1. 什么是System UI

? ? ? ? SystemUI是Android系統(tǒng)級應(yīng)用，負責反饋系統(tǒng)及應(yīng)用狀態(tài)并與用戶保持大量的交互。業(yè)務(wù)主要涉及的組成部分包括狀態(tài)欄(Status Bar)，通知欄(Notification Panel)，鎖屏(Keyguard)，控制中心(Quick Setting)，音量調(diào)節(jié)（VolumeUI）, 近期任務(wù)(Recents)等等。

圖例如下所示：

2. 源碼位置

package name: ? ?com.android.systemui

SystemUI源碼目錄位于： framework/base/packages/SystemUI

Application位于: ? ? ? ?frameworks\base\packages\SystemUI\SystemUIApplication

Service位于： ? ? ? ? ? ?frameworks\base\services\java\com\android\server\

3. systemUI 啟動流程

SystemServer.run()-> startOtherServices()-> startSystemUi()

SystemServer由ZygoteInit進程創(chuàng)建并啟動

frameworks/base/services/java/com/android/server/SystemServer.java

    /**
     * The main entry point from zygote.
     */
    public static void main(String[] args) {
        new SystemServer().run();
    }



    private void run() {
    ......
    // Start services.
        try {
            t.traceBegin("StartServices");
            startBootstrapServices(t);
            startCoreServices(t);
            startOtherServices(t);  //在這里會啟動startSystemUi()
        } catch (Throwable ex) {
            Slog.e("System", "******************************************");
            Slog.e("System", "************ Failure starting system services", ex);
            throw ex;
        } finally {
            t.traceEnd(); // StartServices
        }
    ......
    }


    /**
     * Starts a miscellaneous grab bag of stuff that has yet to be refactored and organized.
     */
        private void startOtherServices(@NonNull TimingsTraceAndSlog t) {
            ......
            try {
                startSystemUi(context, windowManagerF);
            } catch (Throwable e) {
                reportWtf("starting System UI", e);
            }
            ......
        }

    private static void startSystemUi(Context context, WindowManagerService windowManager) {
        PackageManagerInternal pm = LocalServices.getService(PackageManagerInternal.class);
        Intent intent = new Intent();
        intent.setComponent(pm.getSystemUiServiceComponent());  //這里
        intent.addFlags(Intent.FLAG_DEBUG_TRIAGED_MISSING);
        //Slog.d(TAG, "Starting service: " + intent);
        context.startServiceAsUser(intent, UserHandle.SYSTEM);
        windowManager.onSystemUiStarted();
    }

注意看這一段?

intent.setComponent(pm.getSystemUiServiceComponent());

pm是PackageManagerInternal實例，它的getSystemUiServiceComponent()方法是一個抽象方法

frameworks/base/services/core/java/android/content/pm/PackageManagerInternal.java

    /**
     * @return The SystemUI service component name.
     */
    public abstract ComponentName getSystemUiServiceComponent();

PackageManagerService實現(xiàn)了該方法，如下：

frameworks/base/services/core/java/com/android/server/pm/PackageManagerService.java

        @Override
        public ComponentName getSystemUiServiceComponent() {
            return ComponentName.unflattenFromString(mContext.getResources().getString(
                    com.android.internal.R.string.config_systemUIServiceComponent));
        }

?com.android.internal.R.string.config_systemUIServiceComponent的值是

frameworks/base/core/res/res/values/config.xml

<!-- SystemUi service component -->
    <string name="config_systemUIServiceComponent" translatable="false"
            >com.android.systemui/com.android.systemui.SystemUIService</string>

?unflattenFromString返回的是ComponentName(“com.android.systemui”, “com.android.systemui.SystemUIService”);

unflattenFromString?具體方法如下，很簡單，感興趣的可以看看，不感興趣的直接跳過就好，這個方法不是重點。

    /**
     * Recover a ComponentName from a String that was previously created with
     * {@link #flattenToString()}.  It splits the string at the first '/',
     * taking the part before as the package name and the part after as the
     * class name.  As a special convenience (to use, for example, when
     * parsing component names on the command line), if the '/' is immediately
     * followed by a '.' then the final class name will be the concatenation
     * of the package name with the string following the '/'.  Thus
     * "com.foo/.Blah" becomes package="com.foo" class="com.foo.Blah".
     *
     * @param str The String that was returned by flattenToString().
     * @return Returns a new ComponentName containing the package and class
     * names that were encoded in <var>str</var>
     *
     * @see #flattenToString()
     */
    public static @Nullable ComponentName unflattenFromString(@NonNull String str) {
        int sep = str.indexOf('/');
        if (sep < 0 || (sep+1) >= str.length()) {
            return null;
        }
        String pkg = str.substring(0, sep);
        String cls = str.substring(sep+1);
        if (cls.length() > 0 && cls.charAt(0) == '.') {
            cls = pkg + cls;
        }
        return new ComponentName(pkg, cls);
    }

以上代碼功能為通過Intent啟動了SystemUIService。

于是走到SystemUIService的onCreate()方法

frameworks/base/packages/SystemUI/src/com/android/systemui/SystemUIService.java

    public void onCreate() {
        super.onCreate();

        // Start all of SystemUI
        ((SystemUIApplication) getApplication()).startServicesIfNeeded();
        ......
    }


    public void startServicesIfNeeded() {
        String[] names = SystemUIFactory.getInstance().getSystemUIServiceComponents(getResources());
        startServicesIfNeeded(/* metricsPrefix= */ "StartServices", names);
    }

