android系统Launcher启动时机

今天有空研究了一下系统启动流程中的Launcher启动时机，有必要给大家分享一下阅读代码的成果；

首先我们知道系统启动先会生成init进程，init进程会去启动各种守护进程类似usbd，adbd等，同时将启动Zygote进程，而之后Zygote进程会去起SystemServer进程，这个SystemServer是启动大部分服务的进程，同时将这些服务注册到ServiceManager中，其实应该能想到服务启动完之后就会去启动SystemUI和Launcher，于是进入SystemServer.java看一下：

 System.loadLibrary("android_servers");
        nativeInit();

        // Check whether we failed to shut down last time we tried.
        // This call may not return.
        performPendingShutdown();

        // Initialize the system context.
        createSystemContext();

        // Create the system service manager.
        mSystemServiceManager = new SystemServiceManager(mSystemContext);
        LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);

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

        // For debug builds, log event loop stalls to dropbox for analysis.
        if (StrictMode.conditionallyEnableDebugLogging()) {
            Slog.i(TAG, "Enabled StrictMode for system server main thread.");
        }

以上代码可以看到，会先去启动启动引导服务，然后去启动核心服务，其实这两类服务都是去启动平台服务，接下去去启动其他服务，其他服务就是硬件服务，继续看代码startOtherServices

 final CommonTimeManagementService commonTimeMgmtServiceF = commonTimeMgmtService;
        final TextServicesManagerService textServiceManagerServiceF = tsms;
        final StatusBarManagerService statusBarF = statusBar;
        final AssetAtlasService atlasF = atlas;
        final InputManagerService inputManagerF = inputManager;
        final TelephonyRegistry telephonyRegistryF = telephonyRegistry;
        final MediaRouterService mediaRouterF = mediaRouter;
        final AudioService audioServiceF = audioService;
        final MmsServiceBroker mmsServiceF = mmsService;

        // We now tell the activity manager it is okay to run third party
        // code.  It will call back into us once it has gotten to the state
        // where third party code can really run (but before it has actually
        // started launching the initial applications), for us to complete our
        // initialization.
        mActivityManagerService.systemReady(new Runnable() {
            @Override
            public 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);
                }
                try {
                    if (mountServiceF != null) mountServiceF.systemReady();
                } catch (Throwable e) {
                    reportWtf("making Mount Service ready", e);
                }

上面代码中可以看到startSystemUi这个函数，很明显是去启动SystemUI了，启动Launcher代码应该也是差不多这个地方了，继续看方法mActivityManagerService.systemReady

           int i;
                        for (i=0; i

我们看到startHomeActivityLocked这个方法，

boolean startHomeActivityLocked(int userId, String reason) {
        if (mFactoryTest == FactoryTest.FACTORY_TEST_LOW_LEVEL
                && mTopAction == null) {
            // We are running in factory test mode, but unable to find
            // the factory test app, so just sit around displaying the
            // error message and don't try to start anything.
            return false;
        }
        Intent intent = getHomeIntent();
        ActivityInfo aInfo =
            resolveActivityInfo(intent, STOCK_PM_FLAGS, userId);
        if (aInfo != null) {
            intent.setComponent(new ComponentName(
                    aInfo.applicationInfo.packageName, aInfo.name));
            // Don't do this if the home app is currently being
            // instrumented.
            aInfo = new ActivityInfo(aInfo);
            aInfo.applicationInfo = getAppInfoForUser(aInfo.applicationInfo, userId);
            ProcessRecord app = getProcessRecordLocked(aInfo.processName,
                    aInfo.applicationInfo.uid, true);
            if (app == null || app.instrumentationClass == null) {
                intent.setFlags(intent.getFlags() | Intent.FLAG_ACTIVITY_NEW_TASK);
                
            }
        }

        return true;
    }

继续看getHomeIntent方法

                   UsageEvents.Event.MOVE_TO_BACKGROUND);
            }
            synchronized (stats) {
                stats.noteActivityPausedLocked(component.app.uid);
            }
        }
    }

    Intent getHomeIntent() {
        Intent intent = new Intent(mTopAction, mTopData != null ? Uri.parse(mTopData) : null);
        intent.setComponent(mTopComponent);
        if (mFactoryTest != FactoryTest.FACTORY_TEST_LOW_LEVEL) {
            intent.addCategory(Intent.CATEGORY_HOME);
        }
        return intent;
    }

    boolean startHomeActivityLocked(int userId, String reason) {
        if (mFactoryTest == FactoryTest.FACTORY_TEST_LOW_LEVEL
                && mTopAction == null) {
            // We are running in factory test mode, but unable to find
            // the factory test app, so just sit around displaying the
            // error message and don't try to start anything.
            return false;

这里我想大家就能看明白，其实getHomeIntent方法是去配置启动Launcher所需要的Intent，mStackSupervisor.startHomeActivity就是去启动Launcher应用的，StackSupervisor本质就是远端也就是服务端启动Activity的一个类，之前有分析过Activity启动流程，如果对Activity启动流程不是很熟的同学可以去了解一下。