Listener Event Semantics

The arrival of an inbound message is an important event for most Rendezvous programs. It is also an instructive exemplar of an asynchronous event—the receiving program cannot know in advance when a particular message might arrive in the queue. To receive messages, programs create listener events, define callback functions to process the inbound messages, and dispatch events in a loop.

While a program is listening for messages, the event driver queues the listener event each time a message arrives with a matching subject name. Each appearance of the event in the queue leads to a separate invocation of its callback function. At any moment in time, an event queue can contain several references to the same listener event object—each reference paired with a different inbound message.

Each time the callback function runs, it receives an inbound message as an argument. The callback function must process the message in an appropriate application-specific fashion.