Android系统Gps分析（一）

1 GPS架构

2 GPS分析

2.1 头文件

头文件定义在：hardware/libhardware/include/hardware/gps.h，定义了GPS底层相关的结构体和接口

• GpsLocation

GPS位置信息结构体，包含经纬度，高度，速度，方位角等。

/** Flags to indicate which values are valid in a GpsLocation. */typedef uint16_t GpsLocationFlags;// IMPORTANT: Note that the following values must match// constants in GpsLocationProvider.java./** GpsLocation has valid latitude and longitude. */#define GPS_LOCATION_HAS_LAT_LONG   0x0001/** GpsLocation has valid altitude. */#define GPS_LOCATION_HAS_ALTITUDE   0x0002/** GpsLocation has valid speed. */#define GPS_LOCATION_HAS_SPEED      0x0004/** GpsLocation has valid bearing. */#define GPS_LOCATION_HAS_BEARING    0x0008/** GpsLocation has valid accuracy. */#define GPS_LOCATION_HAS_ACCURACY   0x0010/** Represents a location. */typedef struct {    /** set to sizeof(GpsLocation) */    size_t          size;    /** Contains GpsLocationFlags bits. */    uint16_t        flags;    /** Represents latitude in degrees. */    double          latitude;    /** Represents longitude in degrees. */    double          longitude;    /** Represents altitude in meters above the WGS 84 reference     * ellipsoid. */    double          altitude;    /** Represents speed in meters per second. */    float           speed;    /** Represents heading in degrees. */    float           bearing;    /** Represents expected accuracy in meters. */    float           accuracy;    /** Timestamp for the location fix. */    GpsUtcTime      timestamp;} GpsLocation;

• GpsStatus

GPS状态包括5种状态，分别为未知，正在定位，停止定位，启动未定义，未启动。

/** GPS status event values. */typedef uint16_t GpsStatusValue;// IMPORTANT: Note that the following values must match// constants in GpsLocationProvider.java./** GPS status unknown. */#define GPS_STATUS_NONE             0/** GPS has begun navigating. */#define GPS_STATUS_SESSION_BEGIN    1/** GPS has stopped navigating. */#define GPS_STATUS_SESSION_END      2/** GPS has powered on but is not navigating. */#define GPS_STATUS_ENGINE_ON        3/** GPS is powered off. */AgpsCallbacksAgpsInterface#define GPS_STATUS_ENGINE_OFF       4/** Represents the status. */typedef struct {    /** set to sizeof(GpsStatus) */    size_t          size;    GpsStatusValue status;} GpsStatus;

• GpsSvInfo

GPS卫星信息，包含卫星编号，信号强度，卫星仰望角，方位角等。

/** Represents SV information. */typedef struct {    /** set to sizeof(GpsSvInfo) */    size_t          size;    /** Pseudo-random number for the SV. */    int     prn;    /** Signal to noise ratio. */    float   snr;    /** Elevation of SV in degrees. */    float   elevation;    /** Azimuth of SV in degrees. */    float   azimuth;} GpsSvInfo;

• GpsSvStatus

GPS卫星状态，包含可见卫星数和信息，星历时间，年历时间等。

/** Represents SV status. */typedef struct {    /** set to sizeof(GpsSvStatus) */    size_t          size;    /** Number of SVs currently visible. */    int         num_svs;    /** Contains an array of SV information. */    GpsSvInfo   sv_list[GPS_MAX_SVS];    /** Represents a bit mask indicating which SVs     * have ephemeris data.     */    uint32_t    ephemeris_mask;    /** Represents a bit mask indicating which SVs     * have almanac data.     */    uint32_t    almanac_mask;    /**     * Represents a bit mask indicating which SVs     * were used for computing the most recent position fix.     */    uint32_t    used_in_fix_mask;} GpsSvStatus;

