A mutable collection you use to temporarily store transient key-value pairs that are subject to eviction when resources are low.
SDKs
- iOS 4.0+
- macOS 10.6+
- Mac Catalyst 13.0+
- tvOS 9.0+
- watchOS 2.0+
Framework
- Foundation
Declaration
@interface NSCache<__covariant KeyType, __covariant ObjectType> : NSObject
Overview
Cache objects differ from other mutable collections in a few ways:
The
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.
Unlike an
NSMutableobject, a cache does not copy the key objects that are put into it.Dictionary
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 NSDiscardable protocol to improve cache eviction behavior. By default, NSDiscardable objects in a cache are automatically removed if their content is discarded, although this automatic removal policy can be changed. If an NSDiscardable object is put into the cache, the cache calls discard on it upon its removal.