SystemUI启动流程

init -> ServerManager -> Zygote -> SystemServer -> SystemUIService -> SystemUIApplication
SystemServer启动后，会在SystemServer Main Thread启动ActivityManagerService，当ActivityManagerService systemReady后，会去启动SystemUIService。可以看出，startSystemUi不是在SystemServer Main thread，而是在ActivityManagerService Thread。

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

/** * Starts a miscellaneous grab bag of stuff that has yet to be refactored * and organized. */private void startOtherServices() {        if (!disableSystemUI) {            try {                Slog.i(TAG, "Status Bar");                statusBar = new StatusBarManagerService(context, wm);                ServiceManager.addService(Context.STATUS_BAR_SERVICE, statusBar);            } catch (Throwable e) {                reportWtf("starting StatusBarManagerService", e);            }        }    mActivityManagerService.systemReady(new Runnable() {        @Override        public void run() {            try {                startSystemUi(context);            } catch (Throwable e) {                reportWtf("starting System UI", e);            }static final void startSystemUi(Context context) {    Intent intent = new Intent();    intent.setComponent(new ComponentName("com.android.systemui",                "com.android.systemui.SystemUIService"));    //Slog.d(TAG, "Starting service: " + intent);    context.startServiceAsUser(intent, UserHandle.OWNER);

}
备注：
context.startServiceAsUser(…)调用的是\frameworks\base\core\java\android\app\ContextImpl.java中的startServiceAsUser(…)，而startServiceAsUser(…)调用startServiceCommon(…)

SystemServer在startSystemUi启动SystemUIService后，会走到SystemUIService的onCreate函数。
public class SystemUIService extends Service {
@Override
public void onCreate() {
super.onCreate();
((SystemUIApplication) getApplication()).startServicesIfNeeded();
} /* 这个SystemUIService很短，代码不到60行 */
可见SystemUIService调用了SystemUIApplication的startServicesIfNeeded()。
\frameworks\base\packages\SystemUI\src\com\android\systemui\ SystemUIApplication.java
/**
* Application class for SystemUI.
*/
public class SystemUIApplication extends Application {

private static final String TAG = "SystemUIService";private static final boolean DEBUG = false;/** * The classes of the stuff to start. */private final Class<?>[] SERVICES = new Class[] {        com.android.systemui.keyguard.KeyguardViewMediator.class, // 锁屏机制        com.android.systemui.recent.Recents.class, // 近期任务        com.android.systemui.volume.VolumeUI.class, // 音量UI        com.android.systemui.statusbar.SystemBars.class, // 系统栏        com.android.systemui.usb.StorageNotification.class, // 存储信息通知        com.android.systemui.power.PowerUI.class, // 电源UI        com.android.systemui.media.RingtonePlayer.class // 铃声播放}; // 它们并不是真正的Service ，继承了SystemUI.java这个抽象类，复写了start()方法/** * Hold a reference on the stuff we start. */private final SystemUI[] mServices = new SystemUI[SERVICES.length];private boolean mServicesStarted;private boolean mBootCompleted;private final Map<Class<?>, Object> mComponents = new HashMap<Class<?>, Object>();@Overridepublic void onCreate() {    super.onCreate();    // Set the application theme that is inherited by all services. Note that setting the    // application theme in the manifest does only work for activities. Keep this in sync with    // the theme set there.    setTheme(R.style.systemui_theme);    // 注册开机广播    IntentFilter filter = new IntentFilter(Intent.ACTION_BOOT_COMPLETED);    filter.setPriority(IntentFilter.SYSTEM_HIGH_PRIORITY);    registerReceiver(new BroadcastReceiver() {        @Override        public void onReceive(Context context, Intent intent) {            if (mBootCompleted) return;            if (DEBUG) Log.v(TAG, "BOOT_COMPLETED received");            unregisterReceiver(this);            mBootCompleted = true;            if (mServicesStarted) {                final int N = mServices.length;                for (int i = 0; i < N; i++) {                    mServices[i].onBootCompleted();                }            }        }    }, filter);}/** * Makes sure that all the SystemUI services are running. If they are already running, this is a * no-op. This is needed to conditinally start all the services, as we only need to have it in * the main process. * * <p>This method must only be called from the main thread.</p> */public void startServicesIfNeeded() {    if (mServicesStarted) {        return;    }    if (!mBootCompleted) {        // check to see if maybe it was already completed long before we began        // see ActivityManagerService.finishBooting()        if ("1".equals(SystemProperties.get("sys.boot_completed"))) {            mBootCompleted = true;            if (DEBUG) Log.v(TAG, "BOOT_COMPLETED was already sent");        }    }    Log.v(TAG, "Starting SystemUI services.");    final int N = SERVICES.length;    // for循环逐个启动SystemUI的各个服务(电源UI、音量UI、状态栏等)    for (int i=0; i<N; i++) {        Class<?> cl = SERVICES[i];        if (DEBUG) Log.d(TAG, "loading: " + cl);        try {            mServices[i] = (SystemUI)cl.newInstance();        } catch (IllegalAccessException ex) {            throw new RuntimeException(ex);        } catch (InstantiationException ex) {            throw new RuntimeException(ex);        }        mServices[i].mContext = this;        mServices[i].mComponents = mComponents;        if (DEBUG) Log.d(TAG, "running: " + mServices[i]);        mServices[i].start(); // 调用各个”服务”的start()方法        if (mBootCompleted) {            mServices[i].onBootCompleted();        }    }    mServicesStarted = true;}@Overridepublic void onConfigurationChanged(Configuration newConfig) {    if (mServicesStarted) {        int len = mServices.length;        for (int i = 0; i < len; i++) {            mServices[i].onConfigurationChanged(newConfig);        }    }}@SuppressWarnings("unchecked")public <T> T getComponent(Class<T> interfaceType) {    return (T) mComponents.get(interfaceType);}// 暴露的接口public SystemUI[] getServices() {    return mServices;}

