PropertiesChanged signal

Thiago Macieira thiago at
Sat May 8 00:21:49 PDT 2010

Em Sábado 8. Maio 2010, às 04.28.33, David Zeuthen escreveu:
> While it is certainly your prerogative to not support this new signal
> in QtDBus, please note this new signal is mostly a codification of
> existing (and in my humble opinion good) practices. I mean, one way to
> look at it is that if you have a remote object with properties you
> want remote peers to know about these properties. So one way or the
> other you will to transport these values over the transport and that
> includes property changes too. Codifying this into a PropertiesChanged
> signal is helpful because it allows bindings to automatically handle
> this and it does so in a race-free manner and economical manner [0].
> As to whether this signal is inherently wasteful, why is this any
> different than any other (existing) signal? Sure, the way things
> currently work you will always have signal traffic flowing from each
> name owner to the bus even if no clients are listening. While this
> comes with some overhead [1] we can solve this by extending the D-Bus
> protocol with something like this handwavy proposal:

There are two reasons for wanting to know from the bus if anyone is listening:

1) some properties may be changing very fast and relaying that info to the bus 
if no one is interesting will just generate useless traffic, like the position 
of a window/item during motion

2) connecting for a signal makes the signal emission slower than if nothing is 
connected, so it's also a performance issue

Maybe extending the bus protocol like you suggested is the way to go, but it 
requires a more specific "connect to signal" method than AddMatch.

One more suggestion: an annotation, per property, that indicates whether this 
property is contained in the PropertiesChanged signal. This allows a client to 
know whether the PropertiesChanged signal will contain any interesting 
information, or if it will be emitted at all.

Thiago Macieira - thiago (AT) - thiago (AT)
  Senior Product Manager - Nokia, Qt Development Frameworks
      PGP/GPG: 0x6EF45358; fingerprint:
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