Support for retention of event messages emitted from Publisher and retrieve from Subscriber end

[ganeshkumartk]

It will be good if 'Event History' feature is implemented in dart.

Like when publishing an event, the publisher can set an option (retain=True) in the PublishOptions which will cause the Broker to retain the event being published as the most-recent event on this topic.

Same upon subscription, a client can pass (get_retained=True) in the SubscribeOptions. If there is any retained event, it will be immediately sent to the subscriber.

Reference: https://crossbar.io/docs/Event-History/

Python implementation: https://github.com/crossbario/autobahn-python/tree/master/examples/twisted/wamp/pubsub/retained

[konsultaner]

this feature is called event retention and is added in 1.0.6

[ganeshkumartk]

Thanks @konsultaner for this feature implementation ✌️