Android启动过程简析（四）之Lanucher的启动

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前言

通过Android启动过程简析（三）这篇文章我们已经大致了解了服务是如何被启动的，接着本文将会分析系统启动的最后一步，即 Launcher 的启动。

Lanucher的启动流程

在 SystemServer 中启动的这么多服务当中 ActivityManagerService 是负责四大组件的启动、切换、调度的，所以 Launcher 的启动与 ActivityManagerService 离不开关系

frameworks/base/services/java/com/android/server/SystemServer.java

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private void startOtherServices() {
    ....

    // 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() {
            ....
            try {
                startSystemUi(context);
            } catch (Throwable e) {
                reportWtf("starting System UI", e);
            }
            ...
    });
}

可以看到在 startOtherServices 中调用了 mActivityManagerService 的 systemReady 方法，并在传入的 Runnable 对象中启动了 SystemUi，接着继续查看 systemReady 方法

frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

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public void systemReady(final Runnable goingCallback) {
    synchronized(this) {
        //第一次为false不会进入
        if (mSystemReady) {
            // If we're done calling all the receivers, run the next "boot phase" passed in
            // by the SystemServer
            if (goingCallback != null) {
                goingCallback.run();
            }
            return;
        }

        ....
        mSystemReady = true;
    }

    ...

    synchronized(this) {
        // Make sure we have no pre-ready processes sitting around.

        if (mFactoryTest == FactoryTest.FACTORY_TEST_LOW_LEVEL) {
            ....
        }
    }

    ...
    //执行传入的Runnable对象
    if (goingCallback != null) goingCallback.run();

    ...

    synchronized (this) {
        // Only start up encryption-aware persistent apps; once user is
        // unlocked we'll come back around and start unaware apps
        startPersistentApps(PackageManager.MATCH_DIRECT_BOOT_AWARE);

        // Start up initial activity.
        mBooting = true;
        // Enable home activity for system user, so that the system can always boot
        ...
        //启动Lanucher的主Activity
        startHomeActivityLocked(currentUserId, "systemReady");

        ...
        mStackSupervisor.resumeFocusedStackTopActivityLocked();
        mUserController.sendUserSwitchBroadcastsLocked(-1, currentUserId);
    }
}

在 systemReady 方法中会调用 startHomeActivityLocked

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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;
    }
    //构建启动Lanucher主Activity的Intent对象
    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);
            //启动Lanucher主Activity
            mActivityStarter.startHomeActivityLocked(intent, aInfo, reason);
        }
    } else {
        Slog.wtf(TAG, "No home screen found for " + intent, new Throwable());
    }

    return true;
}

进入 startHomeActivityLocked 方法之后可以看到调用了 getHomeIntent 返回了一个 Intent，我们都知道启动一个 Activity 的信息都是通过 Intent 进行传递的，所以继续查看 getHomeIntent 看看 Intent 包含的信息

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

在 getHomeIntent 中首先创建了一个 Intent 对象，然后给这个对象增加了一个 category，category 对应的信息是 android.intent.category.HOME，查找 Lanucher3 的 AndroidManifest.xml 可以看见

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<activity
    android:name="com.android.launcher3.Launcher"
    android:launchMode="singleTask"
    android:clearTaskOnLaunch="true"
    android:stateNotNeeded="true"
    android:theme="@style/Theme"
    android:windowSoftInputMode="adjustPan"
    android:screenOrientation="nosensor"
    android:configChanges="keyboard|keyboardHidden|navigation"
    android:resumeWhilePausing="true"
    android:taskAffinity=""
    android:enabled="true">
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.HOME" />
        <category android:name="android.intent.category.DEFAULT" />
        <category android:name="android.intent.category.MONKEY"/>
    </intent-filter>
</activity>

这说明即将启动的 Activity 是 com.android.launcher3.Launcher，现在回到 startHomeActivityLocked 方法中，继续往下看可以知道在得到 Intent 对象之后，最后是通过 ActivityStarter 的 startHomeActivityLocked 启动了 Launcher。之后的部分由于涉及到 Lanucher 源码和 Activity 启动的细致流程，所以就暂时不做分析了。

总结

通过四篇文章的分析对系统启动的过程有了一个大致的了解，但是限于水平有限，有些部分分析的比较粗糙，希望之后在水平提升之后能够对各个部分进行进一步的完善，例如：Binder、ActivityManagerService、Acitvity 的启动过程等等。 最后借用 Gityuan Android系统开篇中的一张图片说明系统启动的过程

Thanks

  1. Android系统启动流程（四）Launcher启动过程与系统启动流程