Android GPS学习笔记—GpsLP初始化

目录：

frameworks\base\services\core\java\com\android\server\location

GpsLocationProvider本身有一段初始化代码，如下所示：

//GpsLP定义了一些native函数，此处的class_init_native将初始化相关JNI方法
static { class_init_native(); }

下面看一下GpsLocationProvider的构造函数：

public GpsLocationProvider(Context context, ILocationManager ilocationManager,
Looper looper) {
mContext = context;
/**
NTP即Network Time Protocol，它是一种用来同步计算机时间的协议。该时间的源是UTC。在下面这行代码中，GpsLP
将创建一个NtpTrustedTime对象，该对象采用SNTP协议来和指定的NTP服务器通信以获取准确的时间。
**/
 mNtpTime = NtpTrustedTime.getInstance(context);
mILocationManager = ilocationManager;

mLocation.setExtras(mLocationExtras);

// Create a wake lock
mPowerManager = (PowerManager) mContext.getSystemService(Context.POWER_SERVICE);
mWakeLock = mPowerManager.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, WAKELOCK_KEY);
mWakeLock.setReferenceCounted(true);

mAlarmManager = (AlarmManager)mContext.getSystemService(Context.ALARM_SERVICE);
mWakeupIntent = PendingIntent.getBroadcast(mContext, 0, new Intent(ALARM_WAKEUP), 0);
mTimeoutIntent = PendingIntent.getBroadcast(mContext, 0, new Intent(ALARM_TIMEOUT), 0);

mConnMgr = (ConnectivityManager)context.getSystemService(Context.CONNECTIVITY_SERVICE);

// App ops service to keep track of who is accessing the GPS
mAppOpsService = IAppOpsService.Stub.asInterface(ServiceManager.getService(
Context.APP_OPS_SERVICE));

// Battery statistics service to be notified when GPS turns on or off
mBatteryStats = IBatteryStats.Stub.asInterface(ServiceManager.getService(
BatteryStats.SERVICE_NAME));

// 加载GPS配置文件，下文详细介绍函数reloadGpsProperties
mProperties = new Properties();
reloadGpsProperties(mContext, mProperties);

// 主要处理来自GPS HAL层通知的NI事件
mNIHandler = new GpsNetInitiatedHandler(context,
mNetInitiatedListener,
mSuplEsEnabled);

// construct handler, listen for events
mHandler = new ProviderHandler(looper);
//SUPL的初始化可以由两种特殊的短信触发，下文将简单介绍listenForBroadcasts函数
 listenForBroadcasts();

// 对PASSIVE_PROVIDER添加监听
mHandler.post(new Runnable() {
@Override
public void run() {
LocationManager locManager =
(LocationManager) mContext.getSystemService(Context.LOCATION_SERVICE);
final long minTime = 0;
final float minDistance = 0;
final boolean oneShot = false;
LocationRequest request = LocationRequest.createFromDeprecatedProvider(
LocationManager.PASSIVE_PROVIDER,
minTime,
minDistance,
oneShot);

request.setHideFromAppOps(true);
//接收来自NetworkLP的位置更新通知
//当GpsLP收到来自NetworkLP的位置信息后，将把他们传给GPS HAL层去处理
 locManager.requestLocationUpdates(
request,
new NetworkLocationListener(),
mHandler.getLooper());
}
});
}

函数reloadGpsProperties从文件etc/gps.conf中加载了GPS相关参数包括SUPL服务器地址、端口号等，代码如下：

private void reloadGpsProperties(Context context, Properties properties) {
Log.d(TAG, "Reset GPS properties, previous size = " + properties.size());
loadPropertiesFromResource(context, properties);
boolean isPropertiesLoadedFromFile = false;
final String gpsHardware = SystemProperties.get("ro.hardware.gps");
if (!TextUtils.isEmpty(gpsHardware)) {
final String propFilename =
PROPERTIES_FILE_PREFIX + "." + gpsHardware + PROPERTIES_FILE_SUFFIX;
isPropertiesLoadedFromFile =
loadPropertiesFromFile(propFilename, properties);
}
//加载文件中的GPS属性信息
 if (!isPropertiesLoadedFromFile) {
loadPropertiesFromFile(DEFAULT_PROPERTIES_FILE, properties);
}
Log.d(TAG, "GPS properties reloaded, size = " + properties.size());

