【Android架构GPS篇】之定位数据如何从GPS芯片到应用层
2016-12-20 19:05
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原址：http://blog.csdn.net/u013686019/article/details/47444839
写在前面
在漫长的Android源码编译等待过程中，想起之前写过一部分的Android定位实现的探究小品，于是继续探究。
注：代码都是片段化的代码，用来提纲挈领的说明问题。
定位的基础知识：
1、定位芯片和CPU之间通过串口进行通信
2、串口和CPU之间传输的是ASCII格式的NMEA(National Marine Electronics Association)信息，如：
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$GPGGA,092204.999,4250.5589,S,14718.5084,E,1,04,24.4,19.7,M,,,,0000*1F
$GPGLL,4250.5589,S,14718.5084,E,092204.999,A*2D
$GPGSV,3,1,10,20,78,331,45,01,59,235,47,22,41,069,,13,32,252,45*70
$GPRMC,092204.999,A,4250.5589,S,14718.5084,E,0.00,89.68,211200,,*25
基于以上两点，要探知定位数据从GPS芯片到应用层的流程，最好的途径就是从应用层输出NEMA信息的地方开始。
NMEA资料参见：卫星定位数据NMEA介绍
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public class GpsTestActivity extends ActionBarActivity {
/* Other Codes */
/** 获取系统的定位服务，记得在AndroidManifest中赋予定位方面的权限：
* <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION"/>
* <uses-permission android:name="android.permission.ACCESS_LOCATION_EXTRA_COMMANDS"/>
* <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION"/>
*/
LocationManager mLocationService = (LocationManager) getSystemService(Context.LOCATION_SERVICE);
mLocationService.addNmeaListener(mNmeaListener);
private GpsStatus.NmeaListener mNmeaListener = new NmeaListener() {
@Override
public void onNmeaReceived(long timestamp, String nmea) {
System.out.println(nmea + "\n");
}
};
}
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frameworks\base\location\java\android\location\GpsStatus.java
/**
* Used for receiving NMEA sentences from the GPS.
* NMEA 0183 is a standard for communicating with marine electronic devices
* and is a common method for receiving data from a GPS, typically over a serial port.
* See <a href="http://en.wikipedia.org/wiki/NMEA_0183">NMEA 0183</a> for more details.
* You can implement this interface and call {@link LocationManager#addNmeaListener}
* to receive NMEA data from the GPS engine.
*/
public interface NmeaListener {
void onNmeaReceived(long timestamp, String nmea);
}
在上述App中，我们的应用程序实现了该方法，一旦NMEA数据到来，onNmeaReceived()方法就被调用一次，我们在Console上可以看到原始的NEMA信息。
那么接下来，就要寻找nmea数据的来源了。
mNmeaListener通过LocationManager类的addNmeaListener()方法进行注册(register)：
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frameworks\base\location\java\android\location\LocationManager.java
/**
* Adds an NMEA listener.
*
* @param listener a {@link GpsStatus.NmeaListener} object to register
*
* @return true if the listener was successfully added
*
* @throws SecurityException if the ACCESS_FINE_LOCATION permission is not present
*/
public boolean addNmeaListener(GpsStatus.NmeaListener listener) {
boolean result;
/* mNmeaListeners是LocationManager类的成员变量：
* private final HashMap<GpsStatus.NmeaListener, GpsStatusListenerTransport> mNmeaListeners =
* new HashMap<GpsStatus.NmeaListener, GpsStatusListenerTransport>();
*/
if (mNmeaListeners.get(listener) != null) {
// listener is already registered
return true;
}
try {
GpsStatusListenerTransport transport = new GpsStatusListenerTransport(listener);
result = mService.addGpsStatusListener(transport);
if (result) {
mNmeaListeners.put(listener, transport);
}
} catch (RemoteException e) {
Log.e(TAG, "RemoteException in registerGpsStatusListener: ", e);
result = false;
}
return result;
}
这里，先检测定义的NmeaListener有没有被注册过，若果没有，注册之。
注册到哪里去了呢？
由mNmeaListeners成员的定义可知，和GpsStatus.NmeaListener进行关联的是GpsStatusListenerTransport，而它是LocationManager类的一个内部类。
只看相关的部分：
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// This class is used to send GPS status events to the client's main thread.
private class GpsStatusListenerTransport extends IGpsStatusListener.Stub {
private final GpsStatus.NmeaListener mNmeaListener;
// This must not equal any of the GpsStatus event IDs
private static final int NMEA_RECEIVED = 1000;
private class Nmea {
long mTimestamp;
String mNmea;
Nmea(long timestamp, String nmea) {
mTimestamp = timestamp;
mNmea = nmea;
}
}
private ArrayList<Nmea> mNmeaBuffer;
//G psStatusListenerTransport(GpsStatus.Listener listener){}
GpsStatusListenerTransport(GpsStatus.NmeaListener listener) {
mNmeaListener = listener;
mListener = null;
mNmeaBuffer = new ArrayList<Nmea>();
}
@Override
public void onNmeaReceived(long timestamp, String nmea) {
if (mNmeaListener != null) {
synchronized (mNmeaBuffer) {
mNmeaBuffer.add(new Nmea(timestamp, nmea));
}
Message msg = Message.obtain();
msg.what = NMEA_RECEIVED;
// remove any NMEA_RECEIVED messages already in the queue
mGpsHandler.removeMessages(NMEA_RECEIVED);
mGpsHandler.sendMessage(msg);
}
}
private final Handler mGpsHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
if (msg.what == NMEA_RECEIVED) {
synchronized (mNmeaBuffer) {
int length = mNmeaBuffer.size();
for (int i = 0; i < length; i++) {
Nmea nmea = mNmeaBuffer.get(i);
mNmeaListener.onNmeaReceived(nmea.mTimestamp, nmea.mNmea);
}
mNmeaBuffer.clear();
}
} else {
// synchronize on mGpsStatus to ensure the data is copied atomically.
