A connection to the Cloud Pub/Sub service to publish events.
This interface defines pure-virtual methods for each of the user-facing overload sets in BlockingPublisher. That is, all of BlockingPublisher's overloads will forward to the one pure-virtual method declared in this interface. This allows users to inject custom behavior (e.g., with a Google Mock object) in a BlockingPublisher object for use in their own tests.
Applications may define classes derived from PublisherConnection, for example, because they want to mock the class. To avoid breaking all such derived classes when we change the number or type of the arguments to the member functions we define lightweight structures to pass the arguments.