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Android设置应用是Android系统中一个非常重要的系统应用，它允许用户调整和设置系统的各种参数和功能（系统设置/自定义设置/控制应用权限/开发者选项/系统信息等），使用户获得更好的使用体验。同时它一般也是Android系统开发者了解深入的第一个系统级应用，也是用户使用最频繁的系统应用。

源码目录

AOSP源码路径为packages/apps/Settings。src/com/android/settings目录下包含Settings的主要源码。libs目录下的contextualcards.aar包含实现上下文卡片功能的代码和资源，它可以将相关的内容组织在一起，以卡片的形式展现给用户。res目录下包含各种静态资源。Android.bp文件中可以看到模块名为"Settings"。

设计指南

上图是Settings里一个普通的页面，从这个页面可以看出它将许多设置放在一起，设置列表是多个控件的组合。

它有如下优点：

• 提供一个很好的概述。用户应该能够浏览设置屏幕并了解所有单独的设置及其值。
• 直观的设置项目。常用设置放在屏幕顶部。限制一个屏幕上的设置数量。将一些设置移动到单独的屏幕来创建直观的菜单。
• 使用明确的标题和状态。标题简短而有意义。在标题下方，显示状态以突出设置的值，显示具体细节。

关于Settings设计的详细规则及细节，可以参考官网：设计指南

Preference

在Android 常用组件里，存在一个Preference组件，它提供了一个方便的用户界面，用于管理和显示应用程序的各种设置选项，让用户可以轻松浏览和更改应用程序的设置。Preference还通过SharedPreference实现保存读取数据，以其key作为SharedPreference的键，实现持久化数据。Settings中大多数菜单都是通过Preference去实现，且使用的是androidx包的Preference，因此首先了解下Preference的使用。

Preference组件和其它页面组件使用类似，区别在于XML 资源必须放置于 res/xml/ 目录，Preference的根标签必须为PreferenceScreen。举例如下：

<?xml version="1.0" encoding="utf-8"?>
<PreferenceScreen
    xmlns:andoird="http://schemas.android.com/apk/res-auto">
	<PreferenceCategory
        app:key="prefer_category">
   		<Preference
       		andoird:key="feedback"
       		andoird:title="Send feedback"
        	andoird:summary="Report technical issues or suggest new features"/>
	</PreferenceCategory>
</PreferenceScreen>

PreferenceCategory是对Preferences进行分组的标签，显示类别标题，并在视觉上进行分隔Preference。如设计指南中的Settings Display页面的截图中Brightness/Lock display类别标签。

以下是Preference相关属性的介绍：

attr	description
andoird:allowDividerAbove	在菜单上显示一条分割线
andoird:allowDividerBelow	在菜单下显示一条分割线
android:defaultValue	默认值。
android:dependency	设置此元素附属于另一个元素，依赖的可用则当前元素也可用（enable），反之。
andoird:enableCopying	启用长按复制
android:enabled	设置是否可用。
android:fragment	指定跳转fragment。
android:icon	指定左侧的图标。
andoird:iconSpaceReserved	为图标预留位置，菜单向右偏移，默认false
andoird:isPreferenceVisible	菜单是否显示
android:key	选项的名称，也是用来存储时唯一的key。
android:layout	给当前元素指定一个自定义布局。
android:order	偏好的顺序。如果不指定，默认的顺序将字母。
android:persistent	是否将其值持久化。
android:selectable	设置是否可以选择操作。
android:shouldDisableView	当enabled设置为false变暗，同时此属性设置为false时disable但不变暗。
andoird:singleLineTitle	菜单title限制为一行，默认为true
android:summary	摘要，配置的简要说明，显示在标题下面。
android:title	选项的标题，当没有设置summary时自动垂直居中显示。
android:widgetLayout	控件可调小部件的布局。是为一个优先选择的布局，比如一个复选框选择要指定一个自定义布局（注意：包括的只是复选框）在这里。

Setting中扩展的attr如下：

    <declare-styleable name="Preference">
        <!-- 搜索关键词 -->
        <attr name="keywords" format="string" />
        <!-- 是否可搜索，默认为true -->
        <attr name="searchable" format="boolean" />
        <!-- Preference controller类 -->
        <attr name="controller" format="string" />
        <!-- 自定义字幕 -->
        <attr name="unavailableSliceSubtitle" format="string" />
        <!-- Preference针对work profile，默认为false -->
        <attr name="forWork" format="boolean" />
        <!-- 用于在双窗格上突出显示菜单首选项的标识符 -->
        <attr name="highlightableMenuKey" format="string" />
    </declare-styleable>

查看Preference的源码可知，还有一些自定义Preference实现的组件，如CheckBoxPreference/DropDownPreference/EditTextPreference/ListPreference/SwitchPreference等，它是针对不同Android控件(checkbox/dropdown/edittext等)实现的自定义Preference，如需使用只需要在xml引用即可

然后创建一个fragment，继承于PreferenceFragmentCompat。onCreatePreferences方法在PreferenceFragmentCompat的onCreate方法调用，用于创建Prerefence。通过setPreferencesFromResource引用定义的Preference xml资源。这样通过Preference实现的一个简单的菜单就显示在屏幕上了。

public class SettingsFragment extends PreferenceFragmentCompat {

    @Override
    public void onCreatePreferences(Bundle savedInstanceState, String rootKey) {
        setPreferencesFromResource(R.xml.root_preferences, rootKey);
    }
    //preference点击事件，通过key区分
    @Override
    public boolean onPreferenceTreeClick(@NonNull Preference preference) {
        return super.onPreferenceTreeClick(preference);
    }    
}

