UserDefaults Is Cached in Memory

[ahmedk92]

(Originally published 2020-05-22)

UserDefaults caches its contents in memory, and loads it on application startup even if it's not used. This is vaguely documented.
From the docs:

UserDefaults caches the information to avoid having to open the user’s defaults database each time you need a default value.

Let's do an experiment to confirm that.

Let's make a very simple app based on the single view app template.
Before doing anything, let's record how much memory does our app consumes before any UserDefaults-related work.

On the iPhone SE 2 simulator: 8.7 MB.

Good. Let's save 100 MB of data to UserDefaults and see what we get.

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    UserDefaults.standard.set(Data(repeating: 1, count: 100_000_000), forKey: "data")    
    return true
}

After running that I got: 104 MB.

OK, let's run again removing that code; essentially like our very first run, and see if anything changes.

Now we got: 104 MB.

So, even if we no longer save or read from UserDefaults, just having that data there makes our app consumes what that data worth in memory.

Code for the experiment.