Android 10.0 SystemUI启动流程

本文介绍: 可以看到 startSe r v ic esIfNee de d() 循环 start 了config _systemUISer v iceCo mp onents 里的 Ser v ice，这些服务不是四大组件之一的 Service, 而是继承自 Sy ste mUI 接口的服务，我们称之为 Sy ste mUI服务。3、Sy ste mServer会启动系统运行所需的众多核心服务和普通服务、以及一些应用及数据。在Sy ste mServer 中的 main()方法中，就一句代码生成 Sy ste mServer 对象，执行 run 方法。

1、手机开机后，And roid系统首先会创建一个Zy go te（核心进程）。
2、由Zy go te启动Sy ste mServer。
3、Sy ste mServer会启动系统运行所需的众多核心服务和普通服务、以及一些应用及数据。例如：Syste mUI 启动就是从 Sys t emServer 里启动的。
4、进入锁屏界面，开机完成。

Syst emServer 中有一个 main()方法为系统服务的入口；
frameworks/base/services/java/com/android/server/SystemServer.java

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

在SystemServer 中的 main()方法中，就一句代码生成 SystemServer 对象，执行 run 方法。在run()方法里启动了各类服务；
frameworks/base/services/java/com/android/server/SystemServer.java

private void run() {
    //省略部分代码
    // Start services.
    try {
        traceBeginAndSlog("StartServices");
        startBootstrapServices();
        startCoreServices();
        startOtherServices();    // 在该方法里启动了 SystemUI的服务。
        SystemServerInitThreadPool.shutdown();
    } catch (Throwable ex) {
        Slog.e("System", "******************************************");
        Slog.e("System", "************ Failure starting system services", ex);
        throw ex;
    } finally {
        traceEnd();
    }
   //省略部分代码
}
private void startOtherServices() {
    //省略部分代码
    t.traceBegin("StartSystemUI");
    try {
        startSystemUi(context, windowManagerF);
     } catch (Throwable e) {
         reportWtf("starting System UI", e);
     }
     t.traceEnd();
    //省略部分代码
}
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();
 }

SystemServer执行流程图：

SystemUi进入到SystemUIService的onCreate()方法里；在onCreate()方法中获得 SystemUIAppli cat ion 对象并调用其 startServicesIfNee ded() 方法
frameworks/base/packages/SystemUI/src/com/android/systemui/SystemUIService.java

    @Override
    public void onCreate() {
        super.onCreate();
        // Start all of SystemUI
        ((SystemUIApplication) getApplication()).startServicesIfNeeded();
       
        // 省略部分代码...
    }
   /**
     * 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() {
         String[] names = getResources().getStringArray(R.array.config_systemUIServiceComponents);
         startServicesIfNeeded(names);
    }

在SystemUIApplication中查看startServicesIfNeeded() 方法，其中其中 config _systemUIServiceComponents 值在frameworks/base/packages/SystemUI/res/values/config.xml 里：
frameworks/base/packages/SystemUI/src/com/android/systemui/SystemUIApplication.java

private void startServicesIfNeeded(String[] services) {
    if (mServicesStarted) {
        return;
    }
    mServices = new SystemUI[services.length];
    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 for user " +
            Process.myUserHandle().getIdentifier() + ".");
    TimingsTraceLog log = new TimingsTraceLog("SystemUIBootTiming",
            Trace.TRACE_TAG_APP);
    log.traceBegin("StartServices");
    final int N = services.length;
    for (int i = 0; i < N; i++) {
        String clsName = services[i];
        if (DEBUG) Log.d(TAG, "loading: " + clsName);
        log.traceBegin("StartServices" + clsName);
        long ti = System.currentTimeMillis();
        Class cls;
        try {
            cls = Class.forName(clsName);
            mServices[i] = (SystemUI) cls.newInstance();
        } catch(ClassNotFoundException ex){
            throw new RuntimeException(ex);
        } 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();
        log.traceEnd();
        //省略其他代码
    }
}

可以看到 startServicesIfNeeded() 循环 start 了config_systemUIServiceComponents 里的 Service，这些服务不是四大组件之一的 Service, 而是继承自 SystemUI 接口的服务，我们称之为 SystemUI服务。
到此SystemUI 启动流程分析完毕。