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。
[cpp]
/** 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。
[cpp]
/** 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. */AgpsCallbacks
AgpsInterface
#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。
[cpp]
/** 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。
[cpp]
/** 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
[cpp]
/** 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。
[cpp]
/** 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。
[cpp]
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：
[cpp]
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
[cpp]
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：
[cpp]
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：
[cpp]
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：
[cpp]
// Defines the GpsInterface in gps.h
static 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：
[plain]
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：
[cpp]
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：
[cpp]
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：
[cpp]
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：
[cpp]
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：
[cpp]
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:
[plain]
<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：
[java]
//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
[java]
//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：
[java]
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：
[java]
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
[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：
[java]
/**
* 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：
[java]
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
[java]
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：
[java]
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：
[java]
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：
[java]
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：
[java]
// 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：
[plain]
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：
[java]
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：
[java]
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：
[java]
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：
[java]
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：
[java]
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：
[java]
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：
[java]
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：
[java]
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
[cpp]
/** 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：
[cpp]
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：
[cpp]
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）：
[cpp]
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：
[cpp]
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：
[cpp]
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：
[cpp]
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：
[java]
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。
