Описание тега nscache
NSCache is a cache-related class included in Apple's Foundation framework.
NSCache is similar to NSMutableDictionary, with the differences being: [1]
1. It is guaranteed to be thread-safe.
2. It is much slower to access.
3. It may throw out objects from time to time. You can set costs and limits, but they're not guaranteed to be followed.
4. It is not toll-free bridged to anything in CoreFoundation.
5. You can't query the number of objects that are in the cache.
6. You can't enumerate a cache.