[platform-core-dev] New preferences API and event notification

[Christof Marti <christof_marti@xxxxxxxxxx>]

It currently does not appear to be possible to receive events from
preference changes based on a preference's lookup order.

For example: given a preference with the lookup order "project", "instance"
it is not possible to:
1) listen on preference changes from both nodes by registering a listener
only on one object, such that the listener does not have to deal with all
nodes in the lookup order.
2) like 1) but additionally hide events on the preference in the "instance"
node *if* the same preference is set in the "project" node.
3) set a lookup order for a 'qualifier' (E.g., a plugin-id, see
IPreferencesService) instead of a 'qualifier', 'preference key' pair.

Some preferences in JDT/UI (like Task Tags, but also compiler options from
JDT/CORE, probably more in the future) can be set for an "instance" and
selectively on a per "project" basis. In order to simplify the access to
such preferences we would like to:
4) retrieve the preference value from one object, no matter whether the
preference is set in the "project" or the "instance" scope . This is solved
by IPreferencesService.
5) listen for events on one object and receive only visible changes. For
example a change in the "instance" scope can be hidden because the
preference is set in the "project" scope. (corresponds to 1) and 2) above)
6) set a lookup order for the JDT/UI plugin-id (not for each individual
preference). (corresponds to 3) above)

Are these observations correct? Are there any plans to add the
functionality 1), 2) and 3)?

Thanks,

Christof

[Please keep the CCs]