看下Preference的点击事件实现，如果对应的fragment实现了OnPreferenceStartFragmentCallback，重写了onPreferenceStartFragment方法，那么Preference的跳转实现就在onPreferenceStartFragment方法里，并返回处理结果，如果没有实现OnPreferenceStartFragmentCallback，则去获取xml中设置的android:fragment或者setFragment设置的fragment跳转。

public abstract class PreferenceFragmentCompat extends Fragment implements
        PreferenceManager.OnPreferenceTreeClickListener,
        PreferenceManager.OnDisplayPreferenceDialogListener,
        PreferenceManager.OnNavigateToScreenListener,
        DialogPreference.TargetFragment {
	@Override
    public boolean onPreferenceTreeClick(@NonNull Preference preference) {
        if (preference.getFragment() != null) {
            boolean handled = false;
            if (getCallbackFragment() instanceof OnPreferenceStartFragmentCallback) {
                handled = ((OnPreferenceStartFragmentCallback) getCallbackFragment())
                        .onPreferenceStartFragment(this, preference);
            }
            Fragment callbackFragment = this;
            while (!handled && callbackFragment != null) {
                if (callbackFragment instanceof OnPreferenceStartFragmentCallback) {
                    handled = ((OnPreferenceStartFragmentCallback) callbackFragment)
                            .onPreferenceStartFragment(this, preference);
                }
                callbackFragment = callbackFragment.getParentFragment();
            }
            if (!handled && getContext() instanceof OnPreferenceStartFragmentCallback) {
                handled = ((OnPreferenceStartFragmentCallback) getContext())
                        .onPreferenceStartFragment(this, preference);
            }
            if (!handled && getActivity() instanceof OnPreferenceStartFragmentCallback) {
                handled = ((OnPreferenceStartFragmentCallback) getActivity())
                        .onPreferenceStartFragment(this, preference);
            }
            if (!handled) {   
                final FragmentManager fragmentManager = getParentFragmentManager();
                final Bundle args = preference.getExtras();
                final Fragment fragment = fragmentManager.getFragmentFactory().instantiate(
                        requireActivity().getClassLoader(), preference.getFragment());
                fragment.setArguments(args);
                fragment.setTargetFragment(this, 0);
                fragmentManager.beginTransaction()
                        .replace(((View) requireView().getParent()).getId(), fragment)
                        .addToBackStack(null)
                        .commit();
            }
            return true;
        }
        return false;
    }            
}

androidx包中Preference只有针对Fragment的实现，没有针对Activity的实现。还有针对Dialog实现的PreferenceDialogFragmentCompat。在dialog里引用preference。

页面加载分析

本文以Android 13 Settings源码为例进行分析。

首页加载流程

在AndroidManifest.xml中可以看到，启动activity为Settings，Settings中包含大量的静态类继承于SettingsActivity。

 <activity-alias android:name="Settings"
                android:label="@string/settings_label_launcher"
                android:taskAffinity="com.android.settings.root"
                android:launchMode="singleTask"
                android:exported="true"
                android:targetActivity=".homepage.SettingsHomepageActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.DEFAULT" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
            <meta-data android:name="android.app.shortcuts" android:resource="@xml/shortcuts"/>
        </activity-alias>

查看SettingsActivity的onCreate方法，首先读取fragment class和HighlightMenuKey，设置布局为settings_main_prefs.xml，通过intent传递的数据显示不同的布局。切换fragment或根据之前保存状态显示页面。settings_main_prefs.xml中包含顶部的switch bar，底部的button(Back/Skip/Next)，和中间的framelayout显示Fragment，switch bar和button默认隐藏。

public class SettingsActivity extends SettingsBaseActivity
        implements PreferenceManager.OnPreferenceTreeClickListener,
        PreferenceFragmentCompat.OnPreferenceStartFragmentCallback,
        ButtonBarHandler, FragmentManager.OnBackStackChangedListener {
    @Override
    protected void onCreate(Bundle savedState) {
        // Should happen before any call to getIntent()
        getMetaData();
        final Intent intent = getIntent();

        if (shouldShowTwoPaneDeepLink(intent) && tryStartTwoPaneDeepLink(intent)) {
            finish();
            super.onCreate(savedState);
            return;
        }

        super.onCreate(savedState);
        Log.d(LOG_TAG, "Starting onCreate");
        createUiFromIntent(savedState, intent);
    }
    private void getMetaData() {
        try {
            ActivityInfo ai = getPackageManager().getActivityInfo(getComponentName(),
                    PackageManager.GET_META_DATA);
            if (ai == null || ai.metaData == null) return;
            //读取设置的fragment
            mFragmentClass = ai.metaData.getString(META_DATA_KEY_FRAGMENT_CLASS);
            mHighlightMenuKey = ai.metaData.getString(META_DATA_KEY_HIGHLIGHT_MENU_KEY);
        } catch (NameNotFoundException nnfe) {
            // No recovery
            Log.d(LOG_TAG, "Cannot get Metadata for: " + getComponentName().toString());
        }
    }
    protected void createUiFromIntent(Bundle savedState, Intent intent) {
        ...
        setContentView(R.layout.settings_main_prefs);
        ...
        if (savedState != null) {
            // We are restarting from a previous saved state; used that to initialize, instead
            // of starting fresh.
            setTitleFromIntent(intent);

            ArrayList<DashboardCategory> categories =
                    savedState.getParcelableArrayList(SAVE_KEY_CATEGORIES);
            if (categories != null) {
                mCategories.clear();
                mCategories.addAll(categories);
                setTitleFromBackStack();
            }
        } else {
            //加载fragment
            launchSettingFragment(initialFragmentName, intent);
        }
    }
}

AndroidManifest中并没有传递Settings对应的fragment数据，而是指定targetActivity为SettingsHomepageActivity，查看SettingsHomepageActivity的onCreate方法，布局为settings_homepage_container.xml，然后初始化搜索栏。设备不是低内存的情况下加载Suggestion菜单，设置fragment为TopLevelSettings。