}
}
}
};
}
在GpsStatusListenerTransport类中:
定义一个Nmea类型的链表mNmeaBuffer，一旦onNmeaReceived()接收到NMEA数据，新数据被加载到链表mNmeaBuffer中(mNmeaBuffer.add(new Nmea(timestamp, nmea))),然手置消息标志为NMEA_RECEIVED(msg.what = NMEA_RECEIVED)。
mGpsHandler对上述NMEA_RECEIVED消息进行处理，最终把传过来的NMEA数据发往应用层GpsTestActivity中的onNmeaReceived()。
那么，GpsStatusListenerTransport类中onNmeaReceived(long timestamp, String nmea)方法的nmea数据有谁提供呢？
GpsStatusListenerTransport类继承自IGpsStatusListener，由类前的字符"I"我们得知，它是一个扩展名为.aidl的文件。
注：
AIDL：AIDL机制用来完成在进程之间进行通信(在Android中不允许进程间共享数据)，它的详细知识另外Google之。
这里，我们再次见到了onNmeaReceived()：
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frameworks\base\location\java\android\location\IGpsStatusListener.aidl
oneway interface IGpsStatusListener
{
void onGpsStarted();
void onGpsStopped();
void onFirstFix(int ttff);
void onSvStatusChanged(int svCount, in int[] prns, in float[] snrs, in float[] elevations, in float[] azimuths, int ephemerisMask, int almanacMask, int usedInFixMask);
void onNmeaReceived(long timestamp, String nmea);
}
注：
oneway关键字是用来修饰远程调用行为。使用该关键词时，远程调用不是阻塞的，它只是发送事物数据并立即返回。接口的最终实现是把普通的远程调用按照Binder线程池的调用规则来接收，如果oneway是使用在本地调用上，那么不会有任何影响，并且调用依然是异步的。
下面，探究必须进入第三层。
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frameworks\base\services\java\com\android\server\location\GpsLocationProvider.java
public class GpsLocationProvider implements LocationProviderInterface {
// 此处省略1000+N行
private ArrayList<Listener> mListeners = new ArrayList<Listener>();
private final class Listener implements IBinder.DeathRecipient {
final IGpsStatusListener mListener;
Listener(IGpsStatusListener listener) {
mListener = listener;
}
@Override
public void binderDied() {
if (DEBUG) Log.d(TAG, "GPS status listener died");
synchronized (mListeners) {
mListeners.remove(this);
}
if (mListener != null) {
mListener.asBinder().unlinkToDeath(this, 0);
}
}
}
/**
* called from native code to report NMEA data received
*/
private void reportNmea(long timestamp) {
synchronized (mListeners) {
int size = mListeners.size();
if (size > 0) {
// don't bother creating the String if we have no listeners
int length = native_read_nmea(mNmeaBuffer, mNmeaBuffer.length);
String nmea = new String(mNmeaBuffer, 0, length);
for (int i = 0; i < size; i++) {
Listener listener = mListeners.get(i);
try {
listener.mListener.onNmeaReceived(timestamp, nmea);
} catch (RemoteException e) {
Log.w(TAG, "RemoteException in reportNmea");
mListeners.remove(listener);
// adjust for size of list changing
size--;
}
}
}
}
}
}
GPS定位功能最终需要调用硬件实现，操作硬件就必须通过C/C++完成，GpsLocationProvider中包含许多native方法，采用JNI机制为上层提供服务。
在上面的Code Frame中，通过调用本地方法native_read_nmea()获取到NMEA数据，然后传数据到IGpsStatusListener接口类的onNmeaReceived()方法。
reportNmea()是被JNI方法回调的方法,在 JNI 的实现中，通过这些方法的回调来传递JNI层的执行结果。
源码编译出错，解决问题去。。。
native_read_nmea()在GpsLocationProvider类中定义：
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private native int native_read_nmea(byte[] buffer, int bufferSize);
native指明它是本地方法，和它对应的C/C++文件的实现是：
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static jint android_location_GpsLocationProvider_read_nmea(JNIEnv* env, jobject obj, jbyteArray nmeaArray, jint buffer_size);
How？Next...
