How to cache data in Swift using NSCache

Posted on 7 Apr 2023 . 2 min read

When we develop apps for iOS, we need to perform heavy tasks such as loading images from the network, loading files, etc. It’s a very time-consuming task, which affects the performance of the application. Therefore, we cache or store temporary data for them and reuse them appropriately.

Swift uses NSCache to store temporary data in memory. It is a mutable collection that uses key-value pairs to store temporary data.

It evicts objects associated with data when the system is low on memory. The data that is evicted from the memory has to be recreated again. We can add, remove, or query items in the cache.

Syntax

The NSCache class constructor takes two objects: key type and object type.

let cache = NSCache<NSURL, UIImage>()

We can optionally give the cache a name for later use. By default, the value is an empty string.

let cache = NSCache<NSURL, UIImage>()
cache.name = “fetch images”

Storing object

We can set the value to the specified key using the setObject function, which takes two parameters: the object to store and the key name.

let image = UIImage(named: “kitten.png”)!
cache.setObject(image, forKey: “banner”)

Removing cache object

The removeObject function, which takes the key name of the object to be removed as a parameter, can be used to remove an object from cache.

cache.removeObject(forKey: “banner”)

And to remove all objects and empty the cache, use the removeAllObjects function.

cache.removeAllObjects()

Getting object value

The object method returns the value associated with a given key. It gives the value associated with the key or returns nil if there is no value.

if let image = cache.object(forKey: “banner”) {
  print("Image is in the cache")
} else {
  print("Image is not in the cache")
}

Cache Size

We set the maximum number of objects using the countLimit property. If we set countLimit to 0, there is no object. If an object’s limit increases, then it should be evicted from memory instantly.

cache.countLimit = 20

How to set total cost

We can set the totalCostLimit of an object when it is added to the cache. It specifies an object, such as the bytes of an object. The default value is 0; there is no cost value. If an object’s total cost exceeds the limit, it will be automatically removed from memory.

cache.totalCostLimit = 20_000_000

The bottom line

It’s good practice to use cache to store temporary data, which is very expensive and time-consuming. It increases the performance of the app and the resources of the system.

Don’t hesitate to contact me if you have any questions or queries. Follow me on twitter @gurjitpt for any updates.

Thanks!

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Gurjit Singh

I’m Computer Science graduate and an iOS Engineer who writes about Swift and iOS development. Follow me for more updates:

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