• GpsCallbacks

回调函数定义

/** Callback with location information. 向上层传递GPS位置信息 *  Can only be called from a thread created by create_thread_cb. */typedef void (* gps_location_callback)(GpsLocation* location);/** Callback with status information. 向上层传递GPS状态信息 *  Can only be called from a thread created by create_thread_cb. */typedef void (* gps_status_callback)(GpsStatus* status);/** Callback with SV status information. 向上层传递GPS卫星信息 *  Can only be called from a thread created by create_thread_cb. */typedef void (* gps_sv_status_callback)(GpsSvStatus* sv_info);/** Callback for reporting NMEA sentences. 向上层传递MEMA数据 *  Can only be called from a thread created by create_thread_cb. */typedef void (* gps_nmea_callback)(GpsUtcTime timestamp, const char* nmea, int length);/** Callback to inform framework of the GPS engine's capabilities.告知GPS模块可以实现的功能 *  Capability parameter is a bit field of GPS_CAPABILITY_* flags. */typedef void (* gps_set_capabilities)(uint32_t capabilities);/** Callback utility for acquiring the GPS wakelock.上锁，防止处理GPS事件时中止。 *  This can be used to prevent the CPU from suspending while handling GPS events. */typedef void (* gps_acquire_wakelock)();/** Callback utility for releasing the GPS wakelock. */释放锁typedef void (* gps_release_wakelock)();/** Callback for creating a thread that can call into the Java framework code.等待上层请求 *  This must be used to create any threads that report events up to the framework. */typedef pthread_t (* gps_create_thread)(const char* name, void (*start)(void *), void* arg);/** GPS callback structure. */typedef struct {    /** set to sizeof(GpsCallbacks) */    size_t      size;    gps_location_callback location_cb;    gps_status_callback status_cb;    gps_sv_status_callback sv_status_cb;    gps_nmea_callback nmea_cb;    gps_set_capabilities set_capabilities_cb;    gps_acquire_wakelock acquire_wakelock_cb;    gps_release_wakelock release_wakelock_cb;    gps_create_thread create_thread_cb;} GpsCallbacks;

• GpsInterface

GPS接口是最重要的结构体，上层是通过此接口与硬件适配层交互的。

/** 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. 启动定位*/    int   (*start)( void );    /** Stops navigating. 取消定位*/    int   (*stop)( void );    /** Closes the interface. 关闭GPS接口*/    void  (*cleanup)( void );    /** Injects the current time.填入时间 */    int   (*inject_time)(GpsUtcTime time, int64_t timeReference,                         int uncertainty);    /** Injects current location from another location provider填入位置     *  (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删除全部或部分辅助数据，在性能测试时使用     * information defined in the flags. GPS_DELETE_ALL is passed for     * a cold start.     */    void  (*delete_aiding_data)(GpsAidingData flags);    /**设置定位模式和GPS工作模式等     * 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. 自定义的接口*/    const void* (*get_extension)(const char* name);} GpsInterface;

• gps_device_t

GPS设备结构体，继承自hw_device_tcommon，硬件适配接口，向上层提供了重要的get_gps_interface接口。

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);};

2.2硬件适配层

GPS硬件适配层的源码位于：hardware/qcom/gps目录下。

我们看gps/loc_api/llibloc_api/gps.c，首先定义了gps设备模块实例：

const 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,};

这里的methods指向gps.c文件中的gps_module_methods

static struct hw_module_methods_t gps_module_methods = {    .open = open_gps};

gps_module_methods定义了设备的open函数为open_gps，我们看open_gps函数：

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;}

此处可以看作是GPS设备的初始化函数，在使用设备前必须执行此函数。函数里面指定了hw_device_t的module成员，以及gps_device_t的get_gps_interface成员。上层可通过gps_device_t的get_gps_interface调用gps__get_gps_interface函数。gps__get_gps_interface的定义如下：

const GpsInterface* gps__get_gps_interface(struct gps_device_t* dev){    return get_gps_interface();}

用代码跟踪可看到，此函数返回了gps/loc_eng.cpp文件的sLocEngInterface变量，sLocEngInterface定义如下：

// Defines the GpsInterface in gps.hstatic const GpsInterface sLocEngInterface ={    sizeof(GpsInterface),    loc_eng_init,    loc_eng_start,    loc_eng_stop,    loc_eng_cleanup,    loc_eng_inject_time,    loc_eng_inject_location,    loc_eng_delete_aiding_data,    loc_eng_set_position_mode,    loc_eng_get_extension,};

sLocEngInterface指定了GpsInterface结构体的各个回调函数，如启动定位/取消定位等，这些回调函数的实现均在loc_eng.cpp中实现。

2.2 JNI适配层

GPSJNI适配层的源码位于：frameworks/base/services/jni/com_android_server_location_GpsLocationProvider.cpp

首先看注册JNI方法的函数定义：

int register_android_server_location_GpsLocationProvider(JNIEnv* env){    return jniRegisterNativeMethods(env, "com/android/server/location/GpsLocationProvider", sMethods, NELEM(sMethods));}

此函数被同目录下onload.cpp文件调用，调用地方在：

extern "C" jint JNI_OnLoad(JavaVM* vm, void* reserved){    JNIEnv* env = NULL;    jint result = -1;    if (vm->GetEnv((void**) &env, JNI_VERSION_1_4) != JNI_OK) {        LOGE("GetEnv failed!");        return result;    }    LOG_ASSERT(env, "Could not retrieve the env!");    //...省略其他注册代码    register_android_server_location_GpsLocationProvider(env);    return JNI_VERSION_1_4;}