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frameworks\base\services\jni\com_android_server_location_GpsLocationProvider.cpp
static JNINativeMethod sMethods[] = {
/* name, signature, funcPtr */
/* other members... */
{"native_read_nmea", "([BI)I", (void*)android_location_GpsLocationProvider_read_nmea},
/* other members... */
};
JNINativeMethod是Android中采用的Java和C/C++函数的映射方式，并在其中描述了函数的参数和返回值：
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typedef struct {
const char* name; // Java文件中的本地方法
const char* signature; // 述了函数的参数和返回值
void* fnPtr; // 指针，指向具体的C/C++函数
} JNINativeMethod;
详细内容这里还是不展开了。
来看android_location_GpsLocationProvider_read_nmea()的实现：
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static jint android_location_GpsLocationProvider_read_nmea(JNIEnv* env, jobject obj,
jbyteArray nmeaArray, jint buffer_size)
{
// this should only be called from within a call to reportNmea
jbyte* nmea = (jbyte *)env->GetPrimitiveArrayCritical(nmeaArray, 0);
int length = sNmeaStringLength;
if (length > buffer_size)
length = buffer_size;
memcpy(nmea, sNmeaString, length);
env->ReleasePrimitiveArrayCritical(nmeaArray, nmea, JNI_ABORT);
return length;
}
虽然不清楚JNI深入含义，但这个函数意思还是挺明显的，我们推断：
第5行：用来动态分配内存，nmea指向获取到的内存区域，同时把nmea和nmeaArray进行关联；
第6行：sNmeaStringLength指示一次从串口读取到的字节长度
第7、8行：在Java中调用native_read_nmea()方法时指明了我们需要取的数据长度，所以，如果从串口实际读取的数据长度大于我们需要的，我们对串口数据进行截取：即，只取指定长度的数据；
第9行：从串口读出的数据存在sNmeaString中，这里Copy到nmea指向的内存区域；
第10行：nmea指向的内存区域中的数据交给nmeaArray，然后释放nmea指向的内存空间。这里也可以看到，函数调用是通过nmeaArray传递NMEA数据的
下面应该看sNmeaStringLength、sNmeaString的设置过程：
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static void nmea_callback(GpsUtcTime timestamp, const char* nmea, int length)
{
JNIEnv* env = AndroidRuntime::getJNIEnv();
// The Java code will call back to read these values
// We do this to avoid creating unnecessary String objects
sNmeaString = nmea;
sNmeaStringLength = length;
env->CallVoidMethod(mCallbacksObj, method_reportNmea, timestamp);
checkAndClearExceptionFromCallback(env, __FUNCTION__);
}
method_reportNmea、、、有没有熟悉的感觉？
对，在GpsLocationProvider类中见过reportNmea(long timestamp)函数。
下面的代码片段表明，method_reportNmea()和reportNmea()是绑定在一起的，调用C/C++函数method_reportNmea，也就间接调用Java的reportNmea()方法。这中间的机制，就是JNI！
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static void android_location_GpsLocationProvider_class_init_native(JNIEnv* env, jclass clazz) {
/* other definitions... */
method_reportNmea = env->GetMethodID(clazz, "reportNmea", "(J)V");
/* other definitions... */
}
而method_reportNmea是在nmea_callback()函数中被调用的，哪里又调用nmea_callback()函数呢？
Let's go to neXt Layer...
com_android_server_location_GpsLocationProvider.cpp文件中另外出现nmea_callback的地方是：
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GpsCallbacks sGpsCallbacks = {
sizeof(GpsCallbacks),
location_callback,
status_callback,
sv_status_callback,
nmea_callback,
set_capabilities_callback,
acquire_wakelock_callback,
release_wakelock_callback,
create_thread_callback,
request_utc_time_callback,
};
GpsCallbacks结构体封装了所有需要回调的函数(确切的说是函数指针)，sGpsCallbacks调用关系：
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static jboolean android_location_GpsLocationProvider_init(JNIEnv* env, jobject obj)
{
// this must be set before calling into the HAL library
if (!mCallbacksObj)
mCallbacksObj = env->NewGlobalRef(obj);
// fail if the main interface fails to initialize
if (!sGpsInterface || sGpsInterface->init(&sGpsCallbacks) != 0)
return false;
/* other codes */
return true;
}
而android_location_GpsLocationProvider_init()在GpsLocationProvider类中调用native_init()时被调用：
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static JNINativeMethod sMethods[] = {
/* name, signature, funcPtr */
{"native_init", "()Z", (void*)android_location_GpsLocationProvider_init}
}
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这里，我们找到了和上层的关系，和下层如何打交道呢？
下面需要贴一大段代码：
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/** Represents the standard GPS interface. */
typedef struct {
/** set to sizeof(GpsInterface) */
size_t size;
/**
* Opens the interface and provides the callback routines
* to the implemenation of this interface.
*/
int (*init)( GpsCallbacks* callbacks );
/** Starts navigating. */
int (*start)( void );
/** Stops navigating. */
int (*stop)( void );
/** Closes the interface. */
void (*cleanup)( void );
/** Injects the current time. */
int (*inject_time)(GpsUtcTime time, int64_t timeReference,
int uncertainty);
/** Injects current location from another location provider
* (typically cell ID).
* latitude and longitude are measured in degrees
* expected accuracy is measured in meters
*/
int (*inject_location)(double latitude, double longitude, float accuracy);
/**
* Specifies that the next call to start will not use the
* information defined in the flags. GPS_DELETE_ALL is passed for
* a cold start.
*/
void (*delete_aiding_data)(GpsAidingData flags);
/**
* min_interval represents the time between fixes in milliseconds.
* preferred_accuracy represents the requested fix accuracy in meters.
* preferred_time represents the requested time to first fix in milliseconds.
*/
int (*set_position_mode)(GpsPositionMode mode, GpsPositionRecurrence recurrence,
uint32_t min_interval, uint32_t preferred_accuracy, uint32_t preferred_time);
/** Get a pointer to extension information. */
const void* (*get_extension)(const char* name);
} GpsInterface;
GpsInterface结构体封装了GPS实现的标准接口——接口，注意！接口不就时用来连接两端的吗？一端是com_android_server_location_GpsLocationProvider.cpp文件里的实现，那另一端就是。。。都探到这个地步了，另一端应该是串口方式直接和GPS芯片打交道的Linux驱动了吧？
确是，但是还需要一个媒介：
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struct gps_device_t {
struct hw_device_t common;
/**
* Set the provided lights to the provided values.
*
* Returns: 0 on succes, error code on failure.
