update method
Updates the subscription set and send the request to the server in the background.
Calling update is a prerequisite for mutating the subscription set,
using a MutableSubscriptionSet passed to the action
.
If you want to wait for the server to acknowledge and send back the data that matches the updated subscriptions, use waitForSynchronization.
Implementation
@override
void update(void Function(MutableSubscriptionSet mutableSubscriptions) action) {
action(this);
}