从这里可以看到，JNI初始化的时候，即会进行JNI方法的注册，从而使上层应用能通过JNI调用c/c++本地方法。

回到register_android_server_location_GpsLocationProvider函数，变量sMethods定义如下：

static JNINativeMethod sMethods[] = {     /* name, signature, funcPtr */    {"class_init_native", "()V", (void *)android_location_GpsLocationProvider_class_init_native},    {"native_is_supported", "()Z", (void*)android_location_GpsLocationProvider_is_supported},    {"native_init", "()Z", (void*)android_location_GpsLocationProvider_init},    {"native_cleanup", "()V", (void*)android_location_GpsLocationProvider_cleanup},    {"native_set_position_mode", "(IIIII)Z", (void*)android_location_GpsLocationProvider_set_position_mode},    {"native_start", "()Z", (void*)android_location_GpsLocationProvider_start},    {"native_stop", "()Z", (void*)android_location_GpsLocationProvider_stop},    {"native_delete_aiding_data", "(I)V", (void*)android_location_GpsLocationProvider_delete_aiding_data},    {"native_read_sv_status", "([I[F[F[F[I)I", (void*)android_location_GpsLocationProvider_read_sv_status},    {"native_read_nmea", "([BI)I", (void*)android_location_GpsLocationProvider_read_nmea},    {"native_inject_time", "(JJI)V", (void*)android_location_GpsLocationProvider_inject_time},    {"native_inject_location", "(DDF)V", (void*)android_location_GpsLocationProvider_inject_location},    {"native_supports_xtra", "()Z", (void*)android_location_GpsLocationProvider_supports_xtra},    {"native_inject_xtra_data", "([BI)V", (void*)android_location_GpsLocationProvider_inject_xtra_data},    {"native_agps_data_conn_open", "(Ljava/lang/String;)V", (void*)android_location_GpsLocationProvider_agps_data_conn_open},    {"native_agps_data_conn_closed", "()V", (void*)android_location_GpsLocationProvider_agps_data_conn_closed},    {"native_agps_data_conn_failed", "()V", (void*)android_location_GpsLocationProvider_agps_data_conn_failed},    {"native_agps_set_id","(ILjava/lang/String;)V",(void*)android_location_GpsLocationProvider_agps_set_id},    {"native_agps_set_ref_location_cellid","(IIIII)V",(void*)android_location_GpsLocationProvider_agps_set_reference_location_cellid},    {"native_set_agps_server", "(ILjava/lang/String;I)V", (void*)android_location_GpsLocationProvider_set_agps_server},    {"native_send_ni_response", "(II)V", (void*)android_location_GpsLocationProvider_send_ni_response},    {"native_agps_ni_message", "([BI)V", (void *)android_location_GpsLocationProvider_agps_send_ni_message},    {"native_get_internal_state", "()Ljava/lang/String;", (void*)android_location_GpsLocationProvider_get_internal_state},    {"native_update_network_state", "(ZIZLjava/lang/String;)V", (void*)android_location_GpsLocationProvider_update_network_state },};

这里定义了GPS所有向上层提供的JNI本地方法，这些本地方法是如何与硬件适配层交互的呢？我们看其中一个本地方法android_location_GpsLocationProvider_start：

static jboolean android_location_GpsLocationProvider_start(JNIEnv* env, jobject obj){    const GpsInterface* interface = GetGpsInterface(env, obj);    if (interface)        return (interface->start() == 0);    else        return false;}

它调用了GetGpsInterface获得GpsInterface接口，然后直接调用该接口的start回调函数。GetGpsInterface方法定义如下：

static const GpsInterface* GetGpsInterface(JNIEnv* env, jobject obj) {    // this must be set before calling into the HAL library    if (!mCallbacksObj)        mCallbacksObj = env->NewGlobalRef(obj);    if (!sGpsInterface) {        sGpsInterface = get_gps_interface();        if (!sGpsInterface || sGpsInterface->init(&sGpsCallbacks) != 0) {            sGpsInterface = NULL;            return NULL;        }    }    return sGpsInterface;}

这个函数返回了sGpsInterface，而sGpsInterface又是从get_gps_interface()获得的，我们继续查看get_gps_interface()函数的实现：

static const GpsInterface* get_gps_interface() {    int err;    hw_module_t* module;    const GpsInterface* interface = NULL;    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;            interface = gps_device->get_gps_interface(gps_device);        }    }    return interface;}