@Override
protected void onCreate(Bundle savedInstanceState) {
        ...
        setContentView(R.layout.settings_homepage_container);
    	...
        initSearchBarView();
        ...
        if (!getSystemService(ActivityManager.class).isLowRamDevice()) {
            initAvatarView();
            final boolean scrollNeeded = mIsEmbeddingActivityEnabled
                    && !TextUtils.equals(getString(DEFAULT_HIGHLIGHT_MENU_KEY), highlightMenuKey);
            showSuggestionFragment(scrollNeeded);
            if (FeatureFlagUtils.isEnabled(this, FeatureFlags.CONTEXTUAL_HOME)) {
                showFragment(() -> new ContextualCardsFragment(), R.id.contextual_cards_content);
                ((FrameLayout) findViewById(R.id.main_content))
                        .getLayoutTransition().enableTransitionType(LayoutTransition.CHANGING);
            }
        }
        mMainFragment = showFragment(() -> {
            final TopLevelSettings fragment = new TopLevelSettings();
            fragment.getArguments().putString(SettingsActivity.EXTRA_FRAGMENT_ARG_KEY,
                    highlightMenuKey);
            return fragment;
        }, R.id.main_content);
        ...
}
private void initSearchBarView() {
        final Toolbar toolbar = findViewById(R.id.search_action_bar);
        FeatureFactory.getFactory(this).getSearchFeatureProvider()
                .initSearchToolbar(this /* activity */, toolbar, SettingsEnums.SETTINGS_HOMEPAGE);

        if (mIsEmbeddingActivityEnabled) {
            final Toolbar toolbarTwoPaneVersion = findViewById(R.id.search_action_bar_two_pane);
            //初始化搜索实现
            FeatureFactory.getFactory(this).getSearchFeatureProvider()
                    .initSearchToolbar(this /* activity */, toolbarTwoPaneVersion,
                            SettingsEnums.SETTINGS_HOMEPAGE);
        }
    }

在搜索的实现类SearchFeatureProviderImpl中可以看到构造的搜索跳转intent如下，可以发现Settings的搜索核心实现在另外一个app内，包名为com.android.settings.intelligence。在后面会对搜索及这个模块进行深入分析，继续分析Settings页面加载。

    @Override
    public Intent buildSearchIntent(Context context, int pageId) {
        return new Intent(Settings.ACTION_APP_SEARCH_SETTINGS)
                .setPackage(getSettingsIntelligencePkgName(context))
                .putExtra(Intent.EXTRA_REFERRER, buildReferrer(context, pageId));
    }

    default String getSettingsIntelligencePkgName(Context context) {
        return context.getString(R.string.config_settingsintelligence_package_name);
    }

    <!-- Settings intelligence package name -->
    <string name="config_settingsintelligence_package_name" translatable="false">
        com.android.settings.intelligence
    </string>

首先看下TopLevelSettings的继承关系，TopLevelSettings继承于DashboardFragment，DashboardFragment是静态和动态Settings 菜单的基类，Settings中大多数菜单对应的fragment继承于DashboardFragment，它继承于SettingsPreferenceFragment，SettingsPreferenceFragment是Settings fragment的基类，它继承于InstrumentedPreferenceFragment，它记录fragment显示状态，继承于ObservablePreferenceFragment，ObservablePreferenceFragment是在SettingsLib里定义的，模块路径：frameworks/base/packages/SettingsLib，在后面会对这个模块深入分析。ObservablePreferenceFragment继承于PreferenceFragmentCompat。

public class TopLevelSettings extends DashboardFragment implements SplitLayoutListener,
        PreferenceFragmentCompat.OnPreferenceStartFragmentCallback {
            public TopLevelSettings() {
        final Bundle args = new Bundle();
        // Disable the search icon because this page uses a full search view in actionbar.
        args.putBoolean(NEED_SEARCH_ICON_IN_ACTION_BAR, false);
        setArguments(args);
    }
    //设置preference对应xml资源
    @Override
    protected int getPreferenceScreenResId() {
        return R.xml.top_level_settings;
    }
     @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        HighlightableMenu.fromXml(context, getPreferenceScreenResId());
        use(SupportPreferenceController.class).setActivity(getActivity());
    }
}

首先查看onAttach方法，调用DashboardFragment的use方法获取SupportPreferenceController实例。SupportPreferenceController存储在mPreferenceControllers中，通过addPreferenceController方法将PreferenceController添加到mPreferenceControllers中，在DashboardFragment的onAttach方法中会调用addPreferenceController，通过createPreferenceControllers方法将代码设置的controller添加到mControllers集合，然后读取xml中设置的controller添加到mControllers集合。

然后看下加载Preference的onCreatePreferences方法，首先通过getPreferenceScreenResId获取对应的xml资源，TopLevelSettings对应的是top_level_settings.xml，xml中定义了Settings首页菜单，最终通过addPreferencesFromResource方法显示Preference。在TopLevelSettings的onCreatePreferences方法还对图标颜色进行了处理。Preference点击事件调用Preferencecontroller的handlePreferenceTreeClick方法。

public abstract class DashboardFragment extends SettingsPreferenceFragment
        implements CategoryListener, Indexable, PreferenceGroup.OnExpandButtonClickListener,
        BasePreferenceController.UiBlockListener {
        @Override
    	public void onAttach(Context context) {
        	super.onAttach(context);
            ...
             // Load preference controllers from code
        	final List<AbstractPreferenceController> controllersFromCode =
                createPreferenceControllers(context);
        	// Load preference controllers from xml definition
        	final List<BasePreferenceController> controllersFromXml = PreferenceControllerListHelper
                .getPreferenceControllersFromXml(context, 			             getPreferenceScreenResId());
        // Filter xml-based controllers in case a similar controller is created from code already.
        final List<BasePreferenceController> uniqueControllerFromXml =
                PreferenceControllerListHelper.filterControllers(
                        controllersFromXml, controllersFromCode);

