updateUserSelections method
Update the current userSelectionState. The callback stateUpdate
receives the actual version of the state and should return the updated
version.
If the returned state is the same as before, no update is triggered.
Implementation
void updateUserSelections(
UserSelectionState Function(UserSelectionState previousState) stateUpdate,
) {
userSelections.value = stateUpdate(userSelections.value);
}