Android偏好设置(1)概述和Preferences简介

1.Overview

　　Instead of using 

View

 objects to build the user interface, settings are built using various subclasses of the

Preference

 class that you declare in an XML file.

　　A 

Preference

 object is the building block for a single setting. Each 

Preference

 appears as an item in a list and provides the appropriate UI for users to modify the setting. For example, a 

CheckBoxPreference

 creates a list item that shows a checkbox, and a 

ListPreference

 creates an item that opens a dialog with a list of choices.

　　Each 

Preference

 you add has a corresponding key-value pair that the system uses to save the setting in a default 

SharedPreferences

 file for your app's settings. When the user changes a setting, the system updates the corresponding value in the 

SharedPreferences

 file for you. The only time you should directly interact with the associated 

SharedPreferences

 file is when you need to read the value in order to determine your app's behavior based on the user's setting.

　　The value saved in 

SharedPreferences

 for each setting can be one of the following data types:

• Boolean
• Float
• Int
• Long
• String
• String 

  Set

　　Because your app's settings UI is built using 

Preference

 objects instead of 

View

 objects, you need to use a specialized 

Activity

 or 

Fragment

 subclass to display the list settings:

• If your app supports versions of Android older than 3.0 (API level 10 and lower), you must build the activity as an extension of the 

  PreferenceActivity

   class.
• On Android 3.0 and later, you should instead use a traditional 

  Activity

   that hosts a 

  PreferenceFragment

  that displays your app settings. However, you can also use 

  PreferenceActivity

   to create a two-pane layout for large screens when you have multiple groups of settings.

　　How to set up your 

PreferenceActivity

 and instances of 

PreferenceFragment

 is discussed in the sections about Creating a Preference Activity and Using Preference Fragments.

　　Figure 1. Screenshots from the Android Messaging app's settings. Selecting an item defined by a 

Preference

 opens an interface to change the setting.

2.Preferences

　　Every setting for your app is represented by a specific subclass of the 

Preference

 class. Each subclass includes a set of core properties that allow you to specify things such as a title for the setting and the default value. Each subclass also provides its own specialized properties and user interface. For instance, figure 1 shows a screenshot from the Messaging app's settings. Each list item in the settings screen is backed by a different 

Preference

 object.

　　A few of the most common preferences are:

CheckBoxPreference

Shows an item with a checkbox for a setting that is either enabled or disabled. The saved value is a boolean (

true

 if it's checked).

ListPreference

Opens a dialog with a list of radio buttons. The saved value can be any one of the supported value types (listed above).

EditTextPreference

Opens a dialog with an 

EditText

 widget. The saved value is a 

String

.

　　See the 

Preference

 class for a list of all other subclasses and their corresponding properties.

　　Of course, the built-in classes don't accommodate every need and your application might require something more specialized. For example, the platform currently does not provide a 

Preference

 class for picking a number or a date. So you might need to define your own 

Preference

 subclass. For help doing so, see the section aboutBuilding a Custom Preference.