        // Add unique controllers to list.
        if (controllersFromCode != null) {
            mControllers.addAll(controllersFromCode);
        }
        mControllers.addAll(uniqueControllerFromXml);
            for (AbstractPreferenceController controller : mControllers) {
            	addPreferenceController(controller);
        	}	
        }
        //获取对应Preference controller实例
        protected <T extends AbstractPreferenceController> T use(Class<T> clazz) {
        List<AbstractPreferenceController> controllerList = mPreferenceControllers.get(clazz);
        if (controllerList != null) {
            if (controllerList.size() > 1) {
                Log.w(TAG, "Multiple controllers of Class " + clazz.getSimpleName()
                        + " found, returning first one.");
            }
            return (T) controllerList.get(0);
        }

        return null;
    }
    protected void addPreferenceController(AbstractPreferenceController controller) {
        if (mPreferenceControllers.get(controller.getClass()) == null) {
            mPreferenceControllers.put(controller.getClass(), new ArrayList<>());
        }
        mPreferenceControllers.get(controller.getClass()).add(controller);
    }
    @Override
    public void onCreatePreferences(Bundle savedInstanceState, String rootKey) {
        checkUiBlocker(mControllers);
        refreshAllPreferences(getLogTag());
     	...
    }
    private void refreshAllPreferences(final String tag) {
        ...
         // Add resource based tiles.
        displayResourceTiles();
        ...
    }
    private void displayResourceTiles() {
        final int resId = getPreferenceScreenResId();
        if (resId <= 0) {
            return;
        }
        addPreferencesFromResource(resId);
        final PreferenceScreen screen = getPreferenceScreen();
        screen.setOnExpandButtonClickListener(this);
        displayResourceTilesToScreen(screen);
    }
    @Override
    public boolean onPreferenceTreeClick(Preference preference) {
        final Collection<List<AbstractPreferenceController>> controllers =
                mPreferenceControllers.values();
        for (List<AbstractPreferenceController> controllerList : controllers) {
            for (AbstractPreferenceController controller : controllerList) {
                if (controller.handlePreferenceTreeClick(preference)) {
                    // log here since calling super.onPreferenceTreeClick will be skipped
                    writePreferenceClickMetric(preference);
                    return true;
                }
            }
        }
        return super.onPreferenceTreeClick(preference);
    }
}

接下来我们以SupportPreferenceController为例，分析下PreferenceController。它继承于BasePreferenceController，BasePreferenceController继承于AbstractPreferenceController。

public class SupportPreferenceController extends BasePreferenceController {
    //指定显示状态
    @Override
    public int getAvailabilityStatus() {
        return mSupportFeatureProvider == null ? UNSUPPORTED_ON_DEVICE : AVAILABLE;
    }
	//点击事件
    @Override
    public boolean handlePreferenceTreeClick(Preference preference) {
        if (preference == null || mActivity == null ||
                !TextUtils.equals(preference.getKey(), getPreferenceKey())) {
            return false;
        }
        mSupportFeatureProvider.startSupport(mActivity);
        return true;

    }
}

AbstractPreferenceController是一个抽象类，主要方法如下：

public abstract class AbstractPreferenceController {
    //preference是否有效
    public abstract boolean isAvailable();
    //preference点击事件
    public boolean handlePreferenceTreeClick(Preference preference) {
        return false;
    }
    //显示preference
    public void displayPreference(PreferenceScreen screen) {
        final String prefKey = getPreferenceKey();
        if (TextUtils.isEmpty(prefKey)) {
            Log.w(TAG, "Skipping displayPreference because key is empty:" + getClass().getName());
            return;
        }
        if (isAvailable()) {
            setVisible(screen, prefKey, true /* visible */);
            if (this instanceof Preference.OnPreferenceChangeListener) {
                final Preference preference = screen.findPreference(prefKey);
                preference.setOnPreferenceChangeListener(
                        (Preference.OnPreferenceChangeListener) this);
            }
        } else {
            setVisible(screen, prefKey, false /* visible */);
        }
    }
    //更新preference状态(summary)
    public void updateState(Preference preference) {
        refreshSummary(preference);
    }
    //preference key
    public abstract String getPreferenceKey();
}

BasePreferenceController对AbstractPreferenceController进行了简单封装，对Preference状态进行处理，共有6中状态，其次对Preference搜索支持也进行了处理。

public abstract class BasePreferenceController extends AbstractPreferenceController implements Sliceable {
    @Retention(RetentionPolicy.SOURCE)
    @IntDef({AVAILABLE, AVAILABLE_UNSEARCHABLE, UNSUPPORTED_ON_DEVICE, DISABLED_FOR_USER,
            DISABLED_DEPENDENT_SETTING, CONDITIONALLY_UNAVAILABLE})
    public @interface AvailabilityStatus {
    }
    //preference 有效
    public static final int AVAILABLE = 0;
    //preference 有效不能搜索
    public static final int AVAILABLE_UNSEARCHABLE = 1;
    //当前不可用，将来可能可用
    public static final int CONDITIONALLY_UNAVAILABLE = 2;
    //设备不支持
    public static final int UNSUPPORTED_ON_DEVICE = 3;
    //当前用户不支持
    public static final int DISABLED_FOR_USER = 4;
    //preference置灰，无法更改，依赖其它设置
    public static final int DISABLED_DEPENDENT_SETTING = 5;
    //指定Preference状态
    @AvailabilityStatus
    public abstract int getAvailabilityStatus();
    //preference 有效的实现
    @Override
    public final boolean isAvailable() {
        if (mIsForWork && mWorkProfileUser == null) {
            return false;
        }

        final int availabilityStatus = getAvailabilityStatus();
        return (availabilityStatus == AVAILABLE
                || availabilityStatus == AVAILABLE_UNSEARCHABLE
                || availabilityStatus == DISABLED_DEPENDENT_SETTING);
    }
    //针对DISABLED_DEPENDENT_SETTING状态进行置灰
    @Override
    public void displayPreference(PreferenceScreen screen) {
        super.displayPreference(screen);
        if (getAvailabilityStatus() == DISABLED_DEPENDENT_SETTING) {
            // Disable preference if it depends on another setting.
            final Preference preference = screen.findPreference(getPreferenceKey());
            if (preference != null) {
                preference.setEnabled(false);
            }
        }
    }
}

