Best way to initialize non-optional var if optional is nil in Swift

Question

I would like to initialize a variable obj by taking it from UserDefaults, which returns a String?, and if it's nil build the value and assign it.

The following code works, but, at the end, my obj is a String? while I want it to be a String (since it can't be nil at this stage).

var obj = UserDefaults.standard.string(forKey: "my_key")// Here, obj is a String?
if obj == nil {
    obj =  ProcessInfo.processInfo.globallyUniqueString// Returns, a String
    defaults.set(obj, forKey: "my_key")
    defaults.synchronize()
}
// Here, obj is still a String?

Is there a good pattern / best practice for this kind of situation ?

Answer 1

You can use the nil-coalescing operator ?? with an "immediately evaluated closure":

let obj = UserDefaults.standard.string(forKey: "my_key") ?? {
    let obj = ProcessInfo.processInfo.globallyUniqueString
    UserDefaults.standard.set(obj, forKey: "my_key")
    return obj
}()

print(obj) // Type is `String`

If the user default is not set, the closure is executed. The closure creates and sets the user default (using a local obj variable) and returns it to the caller, so that it is assigned to the outer obj variable.