go out to feed the animals in offline mode (they are on a farm)
Then they return and the database writes are executed according to the Offline properties of Firebase
When users are online at feeding time, everything works perfectly, but when feeding in offline mode I have a problem specifically updating the inventory, since the offline user cached inventory may be different than the real inventory (either because new foods have been introduced or because of online updates from other users).
The way I am writing the data to Firebase (using the BLOC pattern) is as follows:
Future<bool> actualizarCantidad(String idAlimento, double cantidadActualizada) async {
try {
db.child('alimento/$idAlimento').update({ "cantidad": cantidadActualizada});
} catch (e) {
print(e);
}
return true;
}
The function where the inventory is read and the update of the database is ordered is the following:
Future<void> _submit() async {
_alimento = await alimentoBloc.cargarAlimento(alimentar.idAlimento);
//To read the inventory for the specific food type (alimentar.idAlimento)
final _cantAlimento = _alimento.cantidad - _consumoTotal;
//_alimento.cantidad is refered to the inventory
//_consumoTotal is the quantity to reduce (eaten food)
_alimentoBloc.actualizarCantidad(alimentar.idAlimento, _cantAlimento);
//Use the BLOC and PROVIDER pattern to Update the Inventory with a new Quantity (_cantAlimento)
}
What I would like to do in Firebase is that instead of assigning _cantAlimento quantity to Inventory, execute something like "decrease _consumoTotal of the number in Inventory" and that way it would not matter if the user is Offline or Online. Is this possible?
Another alternative that I have reviewed is to use Transactions to ensure that you are using the latest data, but transactions are lost when the user is offline so it is not a possibility.
How could I update the Inventory in the correct way, considering that my users are often offline?

Answer 1

Since a few months ago Firebase Realtime Database supports a ServerValue.increment() operation that can be used to atomically increment/decrement a value in the database, even when the client is not connected to the servers.
This new method also just landed in version 4.1 of the FlutterFire firebase_database plugin's ServerValue class. If you have issues with it, I'd file a bug or leave a comment on the feature request.

Answer 2

Based on #puf's answer, the procedure to use ServerValue.increment() to update an inventory or a number in Firebase is as follows:
db.child('alimento/$idAlimento/cantidad')
.set(ServerValue.increment(-consumo.round()));
It's important to note that ServerValue.increment() only support integers (until now, it doesn't support doubles). If you have a double number, you should round it up and you can try to use an smaller unit (grs instead Kgs in my case).