Userdefault

Userdefault, in the context of Apple platforms, refers to the user defaults system, a persistent key-value store used by apps to save user preferences, settings, and other small pieces of data. Known most commonly as UserDefaults (and historically NSUserDefaults), it provides a simple interface for storing basic data and automatically persists changes between launches. The storage is isolated to each app and its extensions, unless configured to share data via app groups.

The user defaults database is designed for property list compatible data types. Typical stored values include strings, numbers (integers and doubles), booleans, dates, data, as well as collections such as arrays and dictionaries that contain only property list types. Data stored here is meant for small pieces of information like appearance settings, last used options, or user preferences. Apps can also register default values so that a sane initial state exists before the user or the app sets any values. For sharing data between an app and its extensions, developers can use a shared defaults suite identified by a group container.

Common operations include reading and writing values via a central defaults object (for example, the standard UserDefaults in Swift). Developers typically set values with set(_:forKey:) and retrieve them with object(forKey:) or type-specific getters like string(forKey:), bool(forKey:), or integer(forKey:). Explicit calls to synchronize are generally unnecessary and have been deprecated; the system writes changes automatically.

Limitations include the small data scope and lack of suitability for large datasets or secure storage. For large assets, binary data, or sensitive information, alternative storage approaches such as file storage, Core Data, or secure enclaves may be more appropriate.