从上面可以看出来，Preferencecontroller它是Preference的控制器，控制Preference的显示，点击事件，搜索。Settings中大多数Preference的控制器都继承于BasePreferenceController。以上就是首页菜单加载的流程。

那SupportPreferenceContoller是那个菜单的控制器呢，它是首页Tips & support菜单的控制器。top_level_settings.xml有定义这个preference。Settings中大多数是以这样的实现构成的，xml定义Preference和引用Preferencecontroller，Preferencecontroller去实现对应菜单的逻辑。

二级页面加载流程

首先查看首页菜单的点击事件，它是获取了Preference controller的handlePreferenceTreeClick方法处理点击事件。

public abstract class DashboardFragment extends SettingsPreferenceFragment
        implements CategoryListener, Indexable, PreferenceGroup.OnExpandButtonClickListener,
        BasePreferenceController.UiBlockListener {
            @Override
    public boolean onPreferenceTreeClick(Preference preference) {
        final Collection<List<AbstractPreferenceController>> controllers =
                mPreferenceControllers.values();
        for (List<AbstractPreferenceController> controllerList : controllers) {
            for (AbstractPreferenceController controller : controllerList) {
                if (controller.handlePreferenceTreeClick(preference)) {
                    // log here since calling super.onPreferenceTreeClick will be skipped
                    writePreferenceClickMetric(preference);
                    return true;
                }
            }
        }
        return super.onPreferenceTreeClick(preference);
    }
}

Preference controller的点击基础实现如下：

public abstract class BasePreferenceController extends AbstractPreferenceController implements  Sliceable {
    @Override
    public boolean handlePreferenceTreeClick(Preference preference) {
        if (!TextUtils.equals(preference.getKey(), getPreferenceKey())) {
            return super.handlePreferenceTreeClick(preference);
        }
        if (!mIsForWork || mWorkProfileUser == null) {
            return super.handlePreferenceTreeClick(preference);
        }
        final Bundle extra = preference.getExtras();
        extra.putInt(EXTRA_USER_ID, mWorkProfileUser.getIdentifier());
        new SubSettingLauncher(preference.getContext())
                .setDestination(preference.getFragment())
                .setSourceMetricsCategory(preference.getExtras().getInt(CATEGORY,
                        SettingsEnums.PAGE_UNKNOWN))
                .setArguments(preference.getExtras())
                .setUserHandle(mWorkProfileUser)
                .launch();
        return true;
    }
}

如果Preference controller不处理，则通过onPreferenceStartFragment方法，TopLevelSettings实现了OnPreferenceStartFragmentCallback，

public class TopLevelSettings extends DashboardFragment implements SplitLayoutListener,
        PreferenceFragmentCompat.OnPreferenceStartFragmentCallback {
       @Override
    public boolean onPreferenceStartFragment(PreferenceFragmentCompat caller, Preference pref) {
        new SubSettingLauncher(getActivity())
                .setDestination(pref.getFragment())
                .setArguments(pref.getExtras())
                .setSourceMetricsCategory(caller instanceof Instrumentable
                        ? ((Instrumentable) caller).getMetricsCategory()
                        : Instrumentable.METRICS_CATEGORY_UNKNOWN)
                .setTitleRes(-1)
                .setIsSecondLayerPage(true)
                .launch();
        return true;
    }
}

可以发现，跳转二级页面的实现都是通过SubSettingLauncher来传递参数并且跳转目标fragment。toIntent方法构造调整intent，可以看到跳转的类是SubSettings，launcher方法进行跳转。

public class SubSettingLauncher {
    public void launch() {
       	...
        final Intent intent = toIntent();

        boolean launchAsUser = mLaunchRequest.mUserHandle != null
                && mLaunchRequest.mUserHandle.getIdentifier() != UserHandle.myUserId();
        boolean launchForResult = mLaunchRequest.mResultListener != null;
        if (launchAsUser && launchForResult) {
            launchForResultAsUser(intent, mLaunchRequest.mUserHandle,
                    mLaunchRequest.mResultListener, mLaunchRequest.mRequestCode);
        } else if (launchAsUser && !launchForResult) {
            launchAsUser(intent, mLaunchRequest.mUserHandle);
        } else if (!launchAsUser && launchForResult) {
            launchForResult(mLaunchRequest.mResultListener, intent, mLaunchRequest.mRequestCode);
        } else {
            launch(intent);
        }
    }
    public Intent toIntent() {
        final Intent intent = new Intent(Intent.ACTION_MAIN);
        copyExtras(intent);
        intent.setClass(mContext, SubSettings.class);
        if (TextUtils.isEmpty(mLaunchRequest.mDestinationName)) {
            throw new IllegalArgumentException("Destination fragment must be set");
        }
        intent.putExtra(SettingsActivity.EXTRA_SHOW_FRAGMENT, mLaunchRequest.mDestinationName);

        if (mLaunchRequest.mSourceMetricsCategory < 0) {
            throw new IllegalArgumentException("Source metrics category must be set");
        }
        intent.putExtra(MetricsFeatureProvider.EXTRA_SOURCE_METRICS_CATEGORY,
                mLaunchRequest.mSourceMetricsCategory);

