Android偏好设置(6)应用和监听各偏好参数

Reading Preferences

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　　By default, all your app's preferences are saved to a file that's accessible from anywhere within your application by calling the static method PreferenceManager.getDefaultSharedPreferences(). This returns theSharedPreferences object containing all the key-value pairs that are associated with the Preference objects used in your PreferenceActivity.

For example, here's how you can read one of the preference values from any other activity in your application:

 SharedPreferences sharedPref = PreferenceManager.getDefaultSharedPreferences(this);
 String syncConnPref = sharedPref.getString(SettingsActivity.KEY_PREF_SYNC_CONN, "");

Listening for preference changes

　　There are several reasons you might want to be notified as soon as the user changes one of the preferences. In order to receive a callback when a change happens to any one of the preferences, implement theSharedPreference.OnSharedPreferenceChangeListener interface and register the listener for theSharedPreferences object by calling registerOnSharedPreferenceChangeListener().

The interface has only one callback method, onSharedPreferenceChanged(), and you might find it easiest to implement the interface as a part of your activity. For example:

 public class SettingsActivity extends PreferenceActivity
                               implements OnSharedPreferenceChangeListener {
     public static final String KEY_PREF_SYNC_CONN = "pref_syncConnectionType";
     ...

     public void onSharedPreferenceChanged(SharedPreferences sharedPreferences,
         String key) {
         if (key.equals(KEY_PREF_SYNC_CONN)) {
             Preference connectionPref = findPreference(key);
             // Set summary to be the user-description for the selected value
             connectionPref.setSummary(sharedPreferences.getString(key, ""));
         }
     }
 }

　　In this example, the method checks whether the changed setting is for a known preference key. It callsfindPreference() to get the Preference object that was changed so it can modify the item's summary to be a description of the user's selection. That is, when the setting is a ListPreference or other multiple choice setting, you should call setSummary() when the setting changes to display the current status (such as the Sleep setting shown in figure 5).

Note: As described in the Android Design document about Settings, we recommend that you update the summary for a ListPreference each time the user changes the preference in order to describe the current setting.

For proper lifecycle management in the activity, we recommend that you register and unregister yourSharedPreferences.OnSharedPreferenceChangeListener during the onResume() and onPause()callbacks, respectively:

@Override
protected void onResume() {
    super.onResume();
    getPreferenceScreen().getSharedPreferences()
            .registerOnSharedPreferenceChangeListener(this);
}

@Override
protected void onPause() {
    super.onPause();
    getPreferenceScreen().getSharedPreferences()
            .unregisterOnSharedPreferenceChangeListener(this);
}

Caution: When you call registerOnSharedPreferenceChangeListener(), the preference manager does not currently store a strong reference to the listener. You must store a strong reference to the listener, or it will be susceptible to garbage collection. We recommend you keep a reference to the listener in the instance data of an object that will exist as long as you need the listener.

For example, in the following code, the caller does not keep a reference to the listener. As a result, the listener will be subject to garbage collection, and it will fail at some indeterminate time in the future:

不要像下面这样用

prefs.registerOnSharedPreferenceChangeListener(
  // Bad! The listener is subject to garbage collection!
  new SharedPreferences.OnSharedPreferenceChangeListener() {
  public void onSharedPreferenceChanged(SharedPreferences prefs, String key) {
    // listener implementation
  }
});

Instead, store a reference to the listener in an instance data field of an object that will exist as long as the listener is needed:

SharedPreferences.OnSharedPreferenceChangeListener listener =
    new SharedPreferences.OnSharedPreferenceChangeListener() {
  public void onSharedPreferenceChanged(SharedPreferences prefs, String key) {
    // listener implementation
  }
};
prefs.registerOnSharedPreferenceChangeListener(listener);