【Android】GPS启动流程及数据流向分析（基于2.3.5）

GPS启动流程及数据流向分析：

 首先在系统init阶段，会通过ServiceManager addService添加很多的Service，这其中就包含LocationService。
代码在SystemServer.java中:

             try {
                  Slog.i(TAG, "Location Manager");
                  location = new LocationManagerService(context);
                  ServiceManager.addService(Context.LOCATION_SERVICE, location);
             } catch (Throwable e) {
                  reportWtf("starting Location Manager", e);
             }

 随后调用LocationManagerService的systemReady函数开启一个线程。

            final LocationManagerService locationF = location;
            try {
                     if (locationF != null) locationF.systemReady();
                 } catch (Throwable e) {
                     reportWtf("making Location Service ready", e);
            }

--LocationManagerService.java

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

在 Thread的run函数中为接收消息做好了准备，并且调用了一个initialize函数：

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

接着看initialize()：

private void initialize() {
          // Create a wake lock, needs to be done before calling loadProviders() below
          PowerManager powerManager = (PowerManager) mContext.getSystemService(Context.POWER_SERVICE);
          mWakeLock = powerManager.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, WAKELOCK_KEY);

          // Load providers
          loadProviders();

          // Register for Network (Wifi or Mobile) updates
          IntentFilter intentFilter = new IntentFilter();
          intentFilter.addAction(ConnectivityManager.CONNECTIVITY_ACTION);
          // Register for Package Manager updates
          intentFilter.addAction(Intent.ACTION_PACKAGE_REMOVED);
          intentFilter.addAction(Intent.ACTION_PACKAGE_RESTARTED);
          intentFilter.addAction(Intent.ACTION_QUERY_PACKAGE_RESTART);
          mContext.registerReceiver(mBroadcastReceiver, intentFilter);
         IntentFilter sdFilter = new IntentFilter(Intent.ACTION_EXTERNAL_APPLICATIONS_UNAVAILABLE);
          mContext.registerReceiver(mBroadcastReceiver, sdFilter);

          // listen for settings changes
          ContentResolver resolver = mContext.getContentResolver();
          Cursor settingsCursor = resolver.query(Settings.Secure.CONTENT_URI, null,
                  "(" + Settings.System.NAME + "=?)",
                  new String[]{Settings.Secure.LOCATION_PROVIDERS_ALLOWED},
                 null);
          mSettings = new ContentQueryMap(settingsCursor, Settings.System.NAME, true, mLocationHandler);
          SettingsObserver settingsObserver = new SettingsObserver();
          mSettings.addObserver(settingsObserver);
     }

其中有两个重要的地方：
 1） loadProviders（），会new一个GpsLocationProvider，并将本GpsLocationProvider添加一个ArrayList<LocationProviderInterface>的链 表中。
 2） new 一个 SettingsObserver对象，该对象应该是负责在Settings中有数据变化时通知本地程序进行相应处理的功能。其中 SettingsObserver类中实现 了Observer接口，该接口中的update函数应该就是一个回调函数，当Settings中有数据变化时会回调进这个函数：

private final class SettingsObserver implements Observer {
          public void update(Observable o, Object arg) {
              synchronized (mLock) {
                   updateProvidersLocked();
              }
          }
     }

 private void updateProvidersLocked() {
          boolean changesMade = false;
          for (int i = mProviders.size() - 1; i >= 0; i--) {
               LocationProviderInterface p = mProviders.get(i);
               boolean isEnabled = p.isEnabled();
               String name = p.getName();
               boolean shouldBeEnabled = isAllowedBySettingsLocked(name);
               If (isEnabled && !shouldBeEnabled) {
                    updateProviderListenersLocked(name, false);
                    changesMade = true;
               } else if (!isEnabled && shouldBeEnabled) {
                    updateProviderListenersLocked(name, true);
                    changesMade = true;
               }
          }
          if (changesMade) {
               mContext.sendBroadcast(