Android的Launcher启动过程分析（1）

Launcher启动前

我们知道init进程是Android系统中用户空间的第一个进程，它创建zygote(孵化器)和属性服务等。接着在Android系统中，DVM(Dalvik虚拟机)、应用程序进程以及运行系统的关键服务的SystemServer进程都是由Zygote进程来创建的，我们也将它称为孵化器。它通过fock(复制进程)的形式来创建应用程序进程和SystemServer进程，由于Zygote进程在启动时会创建DVM，因此通过fock而创建的应用程序进程和SystemServer进程可以在内部获取一个DVM的实例拷贝。当SystemServer启动时它会启动Binder线程池，这样就可以与其他进程进行通信；它同时也创建了SystemServiceManager用于对系统的服务进行创建、启动、和生命周期的管理；SystemServiceManager.startService(Class<T> serviceClass)方法中通过反射创建并启动服务，同时将服务返回。
SystemServer.java中：

 private void run() {
        // Start services.
...........此处省略
        try {
            traceBeginAndSlog("StartServices");
            startBootstrapServices();
            startCoreServices();
            startOtherServices();
            SystemServerInitThreadPool.shutdown();
        } catch (Throwable ex) {
            Slog.e("System", "******************************************");
            Slog.e("System", "************ Failure starting system services", ex);
            throw ex;
        } finally {
            traceEnd();
        }
...........此处省略

SystemServiceManager.java中：

public <T extends SystemService> T startService(Class<T> serviceClass) {
        try {
            final String name = serviceClass.getName();
            Slog.i(TAG, "Starting " + name);
            Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "StartService " + name);

            // Create the service.
            if (!SystemService.class.isAssignableFrom(serviceClass)) {
                throw new RuntimeException("Failed to create " + name
                        + ": service must extend " + SystemService.class.getName());
            }
            final T service;
            try {
                Constructor<T> constructor = serviceClass.getConstructor(Context.class);
                service = constructor.newInstance(mContext);
            } ...//省略部分catch

            startService(service);
            return service;
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
        }
    }

Launcher概述

Android系统启动的最后一步是启动一个Home应用程序，这个应用程序用来显示系统中已经安装的应用程序，这个Home应用程序就叫做Launcher。应用程序Launcher在启动过程中会PackageManagerService返回系统中已经安装的应用程序的信息，并将这些信息封装成一个快捷图标列表显示在系统屏幕上，这样用户可以通过点击这些快捷图标来启动相应的应用程序。

Launcher的启动流程

我们知道SystemServer进程再启动的过程中会启动PackageManagerService，PackageManagerService启动后会将系统中的应用程序安装完成。在此之前已经启动的ActivityManagerService会将Launcher启动起来，Launcher的入口为ActivityManagerService的systemReady函数：
com.android.server.SystemServer.java

 private void startBootstrapServices() {
                ...............   
          // Activity manager runs the show.
        traceBeginAndSlog("StartActivityManager");
        mActivityManagerService = mSystemServiceManager.startService(
                ActivityManagerService.Lifecycle.class).getService();
        mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
        mActivityManagerService.setInstaller(installer);
        traceEnd();
               .....................
}

 private void startOtherServices() {
                  ...................
        mActivityManagerService.systemReady(() -> {
            Slog.i(TAG, "Making services ready");
            traceBeginAndSlog("StartActivityManagerReadyPhase");
            mSystemServiceManager.startBootPhase(
                    SystemService.PHASE_ACTIVITY_MANAGER_READY)
               ...................
})