// 读取配置文件中的属性信息，通过JNI设置到HAL层
setSuplHostPort(properties.getProperty("SUPL_HOST"),
properties.getProperty("SUPL_PORT"));
//C2K是CDMA2000的缩写，C2K_HOST和C2K_PORT主要用于GPS模块的测试，对用户来说没有太大意义
 mC2KServerHost = properties.getProperty("C2K_HOST");
String portString = properties.getProperty("C2K_PORT");
if (mC2KServerHost != null && portString != null) {
try {
mC2KServerPort = Integer.parseInt(portString);
} catch (NumberFormatException e) {
Log.e(TAG, "unable to parse C2K_PORT: " + portString);
}
}

try {
// Convert properties to string contents and send it to HAL.
ByteArrayOutputStream baos = new ByteArrayOutputStream(4096);
properties.store(baos, null);
native_configuration_update(baos.toString());
Log.d(TAG, "final config = " + baos.toString());
} catch (IOException ex) {
Log.w(TAG, "failed to dump properties contents");
}

// SUPL_ES configuration.
String suplESProperty = mProperties.getProperty("SUPL_ES");
if (suplESProperty != null) {
try {
mSuplEsEnabled = (Integer.parseInt(suplESProperty) == 1);
} catch (NumberFormatException e) {
Log.e(TAG, "unable to parse SUPL_ES: " + suplESProperty);
}
}
}

下面我们来看一下listenForBroadcasts函数，其内容如下：

private void listenForBroadcasts() {
IntentFilter intentFilter = new IntentFilter();
//SUPL INIT流程可以由一条特殊的数据短信触发。数据短信与文本短信不同，下面的这个IntentFilter将接收发往
//127.0.0.1的数据短信。7275为OMA-SUPL使用的端口号
 intentFilter.addAction(Intents.DATA_SMS_RECEIVED_ACTION);
intentFilter.addDataScheme("sms");
intentFilter.addDataAuthority("localhost","7275");
mContext.registerReceiver(mBroadcastReceiver, intentFilter, null, mHandler);

//SUPL INIT也可由WAP推送短信触发，该短信包含的数据类型为MIME中的"application/vnd.omaloc-supl-init"
intentFilter = new IntentFilter();
intentFilter.addAction(Intents.WAP_PUSH_RECEIVED_ACTION);
try {
intentFilter.addDataType("application/vnd.omaloc-supl-init");
} catch (IntentFilter.MalformedMimeTypeException e) {
Log.w(TAG, "Malformed SUPL init mime type");
}
mContext.registerReceiver(mBroadcastReceiver, intentFilter, null, mHandler);

//监听ALARM事件、网络事件等
intentFilter = new IntentFilter();
intentFilter.addAction(ALARM_WAKEUP);
intentFilter.addAction(ALARM_TIMEOUT);
intentFilter.addAction(ConnectivityManager.CONNECTIVITY_ACTION);
intentFilter.addAction(PowerManager.ACTION_POWER_SAVE_MODE_CHANGED);
intentFilter.addAction(Intent.ACTION_SCREEN_OFF);
intentFilter.addAction(Intent.ACTION_SCREEN_ON);
intentFilter.addAction(SIM_STATE_CHANGED);
// TODO: remove the use TelephonyIntents. We are using it because SIM_STATE_CHANGED
// is not reliable at the moment.
intentFilter.addAction(TelephonyIntents.ACTION_SUBINFO_CONTENT_CHANGE);
intentFilter.addAction(TelephonyIntents.ACTION_SUBINFO_RECORD_UPDATED);
mContext.registerReceiver(mBroadcastReceiver, intentFilter, null, mHandler);
}

当GpsLP收到指定的数据短信或WAP推送短信后，checkSmsSuplInit或checkWapSuplInit函数将被调用。这两个函数的功能比较简单，就是将短信的内容传递到GPS HAL层，下面是它们的代码：

private void checkSmsSuplInit(Intent intent) {
SmsMessage[] messages = Intents.getMessagesFromIntent(intent);
for (int i=0; i <messages.length; i++) {
byte[] supl_init = messages[i].getUserData();
native_agps_ni_message(supl_init,supl_init.length);
}
}

private void checkWapSuplInit(Intent intent) {
byte[] supl_init = (byte[]) intent.getExtra("data");
native_agps_ni_message(supl_init,supl_init.length);
}

GpsLP初始化完毕。

参考文献：《深入理解Android:WiFi、NFC和GPS卷》