Notifies registered reactions when an event is published. Publishers are also reactors and listen to themselves per default as a convenience.

In order to reduce memory leaks, reactions are weakly referenced by default, unless they implement Reactions.StronglyReferenced. That way, the lifetime of reactions are more easily bound to the registering object, which are reactors in common client code and hold strong references to their reactions. As a result, reactors can be garbage collected even though they still have reactions registered at some publisher, but not vice versa since reactors (strongly) reference publishers they are interested in.