What Makes a Good Preference?

When defining your app’s preferences, it is better to use simple values and data types whenever possible. The preferences system is built around property-list data types such as strings, numbers, and dates. Although you can use an NSData object to store arbitrary objects in preferences, doing so is not recommended in most cases.

Storing objects persistently means that your app has to decode that object at some point. In the case of preferences, a stored object means decoding the object every time you access the preference. It also means that a newer version of your app has to ensure that it is able to decode objects created and written to disk using an earlier version of your app, which is potentially error prone.

A better approach for preferences is to store simple strings and values and use them to create the objects your app needs. Storing simple values means that your app can always access the value. The only thing that changes from release to release is the interpretation of the simple value and the objects your app creates in response.

Providing a Preference Interface

For user-facing preferences, Table 1-1 lists the options for displaying those preferences to the user. As you can see from this table, most options involve the creation of a custom user interface for managing and presenting preferences. If you are creating an iOS app, you can use a Settings bundle to present preferences, but you should do so only for settings the user changes infrequently.

Preferences in Mac apps should be accessible from a Preferences menu item in the app menu. Cocoa apps created using the Xcode templates provide such a menu item for you automatically. It is your responsibility to present an appropriate user interface when the user chooses this menu item. You can provide that user interface by defining an action method in your app delegate that displays a custom preferences window and connecting that action method to the menu item in Interface Builder.

There is no standard way to display custom preferences from inside an iOS app. You can integrate preferences in many ways, including using a separate tab in a tab-bar interface or using a custom button from one of your app’s screens. Preferences should generally be presented using a distinct view controller so that changes in preferences can be recorded when that view controller is dismissed by the user.

The Organization of Preferences

Preferences are grouped into domains, each of which has a name and a specific usage. For example, there’s a domain for app-specific preferences and another for systemwide preferences that apply to all apps. All preferences are stored and accessed on a per-user basis. There is no support for sharing preferences between users.

Each preference has three components:

• The domain in which it is stored

• Its name (specified as an NSString object)

• Its value, which can be any property-list object (NSData, NSString, NSNumber, NSDate, NSArray, or NSDictionary)

The lifetime of a preference depends on which domain you store it in. Some domains store preferences persistently by writing them to the user’s defaults database. Such preferences continue to exist from one app launch to the next. Other domains store preferences in a more volatile way, preserving preference values only for the life of the corresponding user defaults object.

A search for the value of a given preference proceeds through the domains in an NSUserDefaults object’s search list. Only domains in the search list are searched and they are searched in the order shown in Table 1-2, starting with the NSArgumentDomain domain. A search ends when a preference with the specified name is found. If multiple domains contain the same preference, the value is taken from the domain nearest the beginning of the search list.

localhost> Xcode.app/Contents/MacOS/Xcode -IndexOnOpen NO

Viewing Preferences Using the Defaults Tool

In OS X, the defaults command-line tool provides a way for you to examine the contents of the user defaults database. During app development, you might use this tool to validate the preferences your app is writing to disk. To do that, you would use a command of the following form from the Terminal app:

defaults read <application-bundle-identifier>

To read the contents of the global domain, you would use the following command:

defaults read NSGlobalDomain

For more information about using the defaults tool to read and write preference values, see defaults man page.