How to cache data using NSCache

Swift version: 5.1

Paul Hudson @twostraws May 28th 2019

Here's an easy win for you that will make your apps immediately much better: NSCache is a specialized class that behaves similarly to a mutable dictionary with one major difference: iOS will automatically remove objects from the cache if the device is running low on memory.

Helpfully, if the system does encounter memory pressure NSCache will automatically start to remove items without you knowing about it, which means you won't get a memory warning unless even more RAM needs to be cleared. It will also remove items intelligently, trying to keep as much cached as possible.

Here's how to use it, imagining a fictional class called ExpensiveObjectClass that you want to compute as infrequently as you can:

let cache = NSCache<NSString, ExpensiveObjectClass>()
let myObject: ExpensiveObjectClass

if let cachedVersion = cache.object(forKey: "CachedObject") {
    // use the cached version
    myObject = cachedVersion
} else {
    // create it from scratch then store in the cache
    myObject = ExpensiveObjectClass()
    cache.setObject(myObject, forKey: "CachedObject")
}

Available from iOS 4.0