这里面调用hw_get_module加载硬件适配模块.so文件，接着通过hw_device_t接口调用open()函数，实际执行gps/loc_api/llibloc_api/gps.c定义的open_gps函数，而后调用gps_device_t接口的get_gps_interface函数，此函数也是在gps.c中定义的，最后返回硬件适配层中loc_eng.cpp文件的sLocEngInterface，从而打通了上层到底层的通道。

2.3 Java Framework

GPSFramework源码位于：frameworks/base/location

2.3.1接口和类简介

首先对GPSFramework重要的接口和类作一个简单的介绍

• 接口

GpsStatus.Listener

用于当Gps状态发生变化时接收通知

GpsStatus.NmeaListener

用于接收Gps的NMEA数据

LocationListener

用于接收当位置信息发生变化时，LocationManager发出的通知

• 类

Address

地址信息类

Criteria

用于根据设备情况动态选择provider

Geocoder

用于处理地理编码信息

GpsSatellite

用于获取当前卫星状态

GpsStatus

用于获取当前Gps状态

Location

地理位置信息类

LocationManager

用于获取和操作gps系统服务

LocationProvider

抽象类，用于提供位置提供者（Locationprovider）

2.3.2 使用Gps编程接口

下面，我们用一个代码示例说明如何在应用层写一个简单的gps程序。

• 首先在AndroidManifest.xml中添加位置服务权限:

<uses-permission android:name="android.permission.INTERNET" />   <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />   <uses-permission android:name="android.permission.ACCESS_FIND_LOCATION" />  <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION"></uses-permission>  

• 接着获取位置信息：

         //获取位置服务 LocationManager locationManager = (LocationManager) getSystemService(Context.LOCATION_SERVICE);          Criteria criteria = new Criteria();          // 获得最好的定位效果          criteria.setAccuracy(Criteria.ACCURACY_FINE);  //设置为最大精度        criteria.setAltitudeRequired(false);  //不获取海拔信息        criteria.setBearingRequired(false);  //不获取方位信息        criteria.setCostAllowed(false);  //是否允许付费        criteria.setPowerRequirement(Criteria.POWER_LOW);  // 使用省电模式        // 获得当前的位置提供者          String provider = locationManager.getBestProvider(criteria, true);          // 获得当前的位置          Location location = locationManager.getLastKnownLocation(provider);  Geocoder gc = new Geocoder(this);           List<Address> addresses = null;          try {    //根据经纬度获得地址信息              addresses = gc.getFromLocation(location.getLatitude(), location.getLongitude(), 1);          } catch (IOException e) {              e.printStackTrace();          } if (addresses.size() > 0) {//获取address类的成员信息Sring msg = “”;           msg += "AddressLine：" + addresses.get(0).getAddressLine(0)+ "\n";           msg += "CountryName：" + addresses.get(0).getCountryName()+ "\n";           msg += "Locality：" + addresses.get(0).getLocality() + "\n";           msg += "FeatureName：" + addresses.get(0).getFeatureName();           }   

• 设置侦听，当位置信息发生变化时，自动更新相关信息

       //匿名类，继承自LocationListener接口private final LocationListener locationListener = new LocationListener() {            public void onLocationChanged(Location location) {            updateWithNewLocation(location);//更新位置信息            }            public void onProviderDisabled(String provider){            updateWithNewLocation(null);//更新位置信息            }            public void onProviderEnabled(String provider){ }            public void onStatusChanged(String provider, int status,Bundle extras){ }    };//更新位置信息private void updateWithNewLocation(Location location) {                     if (location != null) {//获取经纬度            double lat = location.getLatitude();            double lng = location.getLongitude();    }//添加侦听 locationManager.requestLocationUpdates(provider, 2000, 10,locationListener);

2.3.3接口和类分析

下面对相关的类或接口进行分析，LocationManager的代码文件位于： frameworks/base/location/java/location/LocationManager.java 

我们看其构造函数：

public LocationManager(ILocationManager service) {        mService = service;}

其中mService为ILocationManager接口类型，构造函数的参数为service，外部调用时传入LocationManagerService实例。LocationManager是android系统的gps位置信息系统服务，在稍后将会对其进行分析。由带参构造函数实例化LocationManager类的方式用得不多，一般用的方式是由getSystemService获得LocationManagerService服务，再强制转换为LocationManager。例如在2.3.2中的代码示例中是这样获取gps服务的：

LocationManager locationManager = (LocationManager) getSystemService(Context.LOCATION_SERVICE);  

这里的Context.LOCATION_SERVICE为”location”，标识gps服务。

LocationManagerService服务是整个GpsFramework的核心，首先看它是如何加载的，代码文件位于：frameworks/base/services/java/com/android/server/systemserver.java

