NSCache
Unlike an NSMutableDictionary object, a cache does not copy the key objects that are put into it??
NSDiscardableContent
NSPurgeableData
NSMutableData
NSDictionary
【NSCache】If the behavior you want to add supplements that of the existing class, you could write a category on NSDictionary. Keep in mind, however, that this category will be in effect for all instances of NSDictionary that you use, and this might have unintended consequences. Alternatively, you could use composition to achieve the desired behavior.???
NSMapTable
