dbus activation on signal

[WaLyong Cho]

On 03/19/2015 08:01 PM, Simon McVittie wrote:
> On 19/03/15 10:27, WaLyong Cho wrote:
>> As I knew, dbus auto activation is only working for method call and
>> signal what has destination. But generally, in case of signal, the
>> signal destination is not of listener(what hope to be activated).
> 
> Correct. D-Bus does not have a facility for services to be activated
> automatically as a result of a broadcast signal.
> 
> If it did, there are some nasty ordering constraints - I certainly
> wouldn't want to have to delay delivery of a broadcast until all
> interested services had started up! - so I don't think adding this
> feature would be a good idea.
> 
>> If dbus has no solution for this and it is not acceptable functionality,
>> make a new daemon. The new daemon will listen all of signals and a
>> special signal was sent then activate a set of other processes.
>> But when the activated process can not receive the signal. Because that
>> already sent in the past.
> 
> This approach is certainly one possibility.
> 
> If your D-Bus API is such that the activated process can perform state
> recovery, then there's no problem. For instance, suppose you want to
> start an activated service whenever the networking service
> (NetworkManager or ConnMan or similar) detects a wireless network, which
> does something on all the wireless networks.
> 
> On startup, the activated service should connect to the
> NewWirelessNetwork signal (or whatever your networking service calls
> it), but ignore it if received; then call GetAllWirelessNetworks to see
> what is currently available; then on receiving the reply to
> GetAllWirelessNetworks, behave as if all those networks had just been
> detected, and stop ignoring NewWirelessNetwork.
> 
> If you don't or can't do state-recovery, then in general your system is
> broken *anyway*, because you might miss a wireless network that appears
> after you have started up, but before you have connected to the
> NewWirelessNetwork signal.
> 
> The same applies to any other (signal, state-recovery method) pair.
> 

Maybe I have to request to the interested service to make a new method
call for receive the missed signal as message. But if there are many of
interested services then same number of method call should be sent at once.

Anyway, really thank you for your advise.

WaLyong