…//省略其他代码LocationManagerService location = null;…//省略其他代码try {                Slog.i(TAG, "Location Manager");                location = new LocationManagerService(context);                ServiceManager.addService(Context.LOCATION_SERVICE, location);            } catch (Throwable e) {                Slog.e(TAG, "Failure starting Location Manager", e);            }

此处向ServiceManger系统服务管理器注册了新的服务，其名称为”location”，类型为LocationManagerService。注册此服务后，Java应用程序可通过ServiceManager获得LocationManagerService的代理接口ILocationManager.Stub，从而调用LocationManagerService提供的接口函数。ILocationManager位于：

frameworks/base/location/java/location/ILocationManager.aidl，其代码如下：

/** * System private API for talking with the location service. * * {@hide} */interface ILocationManager{    List<String> getAllProviders();    List<String> getProviders(in Criteria criteria, boolean enabledOnly);    String getBestProvider(in Criteria criteria, boolean enabledOnly);    boolean providerMeetsCriteria(String provider, in Criteria criteria);    void requestLocationUpdates(String provider, in Criteria criteria, long minTime, float minDistance,        boolean singleShot, in ILocationListener listener);    void requestLocationUpdatesPI(String provider, in Criteria criteria, long minTime, float minDistance,        boolean singleShot, in PendingIntent intent);    void removeUpdates(in ILocationListener listener);    void removeUpdatesPI(in PendingIntent intent);    boolean addGpsStatusListener(IGpsStatusListener listener);    void removeGpsStatusListener(IGpsStatusListener listener);    // for reporting callback completion    void locationCallbackFinished(ILocationListener listener);    boolean sendExtraCommand(String provider, String command, inout Bundle extras);    void addProximityAlert(double latitude, double longitude, float distance,        long expiration, in PendingIntent intent);    void removeProximityAlert(in PendingIntent intent);    Bundle getProviderInfo(String provider);    boolean isProviderEnabled(String provider);    Location getLastKnownLocation(String provider);    // Used by location providers to tell the location manager when it has a new location.    // Passive is true if the location is coming from the passive provider, in which case    // it need not be shared with other providers.    void reportLocation(in Location location, boolean passive);    boolean geocoderIsPresent();    String getFromLocation(double latitude, double longitude, int maxResults,        in GeocoderParams params, out List<Address> addrs);    String getFromLocationName(String locationName,        double lowerLeftLatitude, double lowerLeftLongitude,        double upperRightLatitude, double upperRightLongitude, int maxResults,        in GeocoderParams params, out List<Address> addrs);    void addTestProvider(String name, boolean requiresNetwork, boolean requiresSatellite,        boolean requiresCell, boolean hasMonetaryCost, boolean supportsAltitude,        boolean supportsSpeed, boolean supportsBearing, int powerRequirement, int accuracy);    void removeTestProvider(String provider);    void setTestProviderLocation(String provider, in Location loc);    void clearTestProviderLocation(String provider);    void setTestProviderEnabled(String provider, boolean enabled);    void clearTestProviderEnabled(String provider);    void setTestProviderStatus(String provider, int status, in Bundle extras, long updateTime);    void clearTestProviderStatus(String provider);    // for NI support    boolean sendNiResponse(int notifId, int userResponse);}

android系统通过ILocationManager.aidl文件自动生成IlocationManager.Stub代理接口，在Java客户端获取LocationManagerService的方式如下：

ILocationManager mLocationManager;IBinder b = ServiceManager.getService(Context.LOCATION_SERVICE);mLocationManager = IlocationManager.Stub.asInterface(b);

客户端通过mLocationManager即可操作LocationMangerService继承自ILocationManager.Stub的的公共接口。之前提到了通过getSystemSerivice方式也可以获得LocationManagerService，但getSystemService()返回的是Object，必须转换为其他接口，我们可以看到之前的是强制转换为LocationManager类型，而此处由ServiceManager.getService返回IBinder接口，再通过ILocationManager.Stub转换为ILocationManager类型，是更加规范的做法。

LocationMangerService的代码文件位于：

frameworks/base/services/java/com/android/server/LocationMangerService.java

我们首先看其中的systemReady()函数

void systemReady() {        // we defer starting up the service until the system is ready         Thread thread = new Thread(null, this, "LocationManagerService");        thread.start();    }

此处启动自身服务线程，因LocationMangerService继承自Runnable接口，当启动此线程后，会执行继承自Runnable接口的run()函数，我们看run()函数的定义：

public void run()    {        Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);        Looper.prepare();        mLocationHandler = new LocationWorkerHandler();        initialize();        Looper.loop();    }

