[Bugs] #1680 UNSP: sugar-base: add sugar.dispatch.DbusSignal

[Sugar Labs Bugs]

#1680: sugar-base: add sugar.dispatch.DbusSignal
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    Reporter:  sascha_silbe               |          Owner:  sascha_silbe     
        Type:  enhancement                |         Status:  new              
    Priority:  Unspecified by Maintainer  |      Milestone:  0.88             
   Component:  sugar                      |        Version:  Git as of bugdate
    Severity:  Unspecified                |       Keywords:  r?               
Distribution:  Unspecified                |   Status_field:  Assigned         
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 This patch adds the classes sugar.dispatch.RelayedSignal and
 sugar.dispatch.DbusSignal.

 RelayedSignal is a base class for distributing signals from some external
 sources. The external source will usually only be attached if there are
 actually (internal) listeners to prevent unnecessary overhead.

 DbusSignal (deriving from RelayedSignal) relays DBus signals which can be
 passed either via full name specification (looked up and connected to only
 on demand) or as an interface instance (in case the caller already got
 one).

 It is intended to provide data store listeners in
 sugar.datastore.datastore that only listen to the DBus signals if there's
 actually someone listening for it, thus reducing overhead.

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Ticket URL: <http://bugs.sugarlabs.org/ticket/1680>
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