*/
const GpsInterface* (*get_gps_interface)(struct gps_device_t* dev);
};
然后，
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static void android_location_GpsLocationProvider_class_init_native(JNIEnv* env, jclass clazz) {
int err;
hw_module_t* module;
/* other codes..*/
err = hw_get_module(GPS_HARDWARE_MODULE_ID, (hw_module_t const**)&module);
if (err == 0) {
hw_device_t* device;
err = module->methods->open(module, GPS_HARDWARE_MODULE_ID, &device);
if (err == 0) {
gps_device_t* gps_device = (gps_device_t *)device;
sGpsInterface = gps_device->get_gps_interface(gps_device);
}
}
/* other codes..*/
}
static JNINativeMethod sMethods[] = {
/* name, signature, funcPtr */
{"class_init_native", "()V", (void *)android_location_GpsLocationProvider_class_init_native},
}
GpsLocationProvider.java通过class_init_native的调用实现对C/C++文件中android_location_GpsLocationProvider_class_init_native的调用；
com_android_server_location_GpsLocationProvider.cpp通过gps_device_t获取操作GPS芯片的接口。How？？？？
重点来了：GPS_HARDWARE_MODULE_ID
对，就是GPS_HARDWARE_MODULE_ID！
往下看：
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hardware\qcom\gps\loc_api\libloc_api\gps.c
struct hw_module_t HAL_MODULE_INFO_SYM = {
.tag = HARDWARE_MODULE_TAG,
.version_major = 1,
.version_minor = 0,
.id = GPS_HARDWARE_MODULE_ID,
.name = "loc_api GPS Module",
.author = "Qualcomm USA, Inc.",
.methods = &gps_module_methods,
};
有木有？GPS_HARDWARE_MODULE_ID！
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hardware\qcom\gps\loc_api\libloc_api\gps.c
extern const GpsInterface* gps_get_hardware_interface();
const GpsInterface* gps__get_gps_interface(struct gps_device_t* dev)
{
return gps_get_hardware_interface();
}
static int open_gps(const struct hw_module_t* module, char const* name,
struct hw_device_t** device)
{
struct gps_device_t *dev = malloc(sizeof(struct gps_device_t));
memset(dev, 0, sizeof(*dev));
dev->common.tag = HARDWARE_DEVICE_TAG;
dev->common.version = 0;
dev->common.module = (struct hw_module_t*)module;
dev->get_gps_interface = gps__get_gps_interface;
*device = (struct hw_device_t*)dev;
return 0;
}
static struct hw_module_methods_t gps_module_methods = {
.open = open_gps
};
流程很清楚了：
gps_get_hardware_interface()函数在驱动程序中实现
——在gps__get_gps_interface()中被调用
——在open_gps()被调用
——在gps_module_methods中例化
——HAL_MODULE_INFO_SYM
const GpsInterface* gps_get_hardware_interface()函数在其他C文件实现，该C文件是和Linux驱动打交道的应用程序。基本功能：
1、open处理器CPU和GPS芯片连接的串口；
2、read串口NEMA数据，并解析；
3、根据上层传进来的回调函数，打包数据，调用相应Callback，进而发送到Android应用层。
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static const GpsInterface mGpsInterface = {
.size =sizeof(GpsInterface),
.init = gps_init,
|--1、接收从上层传下来的GpsCallbacks变量，用它初始化GpsState->callbacks成员
|--2、GpsState结构体的其他成员初始化
|--3、GpsState->init状态设置为：STATE_INIT
|--4、最重要：启动GPS线程，进行数据的读取、处理：
state->thread = state->callbacks.create_thread_cb("gps", gps_state_thread, state);
--gps_create_thread create_thread_cb;
--typedef pthread_t (* gps_create_thread)(const char* name, void (*start)(void *), void* arg);
.start = gps_start,
--设置GPS的状态为开始：GPS_STATUS_SESSION_BEGIN
.stop = gps_stop,
--设置GPS的状态为结束：GPS_STATUS_SESSION_END
.cleanup = gps_cleanup,
--退出需要进行的一些清理工作，如GpsState->init = STATE_QUIT，GpsCallbacks指针归null，信号量回收
.inject_time = gps_inject_time,
--可为空函数
.inject_location = gps_inject_location,
--可为空函数
.delete_aiding_data = gps_delete_aiding_data,
--可为空函数
.set_position_mode = gps_set_position_mode,
--设置GPS工作模式：单GPS、单BD、GPS/BD双系统
.get_extension = gps_get_extension,
--定位之外的扩展功能实现
};
state->thread = state->callbacks.create_thread_cb("gps", gps_state_thread, state);
--static void gps_state_thread(void* arg)：
1、state通过arg参数传入函数
2、创建了Time和Nmea数据处理两个线程
state->nmea_thread = state->callbacks.create_thread_cb("nmea_thread", gps_nmea_thread, state);
--static void gps_nmea_thread(void* arg)
--gps_opentty(state);
nmea_reader_init(reader);
--nmea_reader_parse(NmeaReader* r) {
if (gps_state->callbacks.nmea_cb) {
struct timeval tv;
unsigned long long mytimems;
gettimeofday(&tv,NULL);
mytimems = tv.tv_sec * 1000 + tv.tv_usec / 1000;
gps_state->callbacks.nmea_cb(mytimems, r->in, r->pos);
D("reader_parse. %.*s ", r->pos, r->in );
}
}
我们是从APP层NMEA信息输出自定向下分析的，APP层信息输出的最终起始是：gps_state->callbacks.nmea_cb(mytimems, r->in, r->pos);
到这里还有个问题：GPS芯片和CPU连接，使用的是哪个串口？这个串口号怎么确定的呢？
打算贴个完整HAL层的实例，考虑到代码很多，下篇在说吧。。。
写在前面
在漫长的Android源码编译等待过程中，想起之前写过一部分的Android定位实现的探究小品，于是继续探究。
注：代码都是片段化的代码，用来提纲挈领的说明问题。
定位的基础知识：
1、定位芯片和CPU之间通过串口进行通信
2、串口和CPU之间传输的是ASCII格式的NMEA(National Marine Electronics Association)信息，如：
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$GPGGA,092204.999,4250.5589,S,14718.5084,E,1,04,24.4,19.7,M,,,,0000*1F
$GPGLL,4250.5589,S,14718.5084,E,092204.999,A*2D
