DefaultsKit does not handle arguments passed on launch

[cfilipov]

One very useful feature when debugging is to use Arguments Passed On Launch to override user defaults. It seems DefaultsKit is not able to handle these keys because of the way it attempts to cast to Key.ValueType.

For example, let's say I have the following:

var wipeDatabaseOnLaunch = Key<Bool>("wipeDatabaseOnLaunch")

This works in the normal case:

let value = defaults.get(for: wipeDatabaseOnLaunch)
// value: Optional<Bool>(true)

However, if we pass -wipeDatabaseOnLaunch true argument on launch, the underlying type returned by user defaults is no longer Optional<Bool> but instead Optional<Any>. Since the implementation of get tries to cast to the key's ValueType this cast will fail and you get nil instead.

let value = defaults.get(for: wipeDatabaseOnLaunch)
// value: nil

As a workaround I had to implement a specialized version of get which uses bool(forKey:)

public func get(for key: Key<Bool>) -> Bool {
    return userDefaults.bool(forKey: key._key)
}

Unfortunately, this needs to be done for each of the types you could use as launch arguments. Haven't found a cleaner solution to this.