}

下面看SystemBars.java的start()方法
@Override
public void start() {
if (DEBUG) Log.d(TAG, “start”);
mServiceMonitor = new ServiceMonitor(TAG, DEBUG,
mContext, Settings.Secure.BAR_SERVICE_COMPONENT, this);
mServiceMonitor.start(); // will call onNoService if no remote service is found
}

\frameworks\base\packages\SystemUI\src\com\android\systemui\statusbar\ServiceMonitor.java
public ServiceMonitor(String ownerTag, boolean debug,
Context context, String settingKey, Callbacks callbacks) {
mTag = ownerTag + “.ServiceMonitor”;
mDebug = debug;
mContext = context;
mSettingKey = settingKey;
mCallbacks = callbacks;
}

public void start() {    // listen for setting changes    ContentResolver cr = mContext.getContentResolver();    cr.registerContentObserver(Settings.Secure.getUriFor(mSettingKey),            false /*notifyForDescendents*/, mSettingObserver, UserHandle.USER_ALL);    // listen for package/component changes    IntentFilter filter = new IntentFilter();    filter.addAction(Intent.ACTION_PACKAGE_ADDED);    filter.addAction(Intent.ACTION_PACKAGE_CHANGED);    filter.addDataScheme("package");    mContext.registerReceiver(mBroadcastReceiver, filter);    mHandler.sendEmptyMessage(MSG_START_SERVICE);}

---

init -> ServerManager -> Zygote -> SystemServer -> SystemUIService -> SystemUIApplication
状态栏启动:
SystemUIApplication -> SystemBars

SystemBars里面createStatusBarFromConfig() {
……
String clsName = mContext.getString(R.string.config_statusBarComponent);
……
cls = mContext.getClassLoader().loadClass(clsName);
……
mStatusBar = (BaseStatusBar) cls.newInstance();
……
mStatusBar.start();
}
config_statusBarComponent在frameworks\base\packages\SystemUI\res\values\config.xml中定义:

com.android.systemui.statusbar.phone.PhoneStatusBar
由此可见，SystemBars里面启动的就是PhoneStatusBar了，PhoneStatusBar继承BaseStatusBar。由Java多态性，mStatusBar.start()实际上就是PhoneStatusBar.start()。下面看一下PhoneStatusBar.start()这个函数的函数体:
@Override
public void start() {
……
super.start(); // calls createAndAddWindows()
// 添加导航栏
addNavigationBar();
}
super.start()也就是BaseStatusBar.start()，里面调用了createAndAddWindows()，但这是一个抽象函数，因此会回调至子类PhoneStatusBar的createAndAddWindows()，下面看一下PhoneStatusBar的createAndAddWindows()函数:
@Override
public void createAndAddWindows() {
……
addStatusBarWindow();
……
}
private void addStatusBarWindow() {
makeStatusBarView();
mStatusBarWindowManager = new StatusBarWindowManager(mContext);
mStatusBarWindowManager.add(mStatusBarWindow, getStatusBarHeight());
}