$GPGSV,3,1,10,20,78,331,45,01,59,235,47,22,41,069,,13,32,252,45*70
$GPRMC,092204.999,A,4250.5589,S,14718.5084,E,0.00,89.68,211200,,*25
基于以上两点，要探知定位数据从GPS芯片到应用层的流程，最好的途径就是从应用层输出NEMA信息的地方开始。
NMEA资料参见：卫星定位数据NMEA介绍
一、GPS定位的应用层实现
Luckily，在应用层我们可以通过onNmeaReceived()方法获取到NMEA信息，如下Code Fragment：[java] view
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public class GpsTestActivity extends ActionBarActivity {
/* Other Codes */
/** 获取系统的定位服务，记得在AndroidManifest中赋予定位方面的权限：
* <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION"/>
* <uses-permission android:name="android.permission.ACCESS_LOCATION_EXTRA_COMMANDS"/>
* <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION"/>
*/
LocationManager mLocationService = (LocationManager) getSystemService(Context.LOCATION_SERVICE);
mLocationService.addNmeaListener(mNmeaListener);
private GpsStatus.NmeaListener mNmeaListener = new NmeaListener() {
@Override
public void onNmeaReceived(long timestamp, String nmea) {
System.out.println(nmea + "\n");
}
};
}
二、GPS定位的Framework层实现
GpsStatus.NmeaListener是一个接口类，来自GpsStatus.Java文件：[java] view
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frameworks\base\location\java\android\location\GpsStatus.java
/**
* Used for receiving NMEA sentences from the GPS.
* NMEA 0183 is a standard for communicating with marine electronic devices
* and is a common method for receiving data from a GPS, typically over a serial port.
* See <a href="http://en.wikipedia.org/wiki/NMEA_0183">NMEA 0183</a> for more details.
* You can implement this interface and call {@link LocationManager#addNmeaListener}
* to receive NMEA data from the GPS engine.
*/
public interface NmeaListener {
void onNmeaReceived(long timestamp, String nmea);
}
在上述App中，我们的应用程序实现了该方法，一旦NMEA数据到来，onNmeaReceived()方法就被调用一次，我们在Console上可以看到原始的NEMA信息。
那么接下来，就要寻找nmea数据的来源了。
mNmeaListener通过LocationManager类的addNmeaListener()方法进行注册(register)：
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frameworks\base\location\java\android\location\LocationManager.java
/**
* Adds an NMEA listener.
*
* @param listener a {@link GpsStatus.NmeaListener} object to register
*
* @return true if the listener was successfully added
*
* @throws SecurityException if the ACCESS_FINE_LOCATION permission is not present
*/
public boolean addNmeaListener(GpsStatus.NmeaListener listener) {
boolean result;
/* mNmeaListeners是LocationManager类的成员变量：
* private final HashMap<GpsStatus.NmeaListener, GpsStatusListenerTransport> mNmeaListeners =
* new HashMap<GpsStatus.NmeaListener, GpsStatusListenerTransport>();
*/
if (mNmeaListeners.get(listener) != null) {
// listener is already registered
return true;
}
try {
GpsStatusListenerTransport transport = new GpsStatusListenerTransport(listener);
result = mService.addGpsStatusListener(transport);
if (result) {
mNmeaListeners.put(listener, transport);
}
} catch (RemoteException e) {
Log.e(TAG, "RemoteException in registerGpsStatusListener: ", e);
result = false;
}
return result;
}
这里，先检测定义的NmeaListener有没有被注册过，若果没有，注册之。
注册到哪里去了呢？
由mNmeaListeners成员的定义可知，和GpsStatus.NmeaListener进行关联的是GpsStatusListenerTransport，而它是LocationManager类的一个内部类。
只看相关的部分：
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// This class is used to send GPS status events to the client's main thread.
private class GpsStatusListenerTransport extends IGpsStatusListener.Stub {
private final GpsStatus.NmeaListener mNmeaListener;
// This must not equal any of the GpsStatus event IDs
private static final int NMEA_RECEIVED = 1000;
private class Nmea {
long mTimestamp;
String mNmea;
Nmea(long timestamp, String nmea) {
mTimestamp = timestamp;
mNmea = nmea;
}
}
private ArrayList<Nmea> mNmeaBuffer;
//G psStatusListenerTransport(GpsStatus.Listener listener){}
GpsStatusListenerTransport(GpsStatus.NmeaListener listener) {
mNmeaListener = listener;
mListener = null;
mNmeaBuffer = new ArrayList<Nmea>();
}
@Override
public void onNmeaReceived(long timestamp, String nmea) {
if (mNmeaListener != null) {
synchronized (mNmeaBuffer) {
mNmeaBuffer.add(new Nmea(timestamp, nmea));
}
Message msg = Message.obtain();
msg.what = NMEA_RECEIVED;
// remove any NMEA_RECEIVED messages already in the queue
mGpsHandler.removeMessages(NMEA_RECEIVED);
mGpsHandler.sendMessage(msg);
}
}
private final Handler mGpsHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
if (msg.what == NMEA_RECEIVED) {
synchronized (mNmeaBuffer) {
int length = mNmeaBuffer.size();
for (int i = 0; i < length; i++) {
Nmea nmea = mNmeaBuffer.get(i);
mNmeaListener.onNmeaReceived(nmea.mTimestamp, nmea.mNmea);
}
mNmeaBuffer.clear();
}
} else {
// synchronize on mGpsStatus to ensure the data is copied atomically.
