上一篇博客我们讲了PackageManagerService的启动过程以及对于应用程序的注册过程，当系统启动完成后，系统需要开启第一个应用程序，这就是Home程序，也就是我们熟知的桌面程序。本篇博客主要介绍Home的启动过程。
通过上一篇博客介绍，我们知道系统在启动的时候会启动SystemServer,并且在SystemServer中会启动一系列的Service，包括PackageManagerService,ActivityManagerService等等，而ActivityManagerService在启动后就会负责Home的启动。所以一开始先来看看ActivityManagerService的启动。
1.ActivityManagerService
通过前一篇博客的介绍，我们知道SystemServer在启动后会开启一个线程ServerThread来启动各种系统级Service，其中就包括ActivityManagerService,ServerThread的run函数可以参看上一篇博客。接下来看看ActivityManagerService启动后都干些什么。
1 ActivityManagerService.main
public final class ActivityManagerService extends ActivityManagerNative
implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {
......
public static final Context main(int factoryTest) {
AThread thr = new AThread();
thr.start();
synchronized (thr) {
while (thr.mService == null) {
try {
thr.wait();
} catch (InterruptedException e) {
}
}
}
ActivityManagerService m = thr.mService;
mSelf = m;
ActivityThread at = ActivityThread.systemMain();
mSystemThread = at;
Context context = at.getSystemContext();
m.mContext = context;
m.mFactoryTest = factoryTest;
m.mMainStack = new ActivityStack(m, context, true);
m.mBatteryStatsService.publish(context);
m.mUsageStatsService.publish(context);
synchronized (thr) {
thr.mReady = true;
thr.notifyAll();
}
m.startRunning(null, null, null, null);
return context;
}
......
}这个函数首先通过AThread线程对象来内部创建了一个ActivityManagerService实例，然后将这个实例保存其成员变量mService中，接着又把这个ActivityManagerService实例保存在ActivityManagerService类的静态成员变量mSelf中，最后初始化其它成员变量，就结束了。
2.ActivityManagerService.setProcessSystem
public final class ActivityManagerService extends ActivityManagerNative
implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {
......
public static void setSystemProcess() {
try {
ActivityManagerService m = mSelf;
ServiceManager.addService("activity", m);
ServiceManager.addService("meminfo", new MemBinder(m));
if (MONITOR_CPU_USAGE) {
ServiceManager.addService("cpuinfo", new CpuBinder(m));
}
ServiceManager.addService("permission", new PermissionController(m));
ApplicationInfo info =
mSelf.mContext.getPackageManager().getApplicationInfo(
"android", STOCK_PM_FLAGS);
mSystemThread.installSystemApplicationInfo(info);
synchronized (mSelf) {
ProcessRecord app = mSelf.newProcessRecordLocked(
mSystemThread.getApplicationThread(), info,
info.processName);
app.persistent = true;
app.pid = MY_PID;
app.maxAdj = SYSTEM_ADJ;
mSelf.mProcessNames.put(app.processName, app.info.uid, app);
synchronized (mSelf.mPidsSelfLocked) {
mSelf.mPidsSelfLocked.put(app.pid, app);
}
mSelf.updateLruProcessLocked(app, true, true);
}
} catch (PackageManager.NameNotFoundException e) {
throw new RuntimeException(
"Unable to find android system package", e);
}
}
......
}这个函数主要做了两件事，第一是将ActivityManagerService实例添加到ServiceManager中，这样其他组件就可以通过getSystemService接口来获取到ActivityManagerSerivce了。
第二件事就是通过调用mSystemThread.installSystemApplicationInfo函数来把应用程序框架层下面的android包加载进来。
3.ActivityManagerService.systemReady
接下来ActivityManagerService会调用systemReadey接口。
public final class ActivityManagerService extends ActivityManagerNative
implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {
......
public void systemReady(final Runnable goingCallback) {
......
synchronized (this) {
......
mMainStack.resumeTopActivityLocked(null);
}
}
......
}这里就是通过mMainStack.resumeTopActivityLocked函数来启动Home应用程序的了，这里的mMainStack是一个ActivityStack类型的实例变量。
2.Home应用程序启动
1.mMainStack.resumeTopActivityLocked
接着上面的指令流继续往下看，前文已经讲到了，mMainStack是一个ActivityStack，即一个activity的栈，每个应用程序都会有一个或多个ActivityStack用来维护activity。而resumeTopActivityLocked就是把栈顶的activity恢复到前台。
public class ActivityStack {
......
final boolean resumeTopActivityLocked(ActivityRecord prev) {
// Find the first activity that is not finishing.
ActivityRecord next = topRunningActivityLocked(null);
......
if (next == null) {
// There are no more activities! Let's just start up the
// Launcher...
if (mMainStack) {
return mService.startHomeActivityLocked();
}
}
......
}
......
}由于当前Home应用程序并没有启动，所以next为null，进而会调用mService.startHomeActivityLocked来启动Home程序。
2.mService.startHomeActivityLocked
public final class ActivityManagerService extends ActivityManagerNative
implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {
......
boolean startHomeActivityLocked() {
......
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 =
intent.resolveActivityInfo(mContext.getPackageManager(),
STOCK_PM_FLAGS);
if (aInfo != null) {
intent.setComponent(new ComponentName(
aInfo.applicationInfo.packageName, aInfo.name));
// Don't do this if the home app is currently being
// instrumented.
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, null, 0, aInfo,
null, null, 0, 0, 0, false, false);
}
}
return true;
}
......
