iOS SDK: Working with NSUserDefaults

Question

With the NSUserDefaults class, you can save settings and properties related to application or user data.

now i have tow question :

1. ( settings and properties ) Where are stored ? if =>The objects will be saved in what is known as the iOS “defaults system" then : What is iOS “defaults system"

2. How long will remain stored ?

Answer 1

Where? It depends on what kind of app you're developing. For AppStore applications, in the sandbox folder of your installed app folder, more precisely in the Library/Preferences folder. The actual file that contains the settings is a PLIST (property list) file whose name matches the bundle identifier of your app.

All in all:

(/var/mobile/Applications/XXXX-XXXX-XXXXXX-XXXXXXXX/MyApp.app/Library/Preferences/com.mycompany.bundleid.plist

(where XXXX etc. is an UUID).

If you're making an app for jailbroken devices (ie a Cydia app), it will be a plist file again, whose name matches the bundle ID, but it will be installed in

/var/mobile/Library/Preferences/com.mycompany.bundleid.plist

The iOS defaults system is the part of the Foundation framework, iOS itself and the files and directories altogether which manage, store and modify your preferences/settings/defaults.

How long? Again, it depends on what kind of app you are writing. For a normal AppStore app, the defaults are persited to the file - but only as long as the user doesn't delete your app. When your app is deleted, the whole sandbox, app bundle and od course the user defaults will also be gone. Forever, irreversibly.

If you develop a jailbroken application which resides in the system apps' directory (/Applications), then the user defaults file will be available in /var/mobile/Library/Preferences even if the user deletes your package/application, as APT (the backing package manager of Cydia) only makes note of the files your package originally contained, and that's not the case with the defaults property list file (which is only created when your app first accesses the NSUserDefaults class.) If the user manually deletes the file or restores to a stock firmware, it'll be gone.

Answer 2

Answers:

1. In the application's sandbox. Where exactly is unimportant as you aren't going to be accessing the defaults directly anyway.

2. As long as your app is on the device. If you delete the app, the sandbox is deleted as well, so that's when they get deleted. If you update your app, the same sandbox is used, so they don't get deleted.

As for what it is, I'm just going to quote you a bit of the documentation :

The NSUserDefaults class provides a programmatic interface for interacting with the defaults system. The defaults system allows an application to customize its behavior to match a user's preferences. For example, you can allow users to determine what units of measurement your application displays or how often documents are automatically saved. Applications record such preferences by assigning values to a set of parameters in a user's defaults database. The parameters are referred to as defaults since they're commonly used to determine an application's default state at startup or the way it acts by default.

At runtime, you use an NSUserDefaults object to read the defaults that your application uses from a user's defaults database. NSUserDefaults caches the information to avoid having to open the user's defaults database each time you need a default value. The synchronize method, which is automatically invoked at periodic intervals, keeps the in-memory cache in sync with a user's defaults database.