NSCoding, archiving

Middle PRO

Essence

Archiving is serializing an object graph into NSData and back with NSKeyedArchiver/NSKeyedUnarchiver. For a class to be archivable it implements the NSCoding protocol:

• -encodeWithCoder:(NSCoder *)coder — write the state by keys;
• -initWithCoder:(NSCoder *)coder — restore it.

NSCoder is the abstract "coder" class; the keyed archiver writes key → value pairs, which lets the set of fields change between versions (old keys are simply ignored, missing new ones make decodeObjectForKey: return nil). This is how UIViews are stored in storyboards/XIBs, state restoration, NSUserDefaults for non-standard types, on-disk caches.