}在这个函数中，可以看到AMS会创建一个intent实例，并且设置其category为HOME。而category为Home当然就是Home程序的启动Activity。接下来通过resolveActivityInfo向PackageManagerService查询对应的Activity，当然就是Home程序的Activity了。接下来通过mMainStack.startActivityLocked就启动了Home程序。
3.Launcher.onCreate
public final class Launcher extends Activity
implements View.OnClickListener, OnLongClickListener, LauncherModel.Callbacks, AllAppsView.Watcher {
......
@Override
protected void onCreate(Bundle savedInstanceState) {
......
if (!mRestoring) {
mModel.startLoader(this, true);
}
......
}
......
}
public class LauncherModel extends BroadcastReceiver {
......
public void startLoader(Context context, boolean isLaunching) {
......
synchronized (mLock) {
......
// Don't bother to start the thread if we know it's not going to do anything
if (mCallbacks != null && mCallbacks.get() != null) {
// If there is already one running, tell it to stop.
LoaderTask oldTask = mLoaderTask;
if (oldTask != null) {
if (oldTask.isLaunching()) {
// don't downgrade isLaunching if we're already running
isLaunching = true;
}
oldTask.stopLocked();
}
mLoaderTask = new LoaderTask(context, isLaunching);
sWorker.post(mLoaderTask);
}
}
}
......
}通过上面两段代码可以发现，在Launcher的onCreate初始化函数中，通过mModel来加载Loader，这里的mModel是一个LauncherModel类型的成员变量。 这里不是直接加载应用程序，而是把加载应用程序的操作作为一个消息来处理。这里的sWorker是一个Handler，通过它的post方式把一个消息放在消息队列中去，然后系统就会调用传进去的参数mLoaderTask的run函数来处理这个消息，这个mLoaderTask是LoaderTask类型的实例，于是，下面就会执行LoaderTask类的run函数了。
4.LoaderTask.loadAllAppsByBatch
在LoaderTask的run函数会调用loadAndBindAllApps,而在这个函数中又会调用loadAllAppsByBatch,所以真正启动各个app的工作是在这个函数中执行的。
public class LauncherModel extends BroadcastReceiver {
......
private class LoaderTask implements Runnable {
......
private void loadAllAppsByBatch() {
......
final Intent mainIntent = new Intent(Intent.ACTION_MAIN, null);
mainIntent.addCategory(Intent.CATEGORY_LAUNCHER);
final PackageManager packageManager = mContext.getPackageManager();
List<ResolveInfo> apps = null;
int N = Integer.MAX_VALUE;
int startIndex;
int i=0;
int batchSize = -1;
while (i < N && !mStopped) {
if (i == 0) {
mAllAppsList.clear();
......
apps = packageManager.queryIntentActivities(mainIntent, 0);
......
N = apps.size();
......
if (mBatchSize == 0) {
batchSize = N;
} else {
batchSize = mBatchSize;
}
......
Collections.sort(apps,
new ResolveInfo.DisplayNameComparator(packageManager));
}
startIndex = i;
for (int j=0; i<N && j<batchSize; j++) {
// This builds the icon bitmaps.
mAllAppsList.add(new ApplicationInfo(apps.get(i), mIconCache));
i++;
}
final boolean first = i <= batchSize;
final Callbacks callbacks = tryGetCallbacks(oldCallbacks);
final ArrayList<ApplicationInfo> added = mAllAppsList.added;
mAllAppsList.added = new ArrayList<ApplicationInfo>();
mHandler.post(new Runnable() {
public void run() {
final long t = SystemClock.uptimeMillis();
if (callbacks != null) {
if (first) {
callbacks.bindAllApplications(added);
} else {
callbacks.bindAppsAdded(added);
}
......
} else {
......
}
}
});
......
}
......
}
......
}
......
}函数首先构造一个CATEGORY_LAUNCHER类型的Intent,接着从mContext变量中获得PackageManagerService的接口。下一步就是通过这个PackageManagerService.queryIntentActivities接口来取回所有Action类型为Intent.ACTION_MAIN，并且Category类型为Intent.CATEGORY_LAUNCHER的Activity了。从queryIntentActivities函数调用处返回所要求的Activity后，便调用函数tryGetCallbacks(oldCallbacks)得到一个返CallBack接口，这个接口是由Launcher类实现的，接着调用这个接口的.bindAllApplications函数来进一步操作。注意，这里又是通过消息来处理加载应用程序的操作的。
5.Launcher.bindAllApplications
public final class Launcher extends Activity
implements View.OnClickListener, OnLongClickListener, LauncherModel.Callbacks, AllAppsView.Watcher {
......
private AllAppsView mAllAppsGrid;
......
public void bindAllApplications(ArrayList<ApplicationInfo> apps) {
mAllAppsGrid.setApps(apps);
}
......
}函数很简单，就是调用了mAllAppsGrid.setApps(apps)。这里的mAllAppsGrid是一个AllAppsView类型的变量，它的实际类型一般就是AllApps2D了。所以这个函数的作用很清晰了，就是在Home界面绘制各个应用的图标。具体的绘制逻辑这里就不多讲了。
到这里Home应用程序的启动过程就介绍完了。虽然函数的调用过程比较复杂，但其实总的逻辑还是比较简单的:
- 创建ActivityManagerService；
- ActivityManagerService通过mMainStack来启动Home程序
- mMainStack向PackageManagerService查询Home程序的Activity，然后启动该Activity，并放入该mMainStack中
- Home程序启动后通过PackageManagerService查询所有应用程序的启动Activity
- Home程序在初始化的时候绘制Home界面
- 当点击某个应用程序图标的时候，启动对应应用程序的启动Activity
