Android GPS架构分析（四）

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言归正传，分析sgpsInterface->init方法。

gps_qume.c

static int

qemu_gps_init(GpsCallbacks* callbacks)

{

GpsState* s = _gps_state;

if (！s->init)

gps_state_init(s);

if (s->fd < 0)

return -1;

s->callbacks = *callbacks;

return 0;

}

在sGpsInterface->init中，也就是在qemu_gps_init方法，首先调用了gps_state_init，其次注册了回调函数，再说一次，这个回调函数就是在JNI层实现的，而且有JNI层传下来的函数。

static void

gps_state_init( GpsState* state )

{

state->init = 1;

state->control[0] = -1;

state->control[1] = -1;

state->fd = -1;

state->fd = qemu_channel_open(&state->channel,

QEMU_CHANNEL_NAME,

O_RDONLY );

if (state->fd < 0) {

D("no gps emulation detected");

return;

}

D("gps emulation will read from '%s'qemud channel", QEMU_CHANNEL_NAME );

if ( socketpair( AF_LOCAL, SOCK_STREAM, 0,state->control ) < 0 ) {

LOGE("could not create thread controlsocket pair: %s", strerror(errno));

goto Fail;

}

if ( pthread_create( &state->thread,NULL, gps_state_thread, state ) ！= 0 ) {

LOGE("could not create gps thread:%s", strerror(errno));

goto Fail;

}

D("gps state initialized");

return;

Fail:

gps_state_done( state );

}

在这个gps_state_init函数中，首先打开串口，然后建立socket通信，然后建立线程监听底层数据上报，分别对应于代码中黄低部分。

3）建立线程监听事件

mEventThread = new GpsEventThread();

mEventThread.start();

来看看GpsEventThread的run函数。

public void run() {

if (DEBUG) Log.d(TAG, "GpsEventThreADStarting");

// Exit as soon as disable() is calledinstead of waiting for the GPS to stop.

while (mEnabled) {

// this will wait for an event from theGPS,

// which will be reported viareportLocation or reportStatus

native_wait_for_event();

}

if (DEBUG) Log.d(TAG, "GpsEventThreadexiting");

}

}

run函数中还是需要调用native函数：JNI：android_location_GpsLocationProvider_wait_for_event函数。这个函数就是在一个while循环里面等待事件的触发（由回调函数触发），然后调用GpsLocationProvider类的数据上报函数（Location数据）。这个在后面还会讲到。

static voidandroid_location_GpsLocationProvider_wait_for_event(JNIEnv* env, jobject obj)

{

pthread_mutex_lock(&sEventMutex);

while (sPendingCallbacks == 0) {

pthread_cond_wait(&sEventCond,&sEventMutex);

}

...

}

分析完了enable函数以后就轮到enableLocationTracking函数了。

GpsLocationProvider.java

public void enableLocationTracking(booleanenable) {

synchronized (mHandler) {

mHandler.removeMessages(ENABLE_TRACKING);

Message m = Message.obtain(mHandler,ENABLE_TRACKING);

m.arg1 = (enable ? 1 : 0);

mHandler.sendMessage(m);

}

}

同样地，也采取Handler的方式。调用的是handleEnableLocationTracking函数。

private voidhandleEnableLocationTracking(boolean enable) {

if (enable) {

mTTFF = 0;

mLastFixTime = 0;

startNavigating();

} else {

mAlARMManager.cancel(mWakeupIntent);

mAlarmManager.cancel(mTimeoutIntent);

stopNavigating();

}

}

调用startNavigating函数。

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 instartNavigating()");

return;

}

...

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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