Android GPS架构分析
Daniel Wood 20101224
转载时请注明出处和作者
文章出处：http://danielwood.
作者：Daniel Wood
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分析完了enable函数以后就轮到enableLocationTracking函数了。
GpsLocationProvider.java
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public void enableLocationTracking(boolean enable) {
synchronized (mHandler) {
mHandler.removeMessages(ENABLE_TRACKING);
Message m = Message.obtain(mHandler, ENABLE_TRACKING);
m.arg1 = (enable ? 1 : 0);
mHandler.sendMessage(m);
}
}
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同样地，也采取Handler的方式。调用的是handleEnableLocationTracking函数。
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private void handleEnableLocationTracking(boolean enable) {
if (enable) {
mTTFF = 0;
mLastFixTime = 0;
startNavigating();
} else {
mAlarmManager.cancel(mWakeupIntent);
mAlarmManager.cancel(mTimeoutIntent);
stopNavigating();
}
}
| 调用startNavigating函数。
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private void startNavigating() {
if (!mStarted) {
if (DEBUG) Log.d(TAG, "startNavigating");
mStarted = true;
int positionMode;
if (Settings.Secure.getInt(mContext.getContentResolver(),
Settings.Secure.ASSISTED_GPS_ENABLED, 1) != 0) {
positionMode = GPS_POSITION_MODE_MS_BASED;
} else {
positionMode = GPS_POSITION_MODE_STANDALONE;
}
if (!native_start(positionMode, false, 1)) {
mStarted = false;
Log.e(TAG, "native_start failed in startNavigating()");
return;
}
...
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在startNavigating函数中，最有作用的语句就是调用native方法native_start。调用到了JNI层的android_location_GpsLocationProvider_start函数。
android_location_GpsLocationProvider.cpp
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static jboolean android_location_GpsLocationProvider_start(JNIEnv* env, jobject obj, jint positionMode,
jboolean singleFix, jint fixFrequency)
{
int result = sGpsInterface->set_position_mode(positionMode, (singleFix ? 0 : fixFrequency));
if (result) {
return false;
}
return (sGpsInterface->start() == 0);
}
| 接下去就会调用sGpsInterface接口的实现gps_qemu.c中具体实现的函数。
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static int
qemu_gps_start()
{
GpsState* s = _gps_state;
if (!s->init) {
D("%s: called with uninitialized state !!", __FUNCTION__);
return -1;
}
D("%s: called", __FUNCTION__);
gps_state_start(s);
return 0;
}
| 通过向底层发送命令，CMD_START来启动gps。其实这个所谓的底层就是在enable/init函数中启动的等待数据的线程。
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static void
gps_state_start( GpsState* s )
{
char cmd = CMD_START;
int ret;
do { ret=write( s->control[0], &cmd, 1 ); }
while (ret < 0 && errno == EINTR);
if (ret != 1)
D("%s: could not send CMD_START command: ret=%d: %s",
__FUNCTION__, ret, strerror(errno));
}
| 数据监听线程
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static void*
gps_state_thread( void* arg )
{
...
// now loop
for (;;) {
...
if (cmd == CMD_QUIT) {
D("gps thread quitting on demand");
goto Exit;
}else
if (cmd == CMD_START) {
if (!started) {
D("gps thread starting location_cb=%p", state>callbacks.location_cb);
started = 1;
nmea_reader_set_callback( reader, state->callbacks.location_cb );
} }
else if (cmd == CMD_STOP) {
...
}
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其实就是注册了一个回调函数，location_cb 这个回调函数就是对底层location数据上报的回调函数。
在enableLocationTracking函数调用完成以后，基本上gps服务已经启动完成了，也就是
LocationManagerService中的updateProvidersLocked函数的完成，也就是loadProviders函数的完
成，也就是initialize函数的完成，也就是run函数的完成，也就是2.2中反馈机制systemReady的完成。
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void systemReady() {
// we defer starting up the service until the system is ready
Thread thread = new Thread(null, this, "LocationManagerService");
thread.start();
}
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