Launcher启动流程

基于Android U

SystemServer进程在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。先前已经启动的AMS会将Launcher启动起来。时序图如下：

启动Launcher的入口为AMS的systemReady()方法，它在SystemServer的startOtherServices()方法中被调用。

/frameworks/base/services/java/com/android/server/SystemServer.java
private void startOtherServices(@NonNull TimingsTraceAndSlog t) {
    ...
    mActivityManagerService.systemReady(() -> {
        Slog.i(TAG, "Making services ready");
        t.traceBegin("StartActivityManagerReadyPhase");
        mSystemServiceManager.startBootPhase(t, SystemService.PHASE_ACTIVITY_MANAGER_READY);
        t.traceEnd();
        ...
    }, t);
    ...
}

ActivityManagerService#systemReady

frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java
public void systemReady(final Runnable goingCallback, @NonNull TimingsTraceAndSlog t) {
    ...
    synchronized (this) {
        ...
        boolean isBootingSystemUser = currentUserId == UserHandle.USER_SYSTEM;
        if (isBootingSystemUser && !UserManager.isHeadlessSystemUserMode()) {
            t.traceBegin("startHomeOnAllDisplays");
            mAtmInternal.startHomeOnAllDisplays(currentUserId, "systemReady");
            t.traceEnd();
        }
        ...
    }
} 

mAtmInternal.startHomeOnAllDisplays最终调用了RootWindowContainer.startHomeOnAllDisplays()。

/frameworks/base/services/core/java/com/android/server/wm/RootWindowContainer.java
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;
}

在每个display上显示对应的Home界面。

/frameworks/base/services/core/java/com/android/server/wm/RootWindowContainer.java
boolean startHomeOnDisplay(int userId, String reason, int displayId) {
    return startHomeOnDisplay(userId, reason, displayId, false /* allowInstrumenting */, false /* fromHomeKey */);
}

boolean startHomeOnDisplay(int userId, String reason, int displayId, boolean allowInstrumenting, boolean fromHomeKey) {
    // Fallback to top focused display or default display if the displayId is invalid.
    if (displayId == INVALID_DISPLAY) {
        final Task rootTask = getTopDisplayFocusedRootTask();
        displayId = rootTask != null ? rootTask.getDisplayId() : DEFAULT_DISPLAY;
    }

    final DisplayContent display = getDisplayContent(displayId);
    return display.reduceOnAllTaskDisplayAreas((taskDisplayArea, result) ->
                    result | startHomeOnTaskDisplayArea(userId, reason, taskDisplayArea, allowInstrumenting, fromHomeKey), false /* initValue */);
}

调用startHomeOnTaskDisplayArea()启动每个显示区域（DisplayArea）的Home界面。

/frameworks/base/services/core/java/com/android/server/wm/RootWindowContainer.java
boolean startHomeOnTaskDisplayArea(int userId, String reason, TaskDisplayArea taskDisplayArea, boolean allowInstrumenting, boolean fromHomeKey) {
    ...

    Intent homeIntent = null;
    ActivityInfo aInfo = null;
    if (taskDisplayArea == getDefaultTaskDisplayArea()
                || mWmService.shouldPlacePrimaryHomeOnDisplay(
                        taskDisplayArea.getDisplayId(), userId)) {
        homeIntent = mService.getHomeIntent();
        aInfo = resolveHomeActivity(userId, homeIntent);
    } else if (shouldPlaceSecondaryHomeOnDisplayArea(taskDisplayArea)) {
        Pair<ActivityInfo, Intent> info = resolveSecondaryHomeActivity(userId, taskDisplayArea);
        aInfo = info.first;
        homeIntent = info.second;
    }
    ...
        
    mService.getActivityStartController().startHomeActivity(homeIntent, aInfo, myReason, taskDisplayArea);
    return true;
}

判断传入的TaskDisplayArea是否是默认的TaskDisplayArea。如果是，mService.getHomeIntent()获取对应的Intent，这就是传统的Launcher的启动意图。

在getHomeIntent()方法中创建了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，最后返回该Intent。

/frameworks/base/services/core/java/com/android/server/wm/ActivityTaskManagerService.java
/**
 * 这里主要是获取Launcher的Activity对应的Intent对象以及对应的ActivityInfo
 */
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;
}

/frameworks/base/services/core/java/com/android/server/wm/ActivityStartController.java
/**
 * 这里主要是进行了一通设置后，调用了ActivityStarter对象的execute方法
 */
void startHomeActivity(Intent intent, ActivityInfo aInfo, String reason,
        TaskDisplayArea taskDisplayArea) {
    ...
    mLastHomeActivityStartResult = obtainStarter(intent, "startHomeActivity: " + reason)
            .setOutActivity(tmpOutRecord)
            .setCallingUid(0)
            .setActivityInfo(aInfo)
            .setActivityOptions(options.toBundle())
            .execute();
    ...
}

startHomeActivity()启动的应用程序就是Launcher，因为Launcher的AndroidManifest.xml文件中的intent-filter标签匹配了Action为Intent.ACTION_MAIN，Category为Intent.CATEGORY_HOME。

execute()处理Activity启动请求的接口，executeRequest()执行一系列权限检查，对于合法的请求才继续。

/frameworks/base/services/core/java/com/android/server/wm/ActivityStarter.java
int execute() {
    ...
    res = executeRequest(mRequest);
}

后面的步骤和《根Activity的启动过程》相同。