What is the best way to get the UID of any selected page item (frames, rectangle etc)?

Question

I need UID of any selected item on page and also array of UID's in case multiple selection happens. How can I achieve it?
One way I know is to write an observer and attach it on every new page Item created, but that way the system might become slow and also
the behavior will be unpredictable in case of multiple selections.
Can some one suggest a better way to do it?

Answer 1

The selection subsystem hides most ideas of direct adressing (e.g. UID of page item(s) or story) into the concrete selection, while from the outside you only work on the suite interfaces of the abstract selection. So you don't ask whether the selection is some kind of page item and you don't peek at the actual items. Instead you ask about their capabilities and applied values.
Attaching observers to page items has nothing to do with the selection, because they only notify about changes on those page items - which could also be caused by scripts or anything else. Even then, the most relevant changes go thru the kDocBoss as single point to attach an observer, rather than thru all those page items.
Regarding speed - the selection has many optimizations, for example "lazy" notifications can be deferred until they become appropriate for an UI update, they might even become aggregated / optimized away.
One starting point into the selection when you come from nowhere would be ISelectionUtils. Your selection observer might also pass interesting parameters.
Dirk