SystemUI启动过程

以Android 8.0代码，首先记载下SystemUI的启动过程。

1.SystemServer.startOtherServices

            traceBeginAndSlog("StartSystemUI");
            try {
                startSystemUi(context, windowManagerF);
            } catch (Throwable e) {
                reportWtf("starting System UI", e);
            }
            traceEnd();

    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();
    }

SystemServer中启动SystemUIService服务，位置在AMS准备工作完成后，可以在这里用来启动第三方的服务等等。

2.SystemUIService.onCreate

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

        // For debugging RescueParty
        if (Build.IS_DEBUGGABLE && SystemProperties.getBoolean("debug.crash_sysui", false)) {
            throw new RuntimeException();
        }
    }

getApplication()获取包含该服务的application，然后强制转化为SystemUIApplication，接着调用其startServicesIfNeeded方法。

3.SystemUIService.startServicesIfNeeded

    public void startServicesIfNeeded() {
        startServicesIfNeeded(SERVICES);
    }

    /**
     * The classes of the stuff to start.
     */
    private final Class<?>[] SERVICES = new Class[] {
            Dependency.class,
            NotificationChannels.class,
            CommandQueue.CommandQueueStart.class,
            KeyguardViewMediator.class,
            Recents.class,
            VolumeUI.class,
            Divider.class,
            SystemBars.class,
            StorageNotification.class,
            PowerUI.class,
            RingtonePlayer.class,
            KeyboardUI.class,
            PipUI.class,
            ShortcutKeyDispatcher.class,
            VendorServices.class,
            GarbageMonitor.Service.class,
            LatencyTester.class,
            GlobalActionsComponent.class,
    };

• startServicesIfNeeded方法的参数是一个class的数组，涵盖了SystemUI里要启动的类，例如recent，keyboard，SystemBar等等都是我们在SystemUI界面非常熟悉的东西。
• 实际上这类class都继承于SystemUI.java，而SystemUI.java是一个抽象类，有点像我们熟知的工厂模式。主要是实现的方法是start()和onBootCompleted()方法。

4.SystemUIService.startServicesIfNeeded

    private void startServicesIfNeeded(Class<?>[] services) {
        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 for user " +
                Process.myUserHandle().getIdentifier() + ".");
        final int N = services.length;
        for (int i = 0; i < N; i++) {
            Class<?> 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();
            }
        }
        Dependency.get(PluginManager.class).addPluginListener(
                new PluginListener<OverlayPlugin>() {
                    private ArraySet<OverlayPlugin> mOverlays;

                    @Override
                    public void onPluginConnected(OverlayPlugin plugin, Context pluginContext) {
                        StatusBar statusBar = getComponent(StatusBar.class);
                        if (statusBar != null) {
                            plugin.setup(statusBar.getStatusBarWindow(),
                                    statusBar.getNavigationBarView());
                        }
                        // Lazy init.
                        if (mOverlays == null) mOverlays = new ArraySet<>();
                        if (plugin.holdStatusBarOpen()) {
                            mOverlays.add(plugin);
                            Dependency.get(StatusBarWindowManager.class).setStateListener(b ->
                                    mOverlays.forEach(o -> o.setCollapseDesired(b)));
                            Dependency.get(StatusBarWindowManager.class).setForcePluginOpen(
                                    mOverlays.size() != 0);

                        }
                    }

                    @Override
                    public void onPluginDisconnected(OverlayPlugin plugin) {
                        mOverlays.remove(plugin);
                        Dependency.get(StatusBarWindowManager.class).setForcePluginOpen(
                                mOverlays.size() != 0);
                    }
                }, OverlayPlugin.class, true /* Allow multiple plugins */);

        mServicesStarted = true;
    }

利用反射机制，依次获取Services里的各个class，并使用onstrat和onBootCompleted方法完成各个类的加载工作。

以上仅做一个SystemUI启动过程的一个初始流程，分布模块较多，后续分析。