NSCache object is a mutable collection that stores key-value pairs, similar to an
NSDictionary object. The
NSCache class provides a programmatic interface to adding and removing objects and setting eviction policies based on the total cost and number of objects in the cache.
- iOS 4.0+
- macOS 10.6+
- tvOS 4.0+
- watchOS 2.0+
NSCache objects differ from other mutable collections in a few ways:
NSCacheclass incorporates various auto-eviction policies, which ensure that a cache doesn’t use too much of the system’s memory. If memory is needed by other applications, these policies remove some items from the cache, minimizing its memory footprint.
You can add, remove, and query items in the cache from different threads without having to lock the cache yourself.
NSMutableDictionaryobject, a cache does not copy the key objects that are put into it.
You typically use
NSCache objects to temporarily store objects with transient data that are expensive to create. Reusing these objects can provide performance benefits, because their values do not have to be recalculated. However, the objects are not critical to the application and can be discarded if memory is tight. If discarded, their values will have to be recomputed again when needed.
Objects that have subcomponents that can be discarded when not being used can adopt the
NSDiscardableContent protocol to improve cache eviction behavior. By default,
NSDiscardableContent objects in a cache are automatically removed if their content is discarded, although this automatic removal policy can be changed. If an
NSDiscardableContent object is put into the cache, the cache calls
discardContentIfPossible() on it upon its removal.