launcher启动
Android开机过程中，会把各种系统服务拉起，并且调用其systemReady()函数，其中最关键的ActivityManagerService拉起后，systemReady()中调用了一个函数startHomeActivityLocked()，
public void systemReady(final Runnable goingCallback, TimingsTraceLog traceLog) {
...
startHomeActivityLocked(currentUserId, "systemReady");
...
}
boolean startHomeActivityLocked(int userId, String reason) {
if (mFactoryTest == FactoryTest.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 = getHomeIntent();
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) {
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);
}
} else {
Slog.wtf(TAG, "No home screen found for " + intent, new Throwable());
}
return true;
}
getHomeIntent函数
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;
}
这里会首先构造一个带有HOME category的Intent，用此intent来从PackageManagerService查询对应的ActivityInfo。Settings、Launcher、setupwizard（开机向导）中有声明，那么会启动哪个呢？看android:priority（[-1000, 1000]），Settings的FallbackHome 优先级高，并且Settings是在Manifest中声明了android:directBootAware=”true”，因此刚开机时，其实只有FallbackHome这个页面会被检索到。
FallbackHome是个没有界面的activity，功能就是一直查找launcher，找到launcher后，启动launcher干掉自己。
private void maybeFinish() {
if (getSystemService(UserManager.class).isUserUnlocked()) {
final Intent homeIntent = new Intent(Intent.ACTION_MAIN)
.addCategory(Intent.CATEGORY_HOME);
final ResolveInfo homeInfo = getPackageManager().resolveActivity(homeIntent, 0);
if (Objects.equals(getPackageName(), homeInfo.activityInfo.packageName)) {
if (UserManager.isSplitSystemUser()
&& UserHandle.myUserId() == UserHandle.USER_SYSTEM) {
// This avoids the situation where the system user has no home activity after
// SUW and this activity continues to throw out warnings. See b/28870689.
return;
}
Log.d(TAG, "User unlocked but no home; let's hope someone enables one soon?");
mHandler.sendEmptyMessageDelayed(0, 500);
} else {
Log.d(TAG, "User unlocked and real home found; let's go!");
getSystemService(PowerManager.class).userActivity(
SystemClock.uptimeMillis(), false);
finish();
}
}
}
finish之后系统会再次调用到AMS的startHomeActivityLocked，这时候就已经可以查询到两个声明CATEGORY_HOME的activity了。但此时依然不会有应用选择对话框。这是因为FallbackHome在manifest中声明了自己的优先级为-1000，PackageManagerService里面对这样的情况是做了处理的。
#开机向导启动
启动FallbackHome后，第一次会启动开机向导，因为开机向导的优先级比launcher高
进入Google开机向导后，首先判断系统是否需要进入开机向导，通过以下三个参数的值：
Settings$Global.DEVICE_PROVISIONED
Settings$Secure.USER_SETUP_COMPLETE
System property: ro.setupwizard.mode (默认配置为OPTIONAL)
DEVICE_PROVISIONED用来标识是否走完开机向导
USER_SETUP_COMPLETE与ro.setupwizard.mode是为多用户系统服务的，所知不多，暂不深入分析。
DEVICE_PROVISIONED为0，表示需要正常进入开机向导流程。
#开机向导为什么只会启动一次
退出开机向导后会把HOME category禁用
