Publish : executes callback defined for a topic.

Subscribe subscribes to a topic.

SubscribeAsync subscribes to a topic with an asynchronous callback Transactional determines whether subsequent callbacks for a topic are run serially (true) or concurrently (false) Returns error if `fn` is not a function.

SubscribeOnce subscribes to a topic once.

SubscribeOnceAsync subscribes to a topic once with an asynchronous callback Handler will be removed after executing.

Unsubscribe removes callback defined for a topic.

Bus : Main event bus.