}
}
}
};
}
在GpsStatusListenerTransport类中:
定义一个Nmea类型的链表mNmeaBuffer，一旦onNmeaReceived()接收到NMEA数据，新数据被加载到链表mNmeaBuffer中(mNmeaBuffer.add(new Nmea(timestamp, nmea))),然手置消息标志为NMEA_RECEIVED(msg.what = NMEA_RECEIVED)。
mGpsHandler对上述NMEA_RECEIVED消息进行处理，最终把传过来的NMEA数据发往应用层GpsTestActivity中的onNmeaReceived()。
那么，GpsStatusListenerTransport类中onNmeaReceived(long timestamp, String nmea)方法的nmea数据有谁提供呢？
GpsStatusListenerTransport类继承自IGpsStatusListener，由类前的字符"I"我们得知，它是一个扩展名为.aidl的文件。
注：
AIDL：AIDL机制用来完成在进程之间进行通信(在Android中不允许进程间共享数据)，它的详细知识另外Google之。
这里，我们再次见到了onNmeaReceived()：
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frameworks\base\location\java\android\location\IGpsStatusListener.aidl
oneway interface IGpsStatusListener
{
void onGpsStarted();
void onGpsStopped();
void onFirstFix(int ttff);
void onSvStatusChanged(int svCount, in int[] prns, in float[] snrs, in float[] elevations, in float[] azimuths, int ephemerisMask, int almanacMask, int usedInFixMask);
void onNmeaReceived(long timestamp, String nmea);
}
注：
oneway关键字是用来修饰远程调用行为。使用该关键词时，远程调用不是阻塞的，它只是发送事物数据并立即返回。接口的最终实现是把普通的远程调用按照Binder线程池的调用规则来接收，如果oneway是使用在本地调用上，那么不会有任何影响，并且调用依然是异步的。
下面，探究必须进入第三层。
三、GPS定位的Lib层实现
和IGpsStatusListener接头的是GpsLocationProvider类：[java] view
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frameworks\base\services\java\com\android\server\location\GpsLocationProvider.java
public class GpsLocationProvider implements LocationProviderInterface {
// 此处省略1000+N行
private ArrayList<Listener> mListeners = new ArrayList<Listener>();
private final class Listener implements IBinder.DeathRecipient {
final IGpsStatusListener mListener;
Listener(IGpsStatusListener listener) {
mListener = listener;
}
@Override
public void binderDied() {
if (DEBUG) Log.d(TAG, "GPS status listener died");
synchronized (mListeners) {
mListeners.remove(this);
}
if (mListener != null) {
mListener.asBinder().unlinkToDeath(this, 0);
}
}
}
/**
* called from native code to report NMEA data received
*/
private void reportNmea(long timestamp) {
synchronized (mListeners) {
int size = mListeners.size();
if (size > 0) {
// don't bother creating the String if we have no listeners
int length = native_read_nmea(mNmeaBuffer, mNmeaBuffer.length);
String nmea = new String(mNmeaBuffer, 0, length);
for (int i = 0; i < size; i++) {
Listener listener = mListeners.get(i);
try {
listener.mListener.onNmeaReceived(timestamp, nmea);
} catch (RemoteException e) {
Log.w(TAG, "RemoteException in reportNmea");
mListeners.remove(listener);
// adjust for size of list changing
size--;
}
}
}
}
}
}
GPS定位功能最终需要调用硬件实现，操作硬件就必须通过C/C++完成，GpsLocationProvider中包含许多native方法，采用JNI机制为上层提供服务。
在上面的Code Frame中，通过调用本地方法native_read_nmea()获取到NMEA数据，然后传数据到IGpsStatusListener接口类的onNmeaReceived()方法。
reportNmea()是被JNI方法回调的方法,在 JNI 的实现中，通过这些方法的回调来传递JNI层的执行结果。
源码编译出错，解决问题去。。。
native_read_nmea()在GpsLocationProvider类中定义：
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private native int native_read_nmea(byte[] buffer, int bufferSize);
native指明它是本地方法，和它对应的C/C++文件的实现是：
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static jint android_location_GpsLocationProvider_read_nmea(JNIEnv* env, jobject obj, jbyteArray nmeaArray, jint buffer_size);
How？Next...
