Android Launcher 启动简述 <2>

Lanucher就是我们平时的开机后的桌面APP(它其实就是一个APP,没什么特别的,看起来有点高大上),作为APP程序,它也需要启动自己的Activity界面(因为桌面是能够看得见的,有界面的).那么开机时如何启动呢?

根据上一篇,开机时SystemServer会将ActivityManagerService启动,那么首先就会启动SystemReady(...)方法.

    public void systemReady(final Runnable goingCallback) {

里面的程序非常的多,大概判断了FactoryTest,Binder,SystemUI一些,还有一些记录信息,但是只需要看到最后面:

mMainStack.resumeTopActivityLocked(null);

看过前面一篇的,对应上面这句话不会陌生的.接下来转到ActivityStack类中:不过记住上面传递的参数为null

final boolean resumeTopActivityLocked(ActivityRecord prev) {
        return resumeTopActivityLocked(prev, null);
    }

    final boolean resumeTopActivityLocked(ActivityRecord prev, Bundle options) {
        // Find the first activity that is not finishing.
        ActivityRecord next = topRunningActivityLocked(null);

        // Remember how we'll process this pause/resume situation, and ensure
        // that the state is reset however we wind up proceeding.
        final boolean userLeaving = mUserLeaving;
        mUserLeaving = false;

        if (next == null) {
            // There are no more activities!  Let's just start up the
            // Launcher...
            if (mMainStack) {
                ActivityOptions.abort(options);
                return mService.startHomeActivityLocked(mCurrentUser);
            }
        }

前面传递的参数为null,发现参数是ActivityRecord,那么就是很容易,现在系统里面还没有任何运行过的Activity记录,当然显而易见,刚开机怎么可能有运行Activity记录呢?

那么下面的程序next==null自然而然就是成立的,然后又要装到ActivityManagerService类:

boolean startHomeActivityLocked(int userId) {
        if (mHeadless) {
            // Added because none of the other calls to ensureBootCompleted seem to fire
            // when running headless.
            ensureBootCompleted();
            return false;
        }

        if (mFactoryTest == SystemServer.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 = new Intent(
            mTopAction,
            mTopData != null ? Uri.parse(mTopData) : null);
        intent.setComponent(mTopComponent);
        if (mFactoryTest != SystemServer.FACTORY_TEST_LOW_LEVEL) {
            intent.addCategory(Intent.CATEGORY_HOME);
        }
        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);
            if (app == null || app.instrumentationClass == null) {
                intent.setFlags(intent.getFlags() | Intent.FLAG_ACTIVITY_NEW_TASK);
                mMainStack.startActivityLocked(null, intent, null, aInfo,
                        null, null, 0, 0, 0, 0, null, false, null);
            }
        }

        return true;
    }

最后的mMainStack.startActivityLocked(...)就会启动Launcher APP,这个在上面的程序中打印出log信息,后面的startActivityLocked和前一篇的是一样的了.

private final void startActivityLocked(ActivityRecord r, boolean newTask,
            boolean doResume, boolean keepCurTransition, Bundle options) {
        final int NH = mHistory.size();

        int addPos = -1;
        
        if (!newTask) {
            // If starting in an existing task, find where that is...
            boolean startIt = true;
            for (int i = NH-1; i >= 0; i--) {
                ActivityRecord p = mHistory.get(i);
                if (p.finishing) {
                    continue;
                }

打印log出上面NH的值会是等于0,因为Launcher是第一个启动的Activity.

这里面特别强调的是mHistory是ArrayList<ActivityRecord>,记录了启机以后所有Activity的运行状态信息,每启动一个Activity就会对应有一个ActivityRecord将其所有的状态和信息记录下来,并且将这个new的ActivityRecord添加到mHistory集合中.启动机器后,mHistory中肯定是空的,所以NH=0.