SystemUI启动

SystemUI的启动是在systemServer（frameworks/base/services/java/com/android/server/SystemServer.java）中启动的，当ActivityManagerService启动完毕之后开始运行：

    static final void startSystemUi(Context context, WindowManagerService windowManager) {
        Intent intent = new Intent();
        intent.setComponent(new ComponentName("com.android.systemui",
                    "com.android.systemui.SystemUIService"));
        intent.addFlags(Intent.FLAG_DEBUG_TRIAGED_MISSING);
        //Slog.d(TAG, "Starting service: " + intent);
        context.startServiceAsUser(intent, UserHandle.SYSTEM);
        windowManager.onSystemUiStarted();
    }

设置好包名和service类名后启动服务。
回到SystemUI的工程中（frameworks/base/packages/SystemUI）在启动SystemUIService之前先启动Application，SystemUIApplication中的onCreate方法

//全套流程
 private final Class<?>[] SERVICES = new Class[] {
            Dependency.class,   // 对其他模块提供组件支持
            NotificationChannels.class,  //
            CommandQueue.CommandQueueStart.class,
            KeyguardViewMediator.class,   // 锁屏界面
            Recents.class,   //最近使用的Activity
            VolumeUI.class,  //音量相关的UI
            Divider.class,  // 多窗口分屏
            SystemBars.class,   //系统状态栏，包括 StatusBar 、 NavigationBar、下拉消息、快捷菜单等等
            StorageNotification.class,   //存储相关的通知
            PowerUI.class,   //电池电量弹窗
            RingtonePlayer.class,  // 铃声控制
            KeyboardUI.class, //外接键盘UI 
            PipUI.class,  
            ShortcutKeyDispatcher.class,   
            VendorServices.class,  
            GarbageMonitor.Service.class,
            LatencyTester.class,
            GlobalActionsComponent.class,
            RoundedCorners.class,  //
    };
//子流程
private final Class<?>[] SERVICES_PER_USER = new Class[] {
            Dependency.class,
            NotificationChannels.class,
            Recents.class
    };
    ...
     @Override
    public void onCreate() {
        super.onCreate();
        if (Process.myUserHandle().equals(UserHandle.SYSTEM)) {
            //如果是由系统启动的
            //注册开机完成广播，保证开机完成后能对每个mServices[i]调用onBootCompleted()
         } else {
           //非由系统启动的
         startServicesIfNeeded(SERVICES_PER_USER);  //只启动一些子流程
         }
    }

再查看SystemUIService类中onCreate方法：

    ((SystemUIApplication) getApplication()).startServicesIfNeeded(); 

继续跟进至SystemUIApplication类

    public void startServicesIfNeeded() {
        startServicesIfNeeded(SERVICES);  //启动全套流程
    }
    private void startServicesIfNeeded(Class<?>[] services) {
        ...
        final int N = services.length;
        for (int i = 0; i < N; i++) {
            mServices[i].start();
            if (mBootCompleted) {
                mServices[i].onBootCompleted();
            }
        }
   ...
   //当有组件连接移除时候的回调
    }

至此SystemUI启动完成，其实就是依次执行每个模块的start方法和onBootCompleted方法。