// ActivityManagerService.java
public void systemReady(final Runnable goingCallback, TimingsTraceAndSlog t) {
    // ① 各种初始化...
    mProcessesReady = true;           // 行 10489: 标记进程可以启动

    // ② 执行 goingCallback（来自 SystemServer）
    goingCallback.run();              // 行 10543
    //  └── startSystemUi(context)    // SystemServer.java:4057 ← SystemUI 在这里启动

    // ③ 启动 Launcher
    startHomeOnAllDisplays(...)       // 行 10609 ← Launcher 在这里启动
}

private static void startSystemUi(Context context, WindowManagerService windowManager) {
    PackageManagerInternal pm = LocalServices.getService(PackageManagerInternal.class);
    Intent intent = new Intent();
    intent.setComponent(pm.getSystemUiServiceComponent());
    intent.addFlags(Intent.FLAG_DEBUG_TRIAGED_MISSING);
    context.startServiceAsUser(intent, UserHandle.SYSTEM);
    windowManager.onSystemUiStarted();
}

@Override
public void onCreate() {
    super.onCreate();
    // Start all of SystemUI
    ((SystemUIApplication) getApplication()).startSystemUserServicesIfNeeded();
}

public void startSystemUserServicesIfNeeded() {
    Map<Class<?>, Provider<CoreStartable>> sortedStartables = new TreeMap<>(
        Comparator.comparing(Class::getName));
    sortedStartables.putAll(mSysUIComponent.getStartables());
    sortedStartables.putAll(mSysUIComponent.getPerUserStartables());
    startServicesIfNeeded(sortedStartables, "StartServices", vendorComponent);
}

if (isBootingSystemUser && !UserManager.isHeadlessSystemUserMode()) {
    t.traceBegin("startHomeOnAllDisplays");
    mAtmInternal.startHomeOnAllDisplays(currentUserId, "systemReady");
    t.traceEnd();
}

boolean startHomeOnAllDisplays(int userId, String reason) {
    boolean homeStarted = false;
    for (int i = getChildCount() - 1; i >= 0; i--) {
        final int displayId = getChildAt(i).mDisplayId;
        homeStarted |= startHomeOnDisplay(userId, reason, displayId);
    }
    return homeStarted;
}

boolean startHomeOnTaskDisplayArea(int userId, String reason,
        TaskDisplayArea taskDisplayArea, ...) {
    // 解析 CATEGORY_HOME Intent，找到 Launcher 组件
    final Pair<ActivityInfo, Intent> homeActivityIntentPair =
            getHomeIntentAndInfoForTaskDisplayArea(taskDisplayArea, userId);

    final ActivityInfo aInfo = homeActivityIntentPair.first;
    final Intent homeIntent = homeActivityIntentPair.second;

    homeIntent.setComponent(new ComponentName(aInfo.applicationInfo.packageName, aInfo.name));
    homeIntent.setFlags(homeIntent.getFlags() | FLAG_ACTIVITY_NEW_TASK);

    // 通过 ActivityStarter 启动 Launcher
    mService.getActivityStartController().startHomeActivity(homeIntent, aInfo, myReason,
            taskDisplayArea, onTop);
    return true;
}

void startHomeActivity(Intent intent, ActivityInfo aInfo, String reason,
        TaskDisplayArea taskDisplayArea, boolean onTop) {
    final ActivityOptions options = ActivityOptions.makeBasic();
    options.setLaunchWindowingMode(WINDOWING_MODE_FULLSCREEN);
    options.setLaunchActivityType(ACTIVITY_TYPE_HOME);
    options.setLaunchDisplayId(displayId);

    rootHomeTask = taskDisplayArea.getOrCreateRootHomeTask(onTop);

    mLastHomeActivityStartResult = obtainStarter(intent, "startHomeActivity: " + reason)
            .setOutActivity(tmpOutRecord)
            .setCallingUid(0)
            .setActivityInfo(aInfo)
            .setActivityOptions(options.toBundle(), ...)
            .execute();
}

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;
}