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frameworks\base\services\jni\com_android_server_location_GpsLocationProvider.cpp
static JNINativeMethod sMethods[] = {
/* name, signature, funcPtr */
/* other members... */
{"native_read_nmea", "([BI)I", (void*)android_location_GpsLocationProvider_read_nmea},
/* other members... */
};
JNINativeMethod是Android中采用的Java和C/C++函数的映射方式，并在其中描述了函数的参数和返回值：
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typedef struct {
const char* name; // Java文件中的本地方法
const char* signature; // 述了函数的参数和返回值
void* fnPtr; // 指针，指向具体的C/C++函数
} JNINativeMethod;
详细内容这里还是不展开了。
来看android_location_GpsLocationProvider_read_nmea()的实现：
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static jint android_location_GpsLocationProvider_read_nmea(JNIEnv* env, jobject obj,
jbyteArray nmeaArray, jint buffer_size)
{
// this should only be called from within a call to reportNmea
jbyte* nmea = (jbyte *)env->GetPrimitiveArrayCritical(nmeaArray, 0);
int length = sNmeaStringLength;
if (length > buffer_size)
length = buffer_size;
memcpy(nmea, sNmeaString, length);
env->ReleasePrimitiveArrayCritical(nmeaArray, nmea, JNI_ABORT);
return length;
}
虽然不清楚JNI深入含义，但这个函数意思还是挺明显的，我们推断：
第5行：用来动态分配内存，nmea指向获取到的内存区域，同时把nmea和nmeaArray进行关联；
第6行：sNmeaStringLength指示一次从串口读取到的字节长度
第7、8行：在Java中调用native_read_nmea()方法时指明了我们需要取的数据长度，所以，如果从串口实际读取的数据长度大于我们需要的，我们对串口数据进行截取：即，只取指定长度的数据；
第9行：从串口读出的数据存在sNmeaString中，这里Copy到nmea指向的内存区域；
第10行：nmea指向的内存区域中的数据交给nmeaArray，然后释放nmea指向的内存空间。这里也可以看到，函数调用是通过nmeaArray传递NMEA数据的
下面应该看sNmeaStringLength、sNmeaString的设置过程：
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static void nmea_callback(GpsUtcTime timestamp, const char* nmea, int length)
{
JNIEnv* env = AndroidRuntime::getJNIEnv();
// The Java code will call back to read these values
// We do this to avoid creating unnecessary String objects
sNmeaString = nmea;
sNmeaStringLength = length;
env->CallVoidMethod(mCallbacksObj, method_reportNmea, timestamp);
checkAndClearExceptionFromCallback(env, __FUNCTION__);
}
method_reportNmea、、、有没有熟悉的感觉？
对，在GpsLocationProvider类中见过reportNmea(long timestamp)函数。
下面的代码片段表明，method_reportNmea()和reportNmea()是绑定在一起的，调用C/C++函数method_reportNmea，也就间接调用Java的reportNmea()方法。这中间的机制，就是JNI！
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static void android_location_GpsLocationProvider_class_init_native(JNIEnv* env, jclass clazz) {
/* other definitions... */
method_reportNmea = env->GetMethodID(clazz, "reportNmea", "(J)V");
/* other definitions... */
}
而method_reportNmea是在nmea_callback()函数中被调用的，哪里又调用nmea_callback()函数呢？
Let's go to neXt Layer...
四、GPS定位HAL层的实现
所谓Android的HAL层，也就是是Linux的应用程序。至于串口具体配置，比如寄存器配置、数据收发等芯片级实现，是在在Linux内核里的。com_android_server_location_GpsLocationProvider.cpp文件中另外出现nmea_callback的地方是：
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GpsCallbacks sGpsCallbacks = {
sizeof(GpsCallbacks),
location_callback,
status_callback,
sv_status_callback,
nmea_callback,
set_capabilities_callback,
acquire_wakelock_callback,
release_wakelock_callback,
create_thread_callback,
request_utc_time_callback,
};
GpsCallbacks结构体封装了所有需要回调的函数(确切的说是函数指针)，sGpsCallbacks调用关系：
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static jboolean android_location_GpsLocationProvider_init(JNIEnv* env, jobject obj)
{
// this must be set before calling into the HAL library
if (!mCallbacksObj)
mCallbacksObj = env->NewGlobalRef(obj);
// fail if the main interface fails to initialize
if (!sGpsInterface || sGpsInterface->init(&sGpsCallbacks) != 0)
return false;
/* other codes */
return true;
}
而android_location_GpsLocationProvider_init()在GpsLocationProvider类中调用native_init()时被调用：
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static JNINativeMethod sMethods[] = {
/* name, signature, funcPtr */
{"native_init", "()Z", (void*)android_location_GpsLocationProvider_init}
}
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这里，我们找到了和上层的关系，和下层如何打交道呢？
下面需要贴一大段代码：
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/** Represents the standard GPS interface. */
typedef struct {
/** set to sizeof(GpsInterface) */
size_t size;
/**
* Opens the interface and provides the callback routines
* to the implemenation of this interface.
*/
int (*init)( GpsCallbacks* callbacks );
/** Starts navigating. */
int (*start)( void );
/** Stops navigating. */
int (*stop)( void );
/** Closes the interface. */
void (*cleanup)( void );
/** Injects the current time. */
int (*inject_time)(GpsUtcTime time, int64_t timeReference,
int uncertainty);
/** Injects current location from another location provider
* (typically cell ID).
* latitude and longitude are measured in degrees
* expected accuracy is measured in meters
*/
int (*inject_location)(double latitude, double longitude, float accuracy);
/**
* Specifies that the next call to start will not use the
* information defined in the flags. GPS_DELETE_ALL is passed for
* a cold start.
*/
void (*delete_aiding_data)(GpsAidingData flags);
/**
* min_interval represents the time between fixes in milliseconds.
* preferred_accuracy represents the requested fix accuracy in meters.
* preferred_time represents the requested time to first fix in milliseconds.
*/
int (*set_position_mode)(GpsPositionMode mode, GpsPositionRecurrence recurrence,
uint32_t min_interval, uint32_t preferred_accuracy, uint32_t preferred_time);
/** Get a pointer to extension information. */
const void* (*get_extension)(const char* name);
} GpsInterface;
GpsInterface结构体封装了GPS实现的标准接口——接口，注意！接口不就时用来连接两端的吗？一端是com_android_server_location_GpsLocationProvider.cpp文件里的实现，那另一端就是。。。都探到这个地步了，另一端应该是串口方式直接和GPS芯片打交道的Linux驱动了吧？
确是，但是还需要一个媒介：
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struct gps_device_t {
struct hw_device_t common;
/**
* Set the provided lights to the provided values.
*
* Returns: 0 on succes, error code on failure.
