【Android架构篇】之定位数据如何从GPS芯片到应用层（一）

Android：V4.2.2
Source Insight

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：

$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。

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：

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：

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)：

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：

// 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()：

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：
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：

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：

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：

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：

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：

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：

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！

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定位的Kernel层实现

com_android_server_location_GpsLocationProvider.cpp文件中另外出现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结构体封装了所有需要回调的函数(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，和下层如何打交道呢？
下面需要贴一大段代码：

/** 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：

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：

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
                
以下，我们一高通GPS的驱动作样板分析。。。