写在前面
在漫长的Android源代码编译等待过程中,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:
$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信息的地方開始。
NMEA资料參见:卫星定位数据NMEA介绍
一、GPS定位的应用层实现
Luckily,在应用层我们能够通过onNmeaReceived()方法获取到NMEA信息。例如以下Code Fragment:
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:
***
*/
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:
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 NMEA 0183 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上能够看到原始的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()方法进行注冊(register):
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 HashMapmNmeaListeners =
* new HashMap();
*/
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,先检測定义的NmeaListener有没有被注冊过。若果没有。注冊之。
注冊到哪里去了呢?
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。
仅仅看相关的部分:
// 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 ArrayListmNmeaBuffer;
//G psStatusListenerTransport(GpsStatus.Listener listener){}
GpsStatusListenerTransport(GpsStatus.NmeaListener listener) {
mNmeaListener = listener;
mListener = null;
mNmeaBuffer = new ArrayList();
}
@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,新数据被载入到链表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。终于把传过来的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机制用来完毕在进程之间进行通信(在Android中不同意进程间共享数据),它的具体知识另外Google之。
这里,我们再次见到了onNmeaReceived():
rameworks\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:
onewaykeyword是用来修饰远程调用行为。使用该关键词时。远程调用不是堵塞的,它仅仅是发送事物数据并马上返回。
接口的终于实现是把普通的远程调用依照Binder线程池的调用规则来接收,假设oneway是使用在本地调用上,那么不会有不论什么影响,而且调用依旧是异步的。
以下。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:
frameworks\base\services\java\com\android\server\location\GpsLocationProvider.java
public class GpsLocationProvider implements LocationProviderInterface {
// 此处省略1000+N行
private ArrayListmListeners = new ArrayList();
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定位功能终于须要调用硬件实现,操作硬件就必须通过C/C++完毕。GpsLocationProvider中包括很多native方法,採用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层的运行结果。
源代码编译出错,解决这个问题去。。。
native_read_nmea()在GpsLocationProvider类中定义:
private native int native_read_nmea(byte[] buffer, int bufferSize);
native指明它是本地方法。和它相应的C/C++文件的实现是:
static jint android_location_GpsLocationProvider_read_nmea(JNIEnv* env, jobject obj, jbyteArray nmeaArray, jint buffer_size);
How?Next...
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中採用的Java和C/C++函数的映射方式,并在当中描写叙述了函数的參数和返回值:
typedef struct {
const char* name; // Java文件里的本地方法
const char* signature; // 述了函数的參数和返回值
void* fnPtr; // 指针,指向具体的C/C++函数
} JNINativeMethod;
具体内容这里还是不展开了。
来看android_location_GpsLocationProvider_read_nmea()的实现:
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;
}
尽管不清楚JNI深入含义,但这个函数意思还是挺明显的。我们判断:
第5行:用来动态分配内存。nmea指向获取到的内存区域,同一时候把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()方法时指明了我们须要取的数据长度,所以,假设从串口实际读取的数据长度大于我们须要的,我们对串口数据进行截取:即。仅仅取指定长度的数据;
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。
这里也能够看到,函数调用是通过nmeaArray传递NMEA数据的
以下应该看sNmeaStringLength、sNmeaString的设置过程:
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。
以下的代码片段表明,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。
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。至于串口详细配置,比方寄存器配置、数据收发等芯片级实现,是在在Linux内核里的。
com_android_server_location_GpsLocationProvider.cpp文件里另外出现nmea_callback的地方是:
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结构体封装了全部须要回调的函数(确切的说是函数指针),sGpsCallbacks调用关系:
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:
static JNINativeMethod sMethods[] = {
/* name, signature, funcPtr */
{"native_init", "()Z", (void*)android_location_GpsLocationProvider_init}
}
这里,我们找到了和上层的关系。和下层怎样打交道呢?
以下须要贴一大段代码:
/** 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文件中的实现,那还有一端就是。。。都探到这个地步了。还有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。可是还须要一个媒介:
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,
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++文件里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:
ardware\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。
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
};
流程非常清楚了:
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()函数在其它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、依据上层传进来的回调函数。打包数据,调用对应Callback。进而发送到Android应用层。
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结构体的其它成员初始化
|--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的状态为開始:GPS_STATUS_SESSION_BEGIN
.stop = gps_stop,
--设置GPS的状态为结束:GPS_STATUS_SESSION_END
.cleanup = gps_cleanup,
--退出须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參数传入函数
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层信息输出的终于起始是: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,考虑到代码非常多,下篇在说吧。
。。