该系列文章总纲链接：专题分纲目录 Android SystemUI组件

本章关键点总结 & 说明：

说明：本章节主要涉及SystemUI的两个启动过程，开机启动和服务启动，最终是启动SystemUI子类的各个服务。

1 SystemUIService开机启动流程

SystemUI是Android系统级的UI组件。在开机启动后，SystemServer启动中 ，开始执行对应的run方法，代码实现如下：

private void run() {//...// Start services.try {startBootstrapServices();startCoreServices();startOtherServices();} catch (Throwable ex) {Slog.e("System", "******************************************");Slog.e("System", "************ Failure starting system services", ex);throw ex;}//...
}

在run方法中，执行了startOtherServices，当SystemServer中核心系统服务启动完成后会调用AMS的systemReady()方法通知AMS系统已经就绪。代码实现如下：

private void startOtherServices() {//...StatusBarManagerService statusBar = null;INotificationManager notification = null;//...mActivityManagerService.systemReady(new Runnable() {@Overridepublic void run() {Slog.i(TAG, "Making services ready");mSystemServiceManager.startBootPhase(SystemService.PHASE_ACTIVITY_MANAGER_READY);try {mActivityManagerService.startObservingNativeCrashes();} catch (Throwable e) {reportWtf("observing native crashes", e);}Slog.i(TAG, "WebViewFactory preparation");WebViewFactory.prepareWebViewInSystemServer();try {startSystemUi(context);} catch (Throwable e) {reportWtf("starting System UI", e);}//...}
}

在SystemServer中startSystemUi的代码实现如下：

static final void startSystemUi(Context context) {Intent intent = new Intent();//设置SystemUIService作为启动目标intent.setComponent(new ComponentName("com.android.systemui","com.android.systemui.SystemUIService"));//启动SystemUIServicecontext.startServiceAsUser(intent, UserHandle.OWNER);
}

这里通过 context的startServiceAsUser方法进行SystemUIService的启动，至此SystemUIService服务的开机启动流程就完成了。

2 SystemUIService服务启动

SystemUIService代码并不多，代码如下：

public class SystemUIService extends Service {//关键方法@Overridepublic void onCreate() {super.onCreate();//从这里可以看出，分析SystemUIApplication的startServicesIfNeeded实现即可((SystemUIApplication) getApplication()).startServicesIfNeeded();}@Overridepublic IBinder onBind(Intent intent) {return null;}@Overrideprotected void dump(FileDescriptor fd, PrintWriter pw, String[] args) {SystemUI[] services = ((SystemUIApplication) getApplication()).getServices();if (args == null || args.length == 0) {for (SystemUI ui: services) {pw.println("dumping service: " + ui.getClass().getName());ui.dump(fd, pw, args);}} else {String svc = args[0];for (SystemUI ui: services) {String name = ui.getClass().getName();if (name.endsWith(svc)) {ui.dump(fd, pw, args);}}}}
}

这里主要分析SystemUIService的onCreate()中startServicesIfNeeded的实现，也就是类SystemUIApplication中 startServicesIfNeeded的代码，如下所示：

public class SystemUIApplication extends Application {//...//成员变量SERVICES存储了各个服务的实例，数组内容如下：private final Class<?>[] SERVICES = new Class[] {com.android.systemui.keyguard.KeyguardViewMediator.class,com.android.systemui.recent.Recents.class,com.android.systemui.volume.VolumeUI.class,com.android.systemui.statusbar.SystemBars.class,com.android.systemui.usb.StorageNotification.class,com.android.systemui.power.PowerUI.class,com.android.systemui.media.RingtonePlayer.class};//...private final SystemUI[] mServices = new SystemUI[SERVICES.length];//...public void startServicesIfNeeded() {//如果SystemUI已经启动过了则不需要再启动if (mServicesStarted) {return;}if (!mBootCompleted) {if ("1".equals(SystemProperties.get("sys.boot_completed"))) {mBootCompleted = true;}}final int N = SERVICES.length;for (int i=0; i<N; i++) {Class<?> cl = SERVICES[i];//...//实例化子服务并将其存储在mServices数组中mServices[i] = (SystemUI)cl.newInstance();//...//设置context，并调用start方法启动该类型的服务。mServices[i].mContext = this;mServices[i].mComponents = mComponents;mServices[i].start();if (mBootCompleted) {mServices[i].onBootCompleted();}}mServicesStarted = true;}//...
}

由上可见，SystemUIService是一个容器。主要为了启动以下服务：

• com.android.systemui.keyguard.KeyguardViewMediator，锁屏管理
• com.android.systemui.recent.Recents，近期应用管理
• com.android.systemui.volume.VolumeUI，显示和控制音量的变化：媒体、铃声、闹钟音量
• com.android.systemui.statusbar.SystemBars，系统 状态栏
• com.android.systemui.usb.StorageNotification，系统 通知栏
• com.android.systemui.power.PowerUI，电池模式切换
• com.android.systemui.media.RingtonePlayer，铃声播放

这些服务类有一个共同特点，均继承于SystemUI抽象类，如下：

public class KeyguardViewMediator extends SystemUI {...}
public class Recents extends SystemUI implements RecentsComponent {...}
public class VolumeUI extends SystemUI {...}
public class SystemBars extends SystemUI implements ServiceMonitor.Callbacks {...}
public class StorageNotification extends SystemUI {...}
public class PowerUI extends SystemUI {...}
public class RingtonePlayer extends SystemUI {...}

SystemUI抽象类的内部实现如下所示：

public abstract class SystemUI {public Context mContext;public Map<Class<?>, Object> mComponents;public abstract void start();protected void onConfigurationChanged(Configuration newConfig) {}public void dump(FileDescriptor fd, PrintWriter pw, String[] args) {}protected void onBootCompleted() {}@SuppressWarnings("unchecked")public <T> T getComponent(Class<T> interfaceType) {return (T) (mComponents != null ? mComponents.get(interfaceType) : null);}public <T, C extends T> void putComponent(Class<T> interfaceType, C component) {if (mComponents != null) {mComponents.put(interfaceType, component);}}
}

可以看到最关键的是start方法，因此分析抽象类SystemUI的几个实现类均从start开始即可。接下来的章节我们 分别从这些子类的start方法开始分析。