reduce abstract method
The reduce method is the action reducer. It may read the action state,
the store state, and then return a new state (or null if no state
change is necessary).
It may be synchronous (returning AppState or null)
or async (returning Future<AppState> or Future<null>).
The StoreConnectors may rebuild only if the reduce method returns
a state which is both not null and different from the previous one
(comparing by identical, not equals).
Implementation
FutureOr<St?> reduce();