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Using shared UserDefaults suites
UserDefaults is a lot more powerful than what it first might seem like. Not only can it store more complex values (like dates and dictionaries) and parse command line arguments - it also enables easy sharing of settings & lightweight data between apps in the same App Group.
let sharedDefaults = UserDefaults(suiteName: "my-app-group")!
let useDarkMode = sharedDefaults.bool(forKey: "dark-mode")
// This value is put into the shared suite.
sharedDefaults.set(true, forKey: "dark-mode")
// If you want to treat the shared settings as read-only (and add
// local overrides on top of them), you can simply add the shared
// suite to the standard UserDefaults.
let combinedDefaults = UserDefaults.standard
combinedDefaults.addSuite(named: "my-app-group")
// This value is a local override, not added to the shared suite.
combinedDefaults.set(true, forKey: "app-specific-override")
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