SharingUserDefaultsBetweenAppAndExtensions.swift

/// This is how you could use standar user defaults for sharing app between your app and your extensions.
/// You should have groups capability enable in your app and extensions and use the same group name everywhere.
/// For more info see: https://developer.apple.com/library/content/documentation/IDEs/Conceptual/AppDistributionGuide/AddingCapabilities/AddingCapabilities.html

/// -initWithSuiteName: initializes an instance of NSUserDefaults that searches the shared preferences search list for the domain 'suitename'. For example, using the identifier of an application group will cause the receiver to search the preferences for that group. Passing the current application's bundle identifier, NSGlobalDomain, or the corresponding CFPreferences constants is an error. Passing nil will search the default search list.

if let defaults = UserDefaults(suiteName: "yourapp.group.name") {
    defaults.set("VALUE", forKey: "KEY")
}