在大多数情况下，添加偏好设置摘要相对比较简单，因为这只涉及使用适当的字符串资源将 android:summary 属性添加到相应的偏好设置中。不过，如果字幕应动态更新，那么可能有必要使用自定义偏好设置控制器。
静态字幕
如需向偏好设置添加静态字幕，请执行以下操作：
- 将
android:summary属性添加到偏好设置中。例如，如需向 L0 显示设置偏好设置添加摘要，请在偏好设置属性中添加如下内容：android:summary="@string/display_settings_summary"例如，考虑下面这个完整的偏好设置代码示例：
<Preference android:fragment="com.android.car.settings.display.DisplaySettingsFragment" android:icon="@drawable/ic_settings_display" android:key="@string/pk_display_settings_entry" android:title="@string/display_settings" android:summary="@string/display_settings_summary" settings:controller="com.android.car.settings.common.DefaultRestrictionsPreferenceController"/>
动态字幕
使用 android:summary 属性指定的字幕是静态的，因此无法根据特定条件进行更新。对于动态字幕，您需要修改相应偏好设置的偏好设置控制器。以下示例修改了 L0 位置信息偏好设置，使其具有如下的字幕：指定位置信息服务是处于开启还是关闭状态；如果处于开启状态，还会指明当前有权使用位置信息的应用数量。
- 定义新字符串：
<!-- Summary for Location settings when location is off [CHAR LIMIT=NONE] --> <string name="location_settings_summary_location_off">Off</string> <!-- Summary for Location settings when location is on, explaining how many apps have location permission [CHAR LIMIT=NONE]--> <plurals name="location_settings_summary_location_on"> <item quantity="one">On - <xliff:g id="count">%1$d</xliff:g> app has access to location</item> <item quantity="other">On - <xliff:g id="count">%1$d</xliff:g> apps have access to location</item> </plurals> <!-- Location settings, loading the number of apps which have location permission [CHAR LIMIT=30] --> <string name="location_settings_loading_app_permission_stats">Loading\u2026</string> - 创建一个新的 PreferenceController
LocationEntryPreferenceController，以便动态设置和更改位置信息偏好设置摘要文本：public class LocationEntryPreferenceController extends PreferenceController<Preference> { private static final Logger LOG = new Logger(LocationEntryPreferenceController.class); private static final IntentFilter INTENT_FILTER_LOCATION_MODE_CHANGED = new IntentFilter(LocationManager.MODE_CHANGED_ACTION); private final Context mContext; private final LocationManager mLocationManager; /** Total number of apps that have location permissions. */ private int mNumTotal = -1; private int mNumTotalLoading = 0; private AtomicInteger mLoadingInProgress = new AtomicInteger(0); private final BroadcastReceiver mReceiver = new BroadcastReceiver() { @Override public void onReceive(Context context, Intent intent) { refreshUi(); } }; public LocationEntryPreferenceController(Context context, String preferenceKey, FragmentController fragmentController, CarUxRestrictions uxRestrictions) { super(context, preferenceKey, fragmentController, uxRestrictions); mContext = context; mLocationManager = (LocationManager) getContext().getSystemService( Service.LOCATION_SERVICE); } @Override protected Class<Preference> getPreferenceType() { return Preference.class; } @Override protected void onStartInternal() { getContext().registerReceiver(mReceiver, INTENT_FILTER_LOCATION_MODE_CHANGED); } @Override protected void onStopInternal() { getContext().unregisterReceiver(mReceiver); } @Override protected void updateState(Preference preference) { super.updateState(preference); updateSummary(preference); if (!mLocationManager.isLocationEnabled() || mLoadingInProgress.get() != 0) { return; } mNumTotalLoading = 0; // Retrieve a list of users inside the current user profile group. List<UserHandle> users = mContext.getSystemService( UserManager.class).getUserProfiles(); mLoadingInProgress.set(users.size()); for (UserHandle user : users) { Context userContext = createPackageContextAsUser(mContext, user.getIdentifier()); if (userContext == null) { if (mLoadingInProgress.decrementAndGet() == 0) { setLocationAppCount(preference, mNumTotalLoading); } continue; } PermissionControllerManager permController = userContext.getSystemService(PermissionControllerManager.class); permController.countPermissionApps( Arrays.asList(ACCESS_FINE_LOCATION, ACCESS_COARSE_LOCATION), PermissionControllerManager.COUNT_ONLY_WHEN_GRANTED, (numApps) -> { mNumTotalLoading += numApps; if (mLoadingInProgress.decrementAndGet() == 0) { setLocationAppCount(preference, mNumTotalLoading); } }, null); } } @VisibleForTesting void setLocationAppCount(Preference preference, int numApps) { mNumTotal = numApps; updateSummary(preference); } private void updateSummary(Preference preference) { String summary = ""; if (mLocationManager.isLocationEnabled()) { if (mNumTotal == -1) { summary = mContext.getString(R.string.location_settings_loading_app_permission_stats); } else { summary = mContext.getResources().getQuantityString( R.plurals.location_settings_summary_location_on, mNumTotal, mNumTotal); } } else { summary = mContext.getString(R.string.location_settings_summary_location_off); } preference.setSummary(summary); } private Context createPackageContextAsUser(Context context, int userId) { try { return context.createPackageContextAsUser( context.getPackageName(), 0 /* flags */, UserHandle.of(userId)); } catch (PackageManager.NameNotFoundException e) { LOG.e("Failed to create user context", e); } return null; } }对于此示例控制器：
- 如果停用了位置信息服务，摘要文本将设置为
location_settings_summary_location_off字符串。 - 如果启用了位置信息服务，系统会添加具有位置信息权限的应用数量。在加载过程中，系统会显示
location_settings_loading_app_permission_stats字符串。数据加载完毕后，控制器会将摘要设置为location_settings_summary_location_on字符串，并指明拥有访问权限的应用数量。 - 偏好设置控制器在启动后会注册接收器，并在位置信息服务状态更改时刷新偏好设置状态。
- 如果停用了位置信息服务，摘要文本将设置为
- 修改 fragment XML 文件，将新控制器附加到相关偏好设置：
<Preference android:fragment="com.android.car.settings.location.LocationSettingsFragment" android:icon="@drawable/ic_settings_location" android:key="@string/pk_location_settings_entry" android:title="@string/location_settings_title" settings:controller="com.android.car.settings.location.LocationEntryPreferenceController"/>