此处调用了initialize()进行初始化，initialize()函数定义如下：

private void initialize() {       //...省略其他代码        loadProviders();//...省略其他代码            }

此处调用了loadProviders()函数，loadProviders()函数调用了_loadProvidersLocked()，其代码如下：

private void _loadProvidersLocked() {        // Attempt to load "real" providers first        if (GpsLocationProvider.isSupported()) {            // Create a gps location provider            GpsLocationProvider gpsProvider = new GpsLocationProvider(mContext, this);            mGpsStatusProvider = gpsProvider.getGpsStatusProvider();            mNetInitiatedListener = gpsProvider.getNetInitiatedListener();            addProvider(gpsProvider);            mGpsLocationProvider = gpsProvider;        }        // create a passive location provider, which is always enabled        PassiveProvider passiveProvider = new PassiveProvider(this);        addProvider(passiveProvider);        mEnabledProviders.add(passiveProvider.getName());        // initialize external network location and geocoder services        if (mNetworkLocationProviderPackageName != null) {            mNetworkLocationProvider =                new LocationProviderProxy(mContext, LocationManager.NETWORK_PROVIDER,                        mNetworkLocationProviderPackageName, mLocationHandler);            addProvider(mNetworkLocationProvider);        }        if (mGeocodeProviderPackageName != null) {            mGeocodeProvider = new GeocoderProxy(mContext, mGeocodeProviderPackageName);        }        updateProvidersLocked();    }

在这里对GpsLocationProvider和NetworkLocationProvider类作了初始化，并添加到provider集合中。GpsLocationProvider和NetworkLocationProvider继承自LocationProviderInterface接口，分别代表两种位置提供者(LocationProvider)：

（1）LocationManager.GPS_PROVIDER：GPS模式，精度比较高，但是慢而且消耗电力，而且可能因为天气原因或者障碍物而无法获取卫星信息，另外设备可能没有GPS模块（2）LocationManager.NETWORK_PROVIDER：通过网络获取定位信息，精度低，耗电少，获取信息速度较快，不依赖GPS模块。

Android提供criteria类，可根据当前设备情况动态选择位置提供者。我们在之前2.3.2的代码示例中，有这样一句代码：

// 获得当前的位置提供者  String provider = locationManager.getBestProvider(criteria, true);  

getBestProvider其实是根据Criteria的条件遍历mProviders集合，返回符合条件的provider名称。我们再看GpsLocationProvider的实现，其代码文件位于：

frameworks/base/services/java/com/android/server/location/GpsLocationProvider.java

在GpsLocationProvider的构造函数中：

public GpsLocationProvider(Context context, ILocationManager locationManager) {       //...省略部分代码        IntentFilter intentFilter = new IntentFilter();        intentFilter.addAction(Intents.DATA_SMS_RECEIVED_ACTION);        intentFilter.addDataScheme("sms");        intentFilter.addDataAuthority("localhost","7275");        context.registerReceiver(mBroadcastReciever, intentFilter);        intentFilter = new IntentFilter();        intentFilter.addAction(Intents.WAP_PUSH_RECEIVED_ACTION);        try {            intentFilter.addDataType("application/vnd.omaloc-supl-init");        } catch (IntentFilter.MalformedMimeTypeException e) {            Log.w(TAG, "Malformed SUPL init mime type");        }        context.registerReceiver(mBroadcastReciever, intentFilter);                //...省略部分代码        // wait until we are fully initialized before returning        mThread = new GpsLocationProviderThread();        mThread.start();        while (true) {            try {                mInitializedLatch.await();                break;            } catch (InterruptedException e) {                Thread.currentThread().interrupt();            }        }    }

这里注册了广播接受者mBroadcastReciever，用于接收广播消息，消息过滤在intentFilter中定义。下面看它接收广播消息时的动作：

private final BroadcastReceiver mBroadcastReciever = new BroadcastReceiver() {        @Override public void onReceive(Context context, Intent intent) {            String action = intent.getAction();            if (action.equals(ALARM_WAKEUP)) {                if (DEBUG) Log.d(TAG, "ALARM_WAKEUP");                startNavigating(false);            } else if (action.equals(ALARM_TIMEOUT)) {                if (DEBUG) Log.d(TAG, "ALARM_TIMEOUT");                hibernate();            } else if (action.equals(Intents.DATA_SMS_RECEIVED_ACTION)) {                checkSmsSuplInit(intent);            } else if (action.equals(Intents.WAP_PUSH_RECEIVED_ACTION)) {                checkWapSuplInit(intent);             }        }    };

