Draft: Event system refactoring proposal [15h]

NOTE this is not a final proposal - it is an entry point for discussion

Some tasks that can be done without any significant problems:

• move admin.execute command to Zmq/Notifd
• wrap response from admin result into some structure
• replace stateless bool with method
• initialize consumers when app starts
• merge heartbeat and regular events
• replace ifs with methods aka pipe, interface change etc

Below is a short description of the proposal for event system refactoring. Detailed discussion will be at the next Tango Kernel meeting (Q'3 2018).

Roughly speaking this refactoring aims to restructure existing classes [1] and code [2,3] preserving existing features:

1. heartbeat
2. reconnect
3. compatibility

[1]

New classes structure [4]:

Below is an example of a new API usage (it is in Java but may give a feeling of how this should look like in c++ as well):

@Test
public void test_subscribe() throws Exception {
        EventSubscriber subscriber = proxy.getEventSubscriber("zmq");//proxy comes from somewhere
        EventSubscription subscription = subscriber.createOrGetSubscription("float_scalar","change");

        try {
            subscriber.getTransport().connect(subscription.getEndpoint());

            subscription.setEventConsumer(EventConsumers.createSimpleCallbackConsumer(new EventCallback() {
                @Override
                public void onEvent(EventData data) {
                    System.out.println("onEvent");
                }

                @Override
                public void onError(EventData data) {
                    System.out.println("onError");
                }
            }));
        } catch (/*EventTransportConnectionFailed*/ IOException ex){
            throw ex;
        }
}

@Test
public void test_unsubscribe() throws Exception {
        EventSubscriber subscriber = proxy.getEventSubscriber("zmq");
        EventSubscription subscription = subscriber.createOrGetSubscription("float_scalar","change");

        subscription.cancel();
}

@Test
public void test_transport_loop() throws Exception {
        while (!Thread.currentThread().isInterrupted()) {
            EventData event = transport.nextEvent();//transport is initialized somewhere i.e. heartbeat subscription

            EventSubscription<?> subscription = EventSubscriptions.getFor(event);

            subscription.getEventConsumer().consume(event);
        }
}

@Test
public void test_heartbeat() throws Exception {
        EventSubscriber subscriber = proxy.getEventSubscriber("zmq");
        EventSubscription subscription = subscriber.createHeartbeatSubscription();

        subscription.getTransport().connect(subscription.getEndpoint());
        subscriber.startTransport();
}

P.S. do we need to support multicast?