        intent.putExtra(SettingsActivity.EXTRA_SHOW_FRAGMENT_ARGUMENTS, mLaunchRequest.mArguments);
        intent.putExtra(SettingsActivity.EXTRA_SHOW_FRAGMENT_TITLE_RES_PACKAGE_NAME,
                mLaunchRequest.mTitleResPackageName);
        intent.putExtra(SettingsActivity.EXTRA_SHOW_FRAGMENT_TITLE_RESID,
                mLaunchRequest.mTitleResId);
        intent.putExtra(SettingsActivity.EXTRA_SHOW_FRAGMENT_TITLE, mLaunchRequest.mTitle);
        intent.addFlags(mLaunchRequest.mFlags);
        intent.putExtra(SettingsBaseActivity.EXTRA_PAGE_TRANSITION_TYPE,
                mLaunchRequest.mTransitionType);
        intent.putExtra(SettingsActivity.EXTRA_IS_SECOND_LAYER_PAGE,
                mLaunchRequest.mIsSecondLayerPage);

        return intent;
    }

    @VisibleForTesting
    void launch(Intent intent) {
        mContext.startActivity(intent);
    }
}

SubSettings继承于SettingsActivity，说明首页菜单除了自定义实现页面跳转逻辑的之外，其它都是跳转到SubSettings这个Activity，这里有一个小技巧，正常情况下我们抓取当前页面的Activity可以通过以下命令：

adb shell dumpsys window | grep mCurrentFocus

但是我们不清楚这个页面对应的fragment，通过上面命令都是SubSettings。在跳转时可以通过以下命令获取fragment：

adb logcat -s "SubSettings"

这样就打印出了具体的启动fragment。

public class SubSettings extends SettingsActivity {

    @Override
    public boolean onNavigateUp() {
        finish();
        return true;
    }

    @Override
    protected boolean isValidFragment(String fragmentName) {
        //打印页面
        Log.d("SubSettings", "Launching fragment " + fragmentName);
        return true;
    }
}

剩下的相关逻辑都和TopLevelSettings类似，这里不继续展开分析了。

动态插入菜单

在Settings里的一些菜单，我们会发现一些菜单在xml和代码中并未添加，但实际上显示在页面上，这是为什么呢？原来是Settings支持动态插入菜单。实现逻辑如下：

在创建Preference的时候，refreshAllPreferences方法刷新Preference，包括来自xml的静态Preference和动态Preference。动态Preference添加实现在refreshDashboardTiles方法中。

public abstract class DashboardFragment extends SettingsPreferenceFragment
        implements CategoryListener, Indexable, PreferenceGroup.OnExpandButtonClickListener,
        BasePreferenceController.UiBlockListener {
    @Override
    public void onCreatePreferences(Bundle savedInstanceState, String rootKey) {
        checkUiBlocker(mControllers);
        refreshAllPreferences(getLogTag());
        ...
    }
    private void refreshAllPreferences(final String tag) {
        ...
        // Add resource based tiles.
        displayResourceTiles();
		//动态Preference
        refreshDashboardTiles(tag);
    }
    private void refreshDashboardTiles(final String tag) {
        final PreferenceScreen screen = getPreferenceScreen();

        final DashboardCategory category =
                mDashboardFeatureProvider.getTilesForCategory(getCategoryKey());
      	...
        final List<Tile> tiles = category.getTiles();
        // Create a list to track which tiles are to be removed.
        final Map<String, List<DynamicDataObserver>> remove = new ArrayMap(mDashboardTilePrefKeys);

        // Install dashboard tiles and collect pending observers.
        final boolean forceRoundedIcons = shouldForceRoundedIcon();
        final List<DynamicDataObserver> pendingObservers = new ArrayList<>();
        for (Tile tile : tiles) {
            final String key = mDashboardFeatureProvider.getDashboardKeyForTile(tile);
            ...
            final List<DynamicDataObserver> observers;
            if (mDashboardTilePrefKeys.containsKey(key)) {
                // Have the key already, will rebind.
                final Preference preference = screen.findPreference(key);
                observers = mDashboardFeatureProvider.bindPreferenceToTileAndGetObservers(
                        getActivity(), this, forceRoundedIcons, preference, tile, key,
                        mPlaceholderPreferenceController.getOrder());
            } else {
                // Don't have this key, add it.
                final Preference pref = createPreference(tile);
                observers = mDashboardFeatureProvider.bindPreferenceToTileAndGetObservers(
                        getActivity(), this, forceRoundedIcons, pref, tile, key,
                        mPlaceholderPreferenceController.getOrder());
                //添加Preference
                screen.addPreference(pref);
                registerDynamicDataObservers(observers);
                mDashboardTilePrefKeys.put(key, observers);
            }
            if (observers != null) {
                pendingObservers.addAll(observers);
            }
            remove.remove(key);
            ...
   //类别key             
   public String getCategoryKey() {
        return DashboardFragmentRegistry.PARENT_TO_CATEGORY_KEY_MAP.get(getClass().getName());
    }
}

首先获取了类别 key，PARENT_TO_CATEGORY_KEY_MAP中实现了页面和key的对应。通过页面class name来确定页面对应的key。

public class DashboardFragmentRegistry {
     static {
        PARENT_TO_CATEGORY_KEY_MAP = new ArrayMap<>();
        PARENT_TO_CATEGORY_KEY_MAP.put(TopLevelSettings.class.getName(),
                CategoryKey.CATEGORY_HOMEPAGE);
        PARENT_TO_CATEGORY_KEY_MAP.put(
                NetworkDashboardFragment.class.getName(), CategoryKey.CATEGORY_NETWORK);
        PARENT_TO_CATEGORY_KEY_MAP.put(ConnectedDeviceDashboardFragment.class.getName(),
                CategoryKey.CATEGORY_CONNECT);
        PARENT_TO_CATEGORY_KEY_MAP.put(AdvancedConnectedDeviceDashboardFragment.class.getName(),
        ...
     }
}