*/
const GpsInterface* (*get_gps_interface)(struct gps_device_t* dev);
};
然后，
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static void android_location_GpsLocationProvider_class_init_native(JNIEnv* env, jclass clazz) {
int err;
hw_module_t* module;
/* other codes..*/
err = hw_get_module(GPS_HARDWARE_MODULE_ID, (hw_module_t const**)&module);
if (err == 0) {
hw_device_t* device;
err = module->methods->open(module, GPS_HARDWARE_MODULE_ID, &device);
if (err == 0) {
gps_device_t* gps_device = (gps_device_t *)device;
sGpsInterface = gps_device->get_gps_interface(gps_device);
}
}
/* other codes..*/
}
static JNINativeMethod sMethods[] = {
/* name, signature, funcPtr */
{"class_init_native", "()V", (void *)android_location_GpsLocationProvider_class_init_native},
}
GpsLocationProvider.java通过class_init_native的调用实现对C/C++文件中android_location_GpsLocationProvider_class_init_native的调用；
com_android_server_location_GpsLocationProvider.cpp通过gps_device_t获取操作GPS芯片的接口。How？？？？
重点来了：GPS_HARDWARE_MODULE_ID
对，就是GPS_HARDWARE_MODULE_ID！
往下看：
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hardware\qcom\gps\loc_api\libloc_api\gps.c
struct hw_module_t HAL_MODULE_INFO_SYM = {
.tag = HARDWARE_MODULE_TAG,
.version_major = 1,
.version_minor = 0,
.id = GPS_HARDWARE_MODULE_ID,
.name = "loc_api GPS Module",
.author = "Qualcomm USA, Inc.",
.methods = &gps_module_methods,
};
有木有？GPS_HARDWARE_MODULE_ID！
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hardware\qcom\gps\loc_api\libloc_api\gps.c
extern const GpsInterface* gps_get_hardware_interface();
const GpsInterface* gps__get_gps_interface(struct gps_device_t* dev)
{
return gps_get_hardware_interface();
}
static int open_gps(const struct hw_module_t* module, char const* name,
struct hw_device_t** device)
{
struct gps_device_t *dev = malloc(sizeof(struct gps_device_t));
memset(dev, 0, sizeof(*dev));
dev->common.tag = HARDWARE_DEVICE_TAG;
dev->common.version = 0;
dev->common.module = (struct hw_module_t*)module;
dev->get_gps_interface = gps__get_gps_interface;
*device = (struct hw_device_t*)dev;
return 0;
}
static struct hw_module_methods_t gps_module_methods = {
.open = open_gps
};
流程很清楚了：
gps_get_hardware_interface()函数在驱动程序中实现
——在gps__get_gps_interface()中被调用
——在open_gps()被调用
——在gps_module_methods中例化
——HAL_MODULE_INFO_SYM
const GpsInterface* gps_get_hardware_interface()函数在其他C文件实现，该C文件是和Linux驱动打交道的应用程序。基本功能：
1、open处理器CPU和GPS芯片连接的串口；
2、read串口NEMA数据，并解析；
3、根据上层传进来的回调函数，打包数据，调用相应Callback，进而发送到Android应用层。
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static const GpsInterface mGpsInterface = {
.size =sizeof(GpsInterface),
.init = gps_init,
|--1、接收从上层传下来的GpsCallbacks变量，用它初始化GpsState->callbacks成员
|--2、GpsState结构体的其他成员初始化
|--3、GpsState->init状态设置为：STATE_INIT
|--4、最重要：启动GPS线程，进行数据的读取、处理：
state->thread = state->callbacks.create_thread_cb("gps", gps_state_thread, state);
--gps_create_thread create_thread_cb;
--typedef pthread_t (* gps_create_thread)(const char* name, void (*start)(void *), void* arg);
.start = gps_start,
--设置GPS的状态为开始：GPS_STATUS_SESSION_BEGIN
.stop = gps_stop,
--设置GPS的状态为结束：GPS_STATUS_SESSION_END
.cleanup = gps_cleanup,
--退出需要进行的一些清理工作，如GpsState->init = STATE_QUIT，GpsCallbacks指针归null，信号量回收
.inject_time = gps_inject_time,
--可为空函数
.inject_location = gps_inject_location,
--可为空函数
.delete_aiding_data = gps_delete_aiding_data,
--可为空函数
.set_position_mode = gps_set_position_mode,
--设置GPS工作模式：单GPS、单BD、GPS/BD双系统
.get_extension = gps_get_extension,
--定位之外的扩展功能实现
};
state->thread = state->callbacks.create_thread_cb("gps", gps_state_thread, state);
--static void gps_state_thread(void* arg)：
1、state通过arg参数传入函数
2、创建了Time和Nmea数据处理两个线程
state->nmea_thread = state->callbacks.create_thread_cb("nmea_thread", gps_nmea_thread, state);
--static void gps_nmea_thread(void* arg)
--gps_opentty(state);
nmea_reader_init(reader);
--nmea_reader_parse(NmeaReader* r) {
if (gps_state->callbacks.nmea_cb) {
struct timeval tv;
unsigned long long mytimems;
gettimeofday(&tv,NULL);
mytimems = tv.tv_sec * 1000 + tv.tv_usec / 1000;
gps_state->callbacks.nmea_cb(mytimems, r->in, r->pos);
D("reader_parse. %.*s ", r->pos, r->in );
}
}
我们是从APP层NMEA信息输出自定向下分析的，APP层信息输出的最终起始是：gps_state->callbacks.nmea_cb(mytimems, r->in, r->pos);
到这里还有个问题：GPS芯片和CPU连接，使用的是哪个串口？这个串口号怎么确定的呢？
打算贴个完整HAL层的实例，考虑到代码很多，下篇在说吧。。。
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