NSRuleEditor looks like a pretty powerful class and it looks like stuff like iTunes Smart Playlists or Finder Smart Folders are using it.
(I reckon it should be called NSSmartRuleEditor...)
But is there any kind of documentation apart form the reference?
Some kind of sample code or guide would be nice but it doesn't look like ADC has anything to offer.. as of now NSRuleEditor seems to be the least documented control in Cocoa!
Anybody knowing of any Open Source projects using the controls, blog articles, etc.?
I think you should probably be looking for NSPredicateEditor rather than NSRuleEditor. NSPredicateEditor is a subclass of NSRuleEditor and is what is used for editing "smart" playlists and the such.
Apple's Documentation: https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/NSPredicateEditor_class/Reference/NSPredicateEditor.html
Sample App: https://developer.apple.com/library/mac/#samplecode/PhotoSearch/Introduction/Intro.html