Class

NSCache

A mutable collection you use to temporarily store transient key-value pairs that are subject to eviction when resources are low.

Declaration

@interface NSCache<__covariant KeyType, __covariant ObjectType> : NSObject

Overview

Cache objects differ from other mutable collections in a few ways:

  • The NSCache class 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 NSMutableDictionary object, 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.

Topics

Managing the Name

name

The name of the cache.

Managing Cache Size

countLimit

The maximum number of objects the cache should hold.

totalCostLimit

The maximum total cost that the cache can hold before it starts evicting objects.

Managing Discardable Content

evictsObjectsWithDiscardedContent

Whether the cache will automatically evict discardable-content objects whose content has been discarded.

NSDiscardableContent

You implement this protocol when a class’s objects have subcomponents that can be discarded when not being used, thereby giving an application a smaller memory footprint.

Managing the Delegate

delegate

The cache’s delegate.

NSCacheDelegate

The delegate of an NSCache object implements this protocol to perform specialized actions when an object is about to be evicted or removed from the cache.

Getting a Cached Value

- objectForKey:

Returns the value associated with a given key.

Adding and Removing Cached Values

- setObject:forKey:

Sets the value of the specified key in the cache.

- setObject:forKey:cost:

Sets the value of the specified key in the cache, and associates the key-value pair with the specified cost.

- removeObjectForKey:

Removes the value of the specified key in the cache.

- removeAllObjects

Empties the cache.

Relationships

Inherits From

See Also

Purgeable Collections

NSPurgeableData

A mutable data object containing bytes that can be discarded when they're no longer needed.