Adds a finalizer to this subscription, so that finalization will be unsubscribed/called when this subscription is unsubscribed. If this subscription is already #closed, because it has already been unsubscribed, then whatever finalizer is passed to it will automatically be executed (unless the finalizer itself is also a closed subscription).
Closed Subscriptions cannot be added as finalizers to any subscription. Adding a closed subscription to a any subscription will result in no operation. (A noop).
Adding a subscription to itself, or adding null or undefined will not perform any
operation at all. (A noop).
Subscription instances that are added to this instance will automatically remove themselves
if they are unsubscribed. Functions and Unsubscribable objects that you wish to remove
will need to be removed manually with #remove
The finalization logic to add to this subscription.
Completes the current handling, triggering actions.complete. If a value for final is provided,
that is emitted via actions.next first. Useful to end the handling but signal a successful completion
to any observers/listeners.
Optional final: TNextTerminates the listener, any of its Observable handlings.
The closed subscription.
Adds a function to be called only once when stop() is invoked