SystemServer启动SystemUI：

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

PackageManagerInternal：

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

res/res/values/config.xml：

<string name="config_systemUIServiceComponent" translatable="false">com.android.systemui/com.android.systemui.SystemUIService</string>

SystemUIService的关键代码：

 <service android:name="SystemUIService"android:exported="true"/>

    @Overridepublic void onCreate() {super.onCreate();// Start all of SystemUI((SystemUIApplication) getApplication()).startServicesIfNeeded();
.....
}

然后会通过反射来创建配置里面的组件：

<!-- 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>

这些 module继承了SystemUI 类 。