Note on UserDefaults, organization of preferences

2 minute read
ios
,

Preferences are grouped into domains, they can be persistent or volatile

  • persistent 👉🏻 stored on memory, not losing when re-boot the app
  • viatile 👉🏻 only valid along UserDefaults instance lifetime.
Domain State
NSArgumentDomain volatile
Application (identified by app’s identifier) persistent
NSGlobalDomain persistent
Languages (identified by language names) volatile
NSRegistrationDomain volatile

a preference has 3 components:

  • domain in which is is stored
  • name (String)
  • value (Data, String, Number, Date, Array, Dictionary)

Whenever retrieving value for a key, a search for value of a given key will go through the domain hierarchy above from top to bottom starting with NSArgumentDomain and return the first value it found

Argument domain

  • contains values that were set from command line arguments (provided when the app launched from command line)
  • preferences set from the command line temporarily override the established values stored in the user’s defaults database.

Application domain

  • contains app-specific preferences that are stored in user defaults database of current user.
  • preferences are automatically placed into this domain when using UserDefaults.standard to write.
  • when the first property was written, a file with format <bundle-identifier>.plist will exist in $HOME/Library/Preferences

Global domain

  • contains preferences that are applicable to all apps
  • used by system frameworks to store system-wide values
  • should NOT be used to store app-specific data
  • write the same preference on application domain to change value of that preference on global domain

Languages domains

  • every language in AppleLanguages preference will be stored in a domain whose name is that language name
  • every language-specific domain has preferences for the corresponding locale, which is useful in some cases.

Registration domain

  • for the first time launching the app, most preferences don’t have value
  • used to define set of default values for the case a given preference has not been set
  • use registerDefaults: of NSUserDefaults

ℹ have to do this on every launch of the app before reading any value from user defaults database since registerDefaults: does NOT persist the default values to memory. ℹ application:didFinishLaunchingWithOptions: is usually the right place

📚 Learned from