The GSCache class is used to maintain a cache of objects in memory for relatively rapid access.
Typical usage might be to keep the results of a database query around for a while in order to re-use them... for instance when application configuration is obtained from a database which might be updated while the application is running.
When the cache is full, old objects are removed to make room for new ones on a least-recently-used basis.
Cache sizes may be limited by the number of objects in the cache, or by the memory used by the cache, or both. Calculation of the size of items in the cache is relatively expensive, so caches are only limited by number of objects in the default case.
Objects stored in the cache may be given a limited lifetime, in which case an attempt to fetch an expired object from the cache will cause it to be removed from the cache instead (subject to control by the delegate).
Cache keys may be objects of any type as long as they are copyable (and the copied keys are immutable) and implement the -hash and -isEqual: methods such that any two keys can be tested for equality and used as dictionary keys.
For object sizing we use the -sizeInBytesExcluding: method, which is declared in the GNUstep-base additions library headers as follows:
- (NSUInteger) sizeInBytesExcluding: (NSHashTable*)exclude;
If you wish to store objects in a size-limited cache, you should implement that method to return an appropriate size for the object you are caching.
NB. GSCache currently does not support subclassing... use it as is or extend it via categories, but do not try to add instance variables.

This protocol defines the messages which may be sent to a delegate of a GSCache object. The messages are only sent if the delegate actually implements them, so a delegate does not need to actually conform to the protocol.