so-5.4.0: so_subscribe().event() methods chaining

Calls to event() methods during event subscription can be chained now:

class my_agent : public so_5::rt::agent_t
{
public :
 
   void so_define_agent() override
   {
      so_subscribe( so_direct_mbox() ).in( st_disconnected )
         .event( so_5::signal< msg_reconnect >, &my_agent::evt_reconnect )
         .event( &my_agent::evt_send_when_disconnected )
         .event( so_5::signal< msg_get_status >,
               []() -> std::string { return "disconnected"; } );
      ...
   }
  ...
};

Note

In SObjectizer v.5.4.0 call to event() create subscription only for the states, specified by previous calls to in() in the same chain. For example in the chain so_subscribe(mbox).in(s1).in(s2).event(e1).in(s3).event(e2) event e1 will be subscribed for states s1 and s2, but not for state s3. Event e2 will be subscribed for the all states: s1, s2, s3. But the chain like so_subscribe(mbox).event(e3).in(s4) has no sence: event e3 will be subscribed only for the default state and the call to in(s4) will be completely ignored. There is no guarantee that this approach will be saved for the future SObjectizer's version. So it is recommended not to mix calls in() and event() in subscription chain – all calls to in() must precede to calls to event().