在startOtherServices函数中，会调用ActivityManagerService的systemReady函数：
com.android.server.am.ActivityManagerService

public void systemReady(final Runnable goingCallback) {
...
synchronized (this) {
           ...
            mStackSupervisor.resumeFocusedStackTopActivityLocked();
            mUserController.sendUserSwitchBroadcastsLocked(-1, currentUserId);
        }
    }

systemReady函数中调用了ActivityStackSupervisor的resumeFocusedStackTopActivityLocked函数：
com.android.server.am.ActivityStackSupervisor

boolean resumeFocusedStackTopActivityLocked(
            ActivityStack targetStack, ActivityRecord target, ActivityOptions targetOptions) {

        if (!readyToResume()) {
            return false;
        }

        if (targetStack != null && isFocusedStack(targetStack)) {
            return targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);
        }

        final ActivityRecord r = mFocusedStack.topRunningActivityLocked();
        if (r == null || !r.isState(RESUMED)) {
            mFocusedStack.resumeTopActivityUncheckedLocked(null, null);
        } else if (r.isState(RESUMED)) {
            // Kick off any lingering app transitions form the MoveTaskToFront operation.
            mFocusedStack.executeAppTransition(targetOptions);
        }

        return false;
    }

targetStack.resumeTopActivityUncheckedLocked(target, targetOptions)中的targetStack(ActivityStack)对象是用来描述Activity堆栈的，resumeTopActivityUncheckedLocked函数如下所示:
com.android.server.am.ActivityStack

 @GuardedBy("mService")
    boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
        if (mStackSupervisor.inResumeTopActivity) {
            // Don't even start recursing.
            return false;
        }

        boolean result = false;
        try {
            // Protect against recursion.
            mStackSupervisor.inResumeTopActivity = true;
            result = resumeTopActivityInnerLocked(prev, options);

            // When resuming the top activity, it may be necessary to pause the top activity (for
            // example, returning to the lock screen. We suppress the normal pause logic in
            // {@link #resumeTopActivityUncheckedLocked}, since the top activity is resumed at the
            // end. We call the {@link ActivityStackSupervisor#checkReadyForSleepLocked} again here
            // to ensure any necessary pause logic occurs. In the case where the Activity will be
            // shown regardless of the lock screen, the call to
            // {@link ActivityStackSupervisor#checkReadyForSleepLocked} is skipped.
            final ActivityRecord next = topRunningActivityLocked(true /* focusableOnly */);
            if (next == null || !next.canTurnScreenOn()) {
                checkReadyForSleep();
            }
        } finally {
            mStackSupervisor.inResumeTopActivity = false;
        }

        return result;
    }

在该函数中如果ActivityStackSupervisor的inResumeTopActivity递归控制 Tag为true 设为false，终止递归。接着调用了resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options)函数，resumeTopActivityInnerLocked函数的代码很长，我们截取我们要分析的关键的一句：调用ActivityStackSupervisor的resumeHomeStackTask函数，代码如下所示：

private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
....
        if (!hasRunningActivity) {
            // There are no activities left in the stack, let's look somewhere else.
            return resumeTopActivityInNextFocusableStack(prev, options, "noMoreActivities");
        }
...
}

接着又调用了resumeTopActivityInNextFocusableStack函数，代码如下所示：

 private boolean resumeTopActivityInNextFocusableStack(ActivityRecord prev,
            ActivityOptions options, String reason) {
        if (adjustFocusToNextFocusableStack(reason)) {
            // Try to move focus to the next visible stack with a running activity if this
            // stack is not covering the entire screen or is on a secondary display (with no home
            // stack).
            return mStackSupervisor.resumeFocusedStackTopActivityLocked(
                    mStackSupervisor.getFocusedStack(), prev, null);
        }

        // Let's just start up the Launcher...
        ActivityOptions.abort(options);
        if (DEBUG_STATES) Slog.d(TAG_STATES,
                "resumeTopActivityInNextFocusableStack: " + reason + ", go home");
        if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
        // Only resume home if on home display
        return isOnHomeDisplay() &&
                mStackSupervisor.resumeHomeStackTask(prev, reason);
    }

接着看继续调用了resumeHomeStackTask(prev, reason)函数：

 boolean resumeHomeStackTask(ActivityRecord prev, String reason) {
        if (!mService.mBooting && !mService.mBooted) {
            // Not ready yet!
            return false;
        }

        mHomeStack.moveHomeStackTaskToTop();
        ActivityRecord r = getHomeActivity();
        final String myReason = reason + " resumeHomeStackTask";

