Provide a thread-safe way for a Consumer to access its Registration

[wilkinsona]

A Consumer's Registration is returned when the Consumer is passed to Reactor.on. As things stand, if the Consumer wants to access its own Registration it cannot do so without being at risk of a race condition: the Consumer's accept() method may be called before Reactor.on has returned so it won't have access to its Registration.

One solution would be for a Consumer to optionally implement RegistrationAware and have the registry pass the Registration to the Consumer before it's actually added to the registry - this would ensure that the Registration is set before accept can be called.

[gembin]

But, if i need to cancel it only if some conditions are satisfied, not necessarily cancel after use, how to handle this?

for example:

new Consumer<Event<?>>() {
  @Override
  public void accept(Event<?> e) {
      if(some conditions are satisfied){
          registration.cancel();
      }
  }
})

[jbrisbin]

instead of registration.cancel(), throw a CancelConsumerException. The Dispatchers will catch this specifically and remove the registration.