当接收ALARM_EAKEUP时，执行startNavigating函数，当接收到ALARM_TIMEOUT广播时，执行hibernate函数。这两个函数很关键，下面看他们的实现：

private void startNavigating(boolean singleShot) {        //...省略部分代码                        if (!native_set_position_mode(mPositionMode, GPS_POSITION_RECURRENCE_PERIODIC,                    interval, 0, 0)) {                mStarted = false;                Log.e(TAG, "set_position_mode failed in startNavigating()");                return;            }            if (!native_start()) {                mStarted = false;                Log.e(TAG, "native_start failed in startNavigating()");                return;            }//...省略部分代码    }

看到没有，这里调用了native_set_position_mode和native_start方法，而这些方法正是我们之前在JNI适配层提到的注册的本地方法。同样的，hibernate函数调用了JNI提供的native_stop方法。我们再看GpsLocationProvider的内部私有函数：

可以看到所有这些本地方法，都是在JNI层注册的，GpsLocationProvider类是从JNI层到Framework层的通道。

下面回到LocationManagerService，分析如何获取最新的位置信息(Location)，获取最新的location的函数是getLastKnownLocation，其实现如下：

private Location _getLastKnownLocationLocked(String provider) {        checkPermissionsSafe(provider);        LocationProviderInterface p = mProvidersByName.get(provider);        if (p == null) {            return null;        }        if (!isAllowedBySettingsLocked(provider)) {            return null;        }        return mLastKnownLocation.get(provider);    }

这里mLastKnownLocation类型为HashMap<String,Location>，所以mLastKnownLocation.get(provider)表示通过provider的名称在哈希字典中获取相应的location，那么这些location是什么时候被存入到哈希字典中的呢？

我们回到LocationManagerService的run函数：

public void run()    {        Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);        Looper.prepare();        mLocationHandler = new LocationWorkerHandler();        initialize();        Looper.loop();    }

这里对类型为LocationWorkerHandler的变量进行初始化，LocationWorkerHandler是在LocationManagerService的一个内部类，它继承自Handler类，Handler是Android系统用于应用程序内部通信的组件，内部通信指同个进程的主线程与其他线程间的通信，Handler通过Message或Runnable对象进行通信。我们继续看LocationWorkerHandler的实现：

private class LocationWorkerHandler extends Handler {        @Override        public void handleMessage(Message msg) {            try {                if (msg.what == MESSAGE_LOCATION_CHANGED) {                    // log("LocationWorkerHandler: MESSAGE_LOCATION_CHANGED!");                    synchronized (mLock) {                        Location location = (Location) msg.obj;                        String provider = location.getProvider();                        boolean passive = (msg.arg1 == 1);                        if (!passive) {                            // notify other providers of the new location                            for (int i = mProviders.size() - 1; i >= 0; i--) {                                LocationProviderInterface p = mProviders.get(i);                                if (!provider.equals(p.getName())) {                                    p.updateLocation(location);                                }                            }                        }                        if (isAllowedBySettingsLocked(provider)) {                            handleLocationChangedLocked(location, passive);                        }                    }                } else if (msg.what == MESSAGE_PACKAGE_UPDATED) {                    //...省略部分代码                    }                }            } catch (Exception e) {                // Log, don't crash!                Slog.e(TAG, "Exception in LocationWorkerHandler.handleMessage:", e);            }        }    }

这里重写Handle类的handleMessage方法，处理用Handle接收的Message对象消息。当接受到位置信息变化的消息MESSAGE_LOCATION_CHANGED时，调用p.updateLocationhandleLocationChangedLocked方法，其实现如下：

private void handleLocationChangedLocked(Location location, boolean passive) {       //...省略部分代码        // Update last known location for provider        Location lastLocation = mLastKnownLocation.get(provider);        if (lastLocation == null) {            mLastKnownLocation.put(provider, new Location(location));        } else {            lastLocation.set(location);        }//...省略部分代码}

可以看到是在handleLocationChangedLocked函数中实现对lastknownlocation的更新的，那么在LocationWorkerHandler类中处理的MESSAGE_LOCATION_CHANGED消息是谁发送出来的呢？答案是在LocationManagerService类的reportLocation函数中：

public void reportLocation(Location location, boolean passive) {        if (mContext.checkCallingOrSelfPermission(INSTALL_LOCATION_PROVIDER)                != PackageManager.PERMISSION_GRANTED) {            throw new SecurityException("Requires INSTALL_LOCATION_PROVIDER permission");        }        mLocationHandler.removeMessages(MESSAGE_LOCATION_CHANGED, location);        Message m = Message.obtain(mLocationHandler, MESSAGE_LOCATION_CHANGED, location);        m.arg1 = (passive ? 1 : 0);        mLocationHandler.sendMessageAtFrontOfQueue(m);    }