        // Only resume home activity if isn't finishing.
        if (r != null && !r.finishing) {
            moveFocusableActivityStackToFrontLocked(r, myReason);
            return resumeFocusedStackTopActivityLocked(mHomeStack, prev, null);
        }
        return mService.startHomeActivityLocked(mCurrentUser, myReason);
    }

我们看到它最后调用了ActivityManagerService的startHomeActivityLocked(mCurrentUser, myReason)函数：

 boolean startHomeActivityLocked(int userId, String reason) {
        if (mFactoryTest == FactoryTest.FACTORY_TEST_LOW_LEVEL
                && mTopAction == null) { //tag1
            // 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();//tag2
        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.instr == null) {  //tag3
                intent.setFlags(intent.getFlags() | FLAG_ACTIVITY_NEW_TASK);
                final int resolvedUserId = UserHandle.getUserId(aInfo.applicationInfo.uid);
                // For ANR debugging to verify if the user activity is the one that actually
                // launched.
                final String myReason = reason + ":" + userId + ":" + resolvedUserId;
                mActivityStartController.startHomeActivity(intent, aInfo, myReason);//tag4
            }
        } else {
            Slog.wtf(TAG, "No home screen found for " + intent, new Throwable());
        }

        return true;
    }

在tag1地方的mFactoryTest代表系统的运行模式，系统的运行模式分为非工厂模式、低级工厂模式、高级工厂模式。mTopAction则用来描述第一个被启动Activity组件的Action，它的值为Intent.ACTION_MAIN。所以当它为null并且是低级工程模式时什么也不启动。
tag2地方调用了getHomeIntent函数代码如下：

Intent getHomeIntent() {
        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;
    }

getHomeIntent函数中创建了Intent，并将mTopAction和mTopData传入。mTopAction的值为Intent.ACTION_MAIN，并且如果系统运行模式不是低级工厂模式则将intent的Category设置为Intent.CATEGORY_HOME。我们再回到ActivityManagerService的startHomeActivityLocked函数，假设系统的运行模式不是低级工厂模式，在注释3处判断符合Action为Intent.ACTION_MAIN，Category为Intent.CATEGORY_HOME的应用程序是否已经启动，如果没启动则调用注释4的方法启动该应用程序。
这个被启动的应用程序就是Launcher，因为Launcher的Manifest文件中的intent-filter标签匹配了Action为Intent.ACTION_MAIN，Category为Intent.CATEGORY_HOME。Launcher的Manifest文件如下所示。
/Volumes/android/WORKING_DIRECTORY/packages/apps/Launcher3/AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest
    xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.android.launcher3">
    <uses-sdk android:targetSdkVersion="23" android:minSdkVersion="21"/>
   
    ....
    <application
        android:backupAgent="com.android.launcher3.LauncherBackupAgent"
        android:fullBackupOnly="true"
        android:fullBackupContent="@xml/backupscheme"
        android:hardwareAccelerated="true"
        android:icon="@drawable/ic_launcher_home"
        android:label="@string/derived_app_name"
        android:theme="@style/AppTheme"
        android:largeHeap="@bool/config_largeHeap"
        android:restoreAnyVersion="true"
        android:supportsRtl="true" >
<!--
        Main launcher activity. When extending only change the name, and keep all the
        attributes and intent filters the same
        -->
        <activity
            android:name="com.android.launcher3.Launcher"
            android:launchMode="singleTask"
            android:clearTaskOnLaunch="true"
            android:stateNotNeeded="true"
            android:windowSoftInputMode="adjustPan"
            android:screenOrientation="unspecified"
            android:configChanges="keyboard|keyboardHidden|mcc|mnc|navigation|orientation|screenSize|screenLayout|smallestScreenSize"
            android:resizeableActivity="true"
            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"/>
                <category android:name="android.intent.category.LAUNCHER_APP" />
            </intent-filter>
        </activity>
....
    </application>
</manifest>

这样，应用程序Launcher就会被启动起来，并执行它的onCreate函数。