Things I Learned Writing Persist

18 Jun 2020

Recently I released the 1.0 version of Persist, a Swift package I’ve been working on for the last few months, and thought it would be interesting to write up some of the things I learned along the way.

UserDefaults

UserDefaults is a very common method of persisting data on Apple’s platforms. Normally I would write code similar to:

UserDefaults.standard.set(123.45, forKey: "my-key")
UserDefaults.standard.double(forKey: "my-key")

However, when adding generics around UserDefaults surfaces some of the internals. For example, setting a Double without a factional part for a key and requesting the value from objectForKey(_:) will return an Int:

UserDefaults.standard.set(123 as Double, forKey: "my-key")
UserDefaults.standard.objectForKey("my-key") // Int(123)

This is also true of Bools:

UserDefaults.standard.set(false, forKey: "my-key")
UserDefaults.standard.objectForKey("my-key") // Int(0)