How to cache data using NSCache

Paul Hudson    September 15th 2016    @twostraws

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

Did this solution work for you? Please pass it on!

Tweet

Other people are reading…

• How to load and save a struct in UserDefaults using Codable
• How to parse JSON using JSONSerialization
• How to run code on the main thread using GCD async()
• How to read names in a string using NSLinguisticTagger
• How to synchronize code to drawing using CADisplayLink

About the Swift Knowledge Base

This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.