此处构造了新的Message对象，然后发送到消息队列的首位置。在GpsLocationProvider类的reportLocation函数中，有这样一段代码：

try     {                mLocationManager.reportLocation(mLocation, false);            } catch (RemoteException e) {                Log.e(TAG, "RemoteException calling reportLocation");           }

所以实际是由GpsLocationProvider主动调用LocationManagerService的reportLocation方法，从而更新最新的位置信息。

实际上，GpsLocationoProvider的reportLocation对应了硬件适配层中的GpsCallbacks结构体中的回调函数gps_location_callback

/** Callback with location information. 向上层传递GPS位置信息 *  Can only be called from a thread created by create_thread_cb. */typedef void (* gps_location_callback)(GpsLocation* location);

那么GpsLocationProvider中的reportLocation函数是如何与GpsCallbacks的gps_location_callback挂钩的呢？我们回到JNI适配层的代码文件：

frameworks/base/services/jni/com_android_server_location_GpsLocationProvider.cpp

其中定义的GetGpsInterface函数：

static const GpsInterface* GetGpsInterface(JNIEnv* env, jobject obj) {    // this must be set before calling into the HAL library    if (!mCallbacksObj)        mCallbacksObj = env->NewGlobalRef(obj);    if (!sGpsInterface) {        sGpsInterface = get_gps_interface();        if (!sGpsInterface || sGpsInterface->init(&sGpsCallbacks) != 0) {            sGpsInterface = NULL;            return NULL;        }    }    return sGpsInterface;}

这里面的sGpsInterface->init(&sGpsCallbacks)调用了GpsInterface的init回调函数，即初始化GpsCallbacks结构体变量sGpsCallbacks，sGpsCallbacks定义如下：

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,};

我们再次看GpsCallbacks的定义（其代码文件在硬件适配层的头文件gps.h中）：

typedef struct {    size_t      size;    gps_location_callback location_cb;    gps_status_callback status_cb;    gps_sv_status_callback sv_status_cb;    gps_nmea_callback nmea_cb;    gps_set_capabilities set_capabilities_cb;    gps_acquire_wakelock acquire_wakelock_cb;    gps_release_wakelock release_wakelock_cb;    gps_create_thread create_thread_cb;} GpsCallbacks;

比较sGpsCallbacks与GpsCallbacks，可以看到location_callback与gps_location_callback对应。再看location_callback函数的定义：

static void location_callback(GpsLocation* location){    JNIEnv* env = AndroidRuntime::getJNIEnv();    env->CallVoidMethod(mCallbacksObj, method_reportLocation, location->flags,            (jdouble)location->latitude, (jdouble)location->longitude,            (jdouble)location->altitude,            (jfloat)location->speed, (jfloat)location->bearing,            (jfloat)location->accuracy, (jlong)location->timestamp);    checkAndClearExceptionFromCallback(env, __FUNCTION__);}

这里面利用JNI调用了Java语言的方法method_reportLocation，method_reportLocation是一个jmethodID变量，表示一个由Java语言定义的方法。下面我们看method_reportLocation的赋值代码：

static void android_location_GpsLocationProvider_class_init_native(JNIEnv* env, jclass clazz) {    method_reportLocation = env->GetMethodID(clazz, "reportLocation", "(IDDDFFFJ)V");//...省略部分代码}

这里表示method_reportLocation指向Java类clazz里的方法reportLocation，那么这个Java类clazz是不是表示GpsLocationProvider呢？我们找到注册JNI方法的方法表：

tatic JNINativeMethod sMethods[] = {     /* name, signature, funcPtr */    {"class_init_native", "()V", (void *)android_location_GpsLocationProvider_class_init_native},//...省略部分代码}

这里说明_GpsLocationProvider_class_init_native对应的native方法名称是class_init_native，下面我们只要确定在Java中的某个类A调用了class_init_native方法，即可以说明A类的reportLocation函数是GpsCallbacks的回调函数。

我们回到GpsLocationProvider的代码文件：

frameworks/base/services/java/com/android/server/location/GpsLocationProvider.java

其中有一段代码：

static { class_init_native(); }

说明是在GpsLocationProvider中调用了class_init_native方法，从而说明GpsLocationProvider的reportLocation函数是GpsCallbacks的回调函数，即当Gps设备的位置信息发生变化时，它调用GpsLocationProvider的回调函数reportLocation，继而调用LocationManagerService的reportLocation函数，从而更新应用层的位置信息。

3 参考文章

基于android的GPS移植——主要结构体及接口介绍

androidGPS定位，定位城市称，经纬度