页面key的定义在CategoryKey类中。这样通过key就清楚当前页面是否动态加载那些菜单。

public final class CategoryKey {
     // Activities in this category shows up in Settings homepage.
    public static final String CATEGORY_HOMEPAGE = "com.android.settings.category.ia.homepage";

    // Top level category.
    public static final String CATEGORY_NETWORK = "com.android.settings.category.ia.wireless";
    public static final String CATEGORY_CONNECT = "com.android.settings.category.ia.connect";
    public static final String CATEGORY_DEVICE = "com.android.settings.category.ia.device";
    public static final String CATEGORY_APPS = "com.android.settings.category.ia.apps";
    ...
}

getTilesForCategory方法的实现在DashboardFeatureProviderImpl类中，它是通过CategoryManager类的getTilesByCategory方法实现。mCategories是获取所有动态菜单的集合。

public class CategoryManager {
    public synchronized DashboardCategory getTilesByCategory(Context context, String categoryKey) {
        tryInitCategories(context);

        return mCategoryByKeyMap.get(categoryKey);
    }
    private synchronized void tryInitCategories(Context context) {
        // Keep cached tiles by default. The cache is only invalidated when InterestingConfigChange
        // happens.
        tryInitCategories(context, false /* forceClearCache */);
    }

    private synchronized void tryInitCategories(Context context, boolean forceClearCache) {
        if (mCategories == null) {
            final boolean firstLoading = mCategoryByKeyMap.isEmpty();
            if (forceClearCache) {
                mTileByComponentCache.clear();
            }
            mCategoryByKeyMap.clear();
            //获取categories list
            mCategories = TileUtils.getCategories(context, mTileByComponentCache);
            for (DashboardCategory category : mCategories) {
                mCategoryByKeyMap.put(category.key, category);
            }
            backwardCompatCleanupForCategory(mTileByComponentCache, mCategoryByKeyMap);
            sortCategories(context, mCategoryByKeyMap);
            filterDuplicateTiles(mCategoryByKeyMap);
            if (firstLoading) {
                logTiles(context);

                final DashboardCategory homepageCategory = mCategoryByKeyMap.get(
                        CategoryKey.CATEGORY_HOMEPAGE);
                if (homepageCategory == null) {
                    return;
                }
                for (Tile tile : homepageCategory.getTiles()) {
                    final String key = tile.getKey(context);
                    if (TextUtils.isEmpty(key)) {
                        Log.w(TAG, "Key hint missing for homepage tile: " + tile.getTitle(context));
                        continue;
                    }
                    HighlightableMenu.addMenuKey(key);
                }
            }
        }
    }
}

从loadActivityTiles方法里可以看出，在Settings里动态插入菜单只能是系统应用。

源码路径：frameworks/base/packages/SettingsLib/Tile/src/com/android/settingslib/drawer/TileUtils.java

public class TileUtils {
    public static final String EXTRA_SETTINGS_ACTION = "com.android.settings.action.EXTRA_SETTINGS";
    public static final String IA_SETTINGS_ACTION = "com.android.settings.action.IA_SETTINGS";
     private static final String SETTINGS_ACTION = "com.android.settings.action.SETTINGS";

    public static List<DashboardCategory> getCategories(Context context,
            Map<Pair<String, String>, Tile> cache) {
        final long startTime = System.currentTimeMillis();
        final boolean setup =
                Global.getInt(context.getContentResolver(), Global.DEVICE_PROVISIONED, 0) != 0;
        final ArrayList<Tile> tiles = new ArrayList<>();
        final UserManager userManager = (UserManager) context.getSystemService(
                Context.USER_SERVICE);
        for (UserHandle user : userManager.getUserProfiles()) {
            // TODO: Needs much optimization, too many PM queries going on here.
            if (user.getIdentifier() == ActivityManager.getCurrentUser()) {
                loadTilesForAction(context, user, SETTINGS_ACTION, cache, null, tiles, true);
                loadTilesForAction(context, user, OPERATOR_SETTINGS, cache,
                        OPERATOR_DEFAULT_CATEGORY, tiles, false);
                loadTilesForAction(context, user, MANUFACTURER_SETTINGS, cache,
                        MANUFACTURER_DEFAULT_CATEGORY, tiles, false);
            }
            if (setup) {
                loadTilesForAction(context, user, EXTRA_SETTINGS_ACTION, cache, null, tiles, false);
                loadTilesForAction(context, user, IA_SETTINGS_ACTION, cache, null, tiles, false);
            }
        }
        ...
        return categories;
    }
    static void loadTilesForAction(Context context,
            UserHandle user, String action, Map<Pair<String, String>, Tile> addedCache,
            String defaultCategory, List<Tile> outTiles, boolean requireSettings) {
        final Intent intent = new Intent(action);
        if (requireSettings) {
	        // 只允许settings通过SETTINGS_ACTION添加
            intent.setPackage(SETTING_PKG);
        }
        loadActivityTiles(context, user, addedCache, defaultCategory, outTiles, intent);
        loadProviderTiles(context, user, addedCache, defaultCategory, outTiles, intent);
    }
    
    private static void loadActivityTiles(Context context,
            UserHandle user, Map<Pair<String, String>, Tile> addedCache,
            String defaultCategory, List<Tile> outTiles, Intent intent) {
        final PackageManager pm = context.getPackageManager();
        final List<ResolveInfo> results = pm.queryIntentActivitiesAsUser(intent,
                PackageManager.GET_META_DATA, user.getIdentifier());
        for (ResolveInfo resolved : results) {
            //系统应用
            if (!resolved.system) {
                // Do not allow any app to add to settings, only system ones.
                continue;
            }
            final ActivityInfo activityInfo = resolved.activityInfo;
            final Bundle metaData = activityInfo.metaData;
            loadTile(user, addedCache, defaultCategory, outTiles, intent, metaData, activityInfo);
        }
    }
    private static void loadTile(UserHandle user, Map<Pair<String, String>, Tile> addedCache,
            String defaultCategory, List<Tile> outTiles, Intent intent, Bundle metaData,
            ComponentInfo componentInfo) {
        // Skip loading tile if the component is tagged primary_profile_only but not running on
        // the current user.
        if (user.getIdentifier() != ActivityManager.getCurrentUser()
                && Tile.isPrimaryProfileOnly(componentInfo.metaData)) {
            Log.w(LOG_TAG, "Found " + componentInfo.name + " for intent "
                    + intent + " is primary profile only, skip loading tile for uid "
                    + user.getIdentifier());
            return;
        }