通過工廠模式獲得SystemUI組件列表

frameworks/base/packages/SystemUI/src/com/android/systemui/SystemUIFactory.java
    public String[] getSystemUIServiceComponents(Resources resources) {
        return resources.getStringArray(R.array.config_systemUIServiceComponents);
    }

?列表如下：

frameworks/base/packages/SystemUI/res/values/config.xml
    <!-- SystemUI Services: The classes of the stuff to start. -->
    <string-array name="config_systemUIServiceComponents" translatable="false">
        <item>com.android.systemui.util.NotificationChannels</item>
        <item>com.android.systemui.keyguard.KeyguardViewMediator</item>
        <item>com.android.systemui.recents.Recents</item>
        <item>com.android.systemui.volume.VolumeUI</item>
        <item>com.android.systemui.stackdivider.Divider</item>
        <item>com.android.systemui.statusbar.phone.StatusBar</item>
        <item>com.android.systemui.usb.StorageNotification</item>
        <item>com.android.systemui.power.PowerUI</item>
        <item>com.android.systemui.media.RingtonePlayer</item>
        <item>com.android.systemui.keyboard.KeyboardUI</item>
        <item>com.android.systemui.pip.PipUI</item>
        <item>com.android.systemui.shortcut.ShortcutKeyDispatcher</item>
        <item>@string/config_systemUIVendorServiceComponent</item>
        <item>com.android.systemui.util.leak.GarbageMonitor$Service</item>
        <item>com.android.systemui.LatencyTester</item>
        <item>com.android.systemui.globalactions.GlobalActionsComponent</item>
        <item>com.android.systemui.ScreenDecorations</item>
        <item>com.android.systemui.biometrics.AuthController</item>
        <item>com.android.systemui.SliceBroadcastRelayHandler</item>
        <item>com.android.systemui.SizeCompatModeActivityController</item>
        <item>com.android.systemui.statusbar.notification.InstantAppNotifier</item>
        <item>com.android.systemui.theme.ThemeOverlayController</item>
        <item>com.android.systemui.accessibility.WindowMagnification</item>
        <item>com.android.systemui.accessibility.SystemActions</item>
        <item>com.android.systemui.toast.ToastUI</item>
    </string-array>

?這個列表是要傳到上面的String[] names里的，作為參數(shù)，下一步執(zhí)行startServicesIfNeeded(/* metricsPrefix= */ "StartServices", names);

方法如下：

大致內(nèi)容就是通過反射獲得上面列表的類的構(gòu)造函數(shù)，然后通過構(gòu)造函數(shù)創(chuàng)建上面那些類的實例，然后調(diào)用這些類的start方法，啟動這些systemuI組件。

private void startServicesIfNeeded(String metricsPrefix, String[] services) {
        if (mServicesStarted) {
            return;
        }
        mServices = new SystemUI[services.length];

        if (!mBootCompleteCache.isBootComplete()) {
            // check to see if maybe it was already completed long before we began
            // see ActivityManagerService.finishBooting()
            if ("1".equals(SystemProperties.get("sys.boot_completed"))) {
                mBootCompleteCache.setBootComplete();
                if (DEBUG) {
                    Log.v(TAG, "BOOT_COMPLETED was already sent");
                }
            }
        }

        final DumpManager dumpManager = mRootComponent.createDumpManager();

        Log.v(TAG, "Starting SystemUI services for user " +
                Process.myUserHandle().getIdentifier() + ".");
        TimingsTraceLog log = new TimingsTraceLog("SystemUIBootTiming",
                Trace.TRACE_TAG_APP);
        log.traceBegin(metricsPrefix);
        final int N = services.length;
        for (int i = 0; i < N; i++) {
            String clsName = services[i];  //獲取類名
            if (DEBUG) Log.d(TAG, "loading: " + clsName);
            log.traceBegin(metricsPrefix + clsName);
            long ti = System.currentTimeMillis();
            try {
                SystemUI obj = mComponentHelper.resolveSystemUI(clsName);
                if (obj == null) {
                    Constructor constructor = Class.forName(clsName).getConstructor(Context.class);  //獲取構(gòu)造函數(shù)
                    obj = (SystemUI) constructor.newInstance(this); //通過構(gòu)造函數(shù)獲取實例
                }
                mServices[i] = obj; //把實例放入到mServices數(shù)組中
            } catch (ClassNotFoundException
                    | NoSuchMethodException
                    | IllegalAccessException
                    | InstantiationException
                    | InvocationTargetException ex) {
                throw new RuntimeException(ex);
            }

            if (DEBUG) Log.d(TAG, "running: " + mServices[i]);
            mServices[i].start(); //執(zhí)行start方法
            log.traceEnd();

            // Warn if initialization of component takes too long
            ti = System.currentTimeMillis() - ti;
            if (ti > 1000) {
                Log.w(TAG, "Initialization of " + clsName + " took " + ti + " ms");
            }
            if (mBootCompleteCache.isBootComplete()) {
                mServices[i].onBootCompleted();
            }

            dumpManager.registerDumpable(mServices[i].getClass().getName(), mServices[i]);
        }
        mRootComponent.getInitController().executePostInitTasks();
        log.traceEnd();

        mServicesStarted = true;
    }

SystemUI的啟動就是這樣。剩下的就是啟動具體組件了，例如com.android.systemui.statusbar.phone.StatusBar。

StatusBar也就是狀態(tài)欄，想看狀態(tài)欄怎么啟動的可以跟到它的start方法，這篇就先到這里。

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