SystemUI启动流程

本文我们来分析下高通Android7.1代码SystemUI的启动流程。ZygoteInit中handleSystemServerProcess函数的最后一步，会调用SystemServer的静态函数main方法。我们来看下那段有关启动SystemUI的代码：

frameworks\base\services\java\com\android\server\SystemServer.java
       mActivityManagerService.systemReady(new Runnable() {
            @Override
            public void run() {
                ...
                Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "StartSystemUI");
                try {
                    startSystemUi(context);
                } catch (Throwable e) {
                    reportWtf("starting System UI", e);
                }
                Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
                ...
            }
        }

    static final void startSystemUi(Context context) {
        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);
    }

可以看到当ActivityManagerService启动成功准备好后，就调用startSystemUI方法启动SystemUIService服务。此过程会创建com.android.systemui进程。具体的创建过程请参考博客：https://www.jianshu.com/p/327f583f970b

frameworks\base\packages\SystemUI\src\com\android\systemui\SystemUIService.java
public class SystemUIService extends Service {

    @Override
    public void onCreate() {
        super.onCreate();
        ((SystemUIApplication) getApplication()).startServicesIfNeeded();
    }

直接调用SystemUIApplication的startServicesIfNeeded()方法。

frameworks\base\packages\SystemUI\src\com\android\systemui\SystemUIApplication.java
   /**
     * The classes of the stuff to start.
     */
    private final Class[] SERVICES = new Class[] {
            com.android.systemui.tuner.TunerService.class,
            com.android.systemui.keyguard.KeyguardViewMediator.class,
            com.android.systemui.recents.Recents.class,
            com.android.systemui.volume.VolumeUI.class,
            Divider.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,
            com.android.systemui.keyboard.KeyboardUI.class,
            com.android.systemui.tv.pip.PipUI.class,
            com.android.systemui.shortcut.ShortcutKeyDispatcher.class,
            com.android.systemui.VendorServices.class
    };

    private void startServicesIfNeeded(Class[] services) {
        ...
        final int N = services.length;
        for (int i=0; i cl = services[i];
            if (DEBUG) Log.d(TAG, "loading: " + cl);
            try {
                Object newService = SystemUIFactory.getInstance().createInstance(cl);
                mServices[i] = (SystemUI) ((newService == null) ? cl.newInstance() : newService);
            } 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();

            if (mBootCompleted) {
                mServices[i].onBootCompleted();
            }
        }
        mServicesStarted = true;
    }

startServicesIfNeeded通过反射启动了一系列的服务。这里的服务不是Android的service，它们其实是一个普通的java文件，里面的start方法是用来进行一些初始化操作。我们来关注一下SystemBars的实现，通过它我们才创建了状态栏与导航栏。

public class SystemBars extends SystemUI implements ServiceMonitor.Callbacks {
    ...
    @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
    }
    ...
    @Override
    public void onNoService() {
        if (DEBUG) Log.d(TAG, "onNoService");
        createStatusBarFromConfig();  // fallback to using an in-process implementation
    }
    

SystemBars调用ServiceMonitor的start后,ServiceMonitor又会通过消息机制回调SystemBars的onNoService方法，从而读取配置文件里面配置的具体内容，我的系统配置的是PhoneStatusBar：

com.android.systemui.statusbar.phone.PhoneStatusBar

PhoneStatusBar的start方法将对状态栏与导航栏进行创建，不过这是后话了。到这里SystemUI的启动我们就分析完了，欢迎大家留言指正。

参考博客：

https://blog.csdn.net/kebelzc24/article/details/53690121