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Subscription
------------
The subscription mechanism would involve sending a Subscribe()
(preferably without reply) message to the subscriber, and one AddMatch()
message to the D-Bus daemon for each property subscribed. Unsubscribing
is similarly straightforward.
This is based on the fact that the provider mainly is interested in the
fact whether there exists someone at all interested in their exposed
properties.
However, to handle ungraceful subscribers who don't send Unsubscribe()
messages when they go away, the provider needs to know its peers. It
can do so by maintaining an association of property => subscribers
(using the unique D-Bus name of the sender as subscriber). On receipt
of an Unsubscribe() message, or a NameOwnerChanged() signal, it needs to
check if the sender was subscribed to some properties and remove it from
the mapping. If noone is interested in a property, it should cease
updating the property to save resources.
Signaling changes
-----------------
Properties would be represented as object paths. A subscriber emits a
ValueChanged() signal whenever a value changes.
One concern is the number of match rules in the D-Bus daemon. This
approach effectively means one rule per each (subscriber, property)
pair, contrary to the current way of one rule per subscriber.
Suspending notifications
Commanding
----------
Performance speculations
------------------------
However the number of messages
With the current implementation, for N subscribers, there are N messages
emitted (with different object paths). Including the hops between the
D-Bus daemon that totals in 2*N messages, with the D-Bus daemon having
to dispatch N incoming messages. Using the new scheme it's only N+1
messages, and the daemon only has to dispatch 1 message.
The D-Bus interface
-------------------
METHOD VOID Subscribe(ARRAY OF STRING properties)
[ If a subscriber needs to immediately get the value of a property, it
should call the Get() method. ]
SIGNAL ValueChanged(BOOL unknownp, VARIANT value)
[ The old signature obviously has to change, as now we don't try to
bundle several properties into one message. If the first boolean
argument determines whether the value is known or unknown. In the
latter case, the second argument is to be ignored. Alternatively, the
unknown value could be represented specially, like an emtpy STRUCT on
D-Bus, then the BOOL is omitted. ]
METHOD VOID Unsubscribe(ARRAY OF STRING properties)
METHOD VARIANT Get(STRING property)
[ The provider shall _immediately_ return whatever value it has for the
named property. If it's unknown, then that, but it must not block. ]
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