How well does NSCache and NSPurgeableData work for caching Contacts data or anything else?

Anyone have any words of wisdom about using NSCache and NSPurgeableData for caching in an iOS app, specifically to cache Contacts data from the device contact store? Under what conditions does this work well?

How well does NSCache and NSPurgeableData work for caching Contacts data or anything else?
 
 
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