        String categoryKey = defaultCategory;
        // Load category
        categoryKey = metaData.getString(EXTRA_CATEGORY_KEY);
        final boolean isProvider = componentInfo instanceof ProviderInfo;
        final Pair<String, String> key = isProvider
                ? new Pair<>(((ProviderInfo) componentInfo).authority,
                        metaData.getString(META_DATA_PREFERENCE_KEYHINT))
                : new Pair<>(componentInfo.packageName, componentInfo.name);
        Tile tile = addedCache.get(key);
        if (tile == null) {
            tile = isProvider
                    ? new ProviderTile((ProviderInfo) componentInfo, categoryKey, metaData)
                    : new ActivityTile((ActivityInfo) componentInfo, categoryKey);
            addedCache.put(key, tile);
        } else {
            tile.setMetaData(metaData);
        }
        if (!tile.userHandle.contains(user)) {
            tile.userHandle.add(user);
        }
        if (!outTiles.contains(tile)) {
            outTiles.add(tile);
        }
    }
}

然后遍历tiles集合，Tile类里包含Preference的数据(Key/order/intent等等)，也可以设置这些字段的key。

最后一个动态菜单就被成功添加到当前页面了。我们以System->Developer options 菜单为例，它是被动态添加到Settings里的菜单。它定义在Settings的AndroidManifest.xml中。

<activity
            android:name="Settings$DevelopmentSettingsDashboardActivity"
            android:label="@string/development_settings_title"
            android:icon="@drawable/ic_settings_development"
            android:exported="true"
            android:enabled="false">
            <intent-filter android:priority="1">
                <action android:name="android.settings.APPLICATION_DEVELOPMENT_SETTINGS" />
                <action android:name="com.android.settings.APPLICATION_DEVELOPMENT_SETTINGS" />
                <action android:name="android.service.quicksettings.action.QS_TILE_PREFERENCES"/>
                <category android:name="android.intent.category.DEFAULT" />
            </intent-filter>
            <intent-filter>
                <action android:name="com.android.settings.action.SETTINGS" />
            </intent-filter>
            <meta-data android:name="com.android.settings.order" android:value="-40"/>
            <meta-data android:name="com.android.settings.category"
                       android:value="com.android.settings.category.ia.system" />
            <meta-data android:name="com.android.settings.summary"
                       android:resource="@string/summary_empty"/>
            <meta-data android:name="com.android.settings.icon"
                       android:resource="@drawable/ic_settings_development" />
            <meta-data android:name="com.android.settings.FRAGMENT_CLASS"
                       android:value="com.android.settings.development.DevelopmentSettingsDashboardFragment" />
            <meta-data android:name="com.android.settings.HIGHLIGHT_MENU_KEY"
                       android:value="@string/menu_key_system"/>
            <meta-data android:name="com.android.settings.PRIMARY_PROFILE_CONTROLLED"
                       android:value="true" />
        </activity>

首先它设置action为com.android.settings.action.SETTING，从前面TileUtils类分析知道这个action只能Settings里添加时设置。然后设置菜单顺序，菜单category为com.android.settings.category.ia.system，查阅DashboardFragmentRegistry类中PARENT_TO_CATEGORY_KEY_MAP的对应关系，可知对应页面fragment为SystemDashboardFragment，即System页面，接着指定了Summary，icon，fragment等等。这样开发者选项菜单就被插入到了System页面下。

Settings中还存在其它动态插入的选项，例如Google GMS插入的首页菜单Google和Digital Wellbeing & parental controlls。

因为很多应用需要在Settings中增加菜单，作为应用的入口，这种不需要修改Settings代码，而直接修改应用的AndroidManifest.xml文件，实现解耦并自动适配。当然只有系统应用可以动态在Settings插入菜单。

SettingsLib

在分析Settings页面加载分析的时候，发现有部分类来自SettingsLib模块，这个模块是干嘛的呢？

SettingsLib是Android系统中一个专注于为Settings应用提供服务的库。它包含了许多Settings基础功能，并封装了一些操作。

源码路径：frameworks/base/packages/SettingsLib

从bp文件可知，编译后会生成一个SettingsLib的jar包。SettingsLib下根据不同功能，UI基础实现有许多模块，SettingsLib引用这些模块。

SettingsLib模块只有具有系统级别权限如系统应用，framework等才可以调用，第三方应用无法使用。

此时在想，为什么不直接在Settings中直接实现呢？因为将不同功能，UI等的基础实现放在一个公共模块中，可以方便其它与Settings交互的模块或framework使用，进行定制使用，因此，SettingsLib虽专注于为Settings，但它服务于系统，供系统进行Settings相关扩展使用，例如SystemUI模块就在使用SettingsLib相关实现。

相关资料

官方文档：Android“设置”菜单

总结

AndroidSettings具有以下优势：

• 界面。引入Preference显示菜单设置项。统一的页面风格，页面简单，标题状态清晰。

• 扩展性。Android Settings页面采用单个Activity(SubSettings)，多Fragment，支持其它系统应用在Settings添加菜单，可扩展性强。

  Preference和PreferenceController的配合使用，方便定制新的设置项和页面，厂商定制性高。

• 使用。添加了搜索，让用户可以轻松快速找到设置项。界面也决定了用户可以轻松修改各种设置项。