[LAD] MIDI and audio patchbays

[Pedro Lopez-Cabanillas]

> Hello folks!
>    One question, I hope it's not too dumb. :-(
>    If you have your average patchbay, how does it know, when new MIDI/audio
> ports/clients come to live or die? And how does it know, that some
> connection was killed by some other application.
>    Does it simply query it all the time? I wouldn't think so... But perhaps
> I'm wrong...

For ALSA MIDI sequencer, the patchbay program can create an input sequencer 
port, and subscribe it to System:announce (0:1) to receive ALSA notifications 
about these events: client start/exit, port start/exit, port 
subscribed/unsubscribed. You can try this with aseqdump, like the following 
sample session:

$ aseqdump -p 0:1
Waiting for data. Press Ctrl+C to end.
Source  Event                  Ch  Data
  0:1   Port subscribed            0:1 -> 128:0
  0:1   Client start               client 129
  0:1   Client start               client 130
  0:1   Port start                 129:0
  0:1   Port start                 130:0
  0:1   Client start               client 131
  0:1   Port start                 131:0
  0:1   Port subscribed            129:0 -> 131:0
...
  0:1   Port unsubscribed          129:0 -> 131:0
  0:1   Port exit                  129:0
  0:1   Client exit                client 129
  0:1   Port exit                  130:0
  0:1   Client exit                client 130
  0:1   Port exit                  131:0
  0:1   Client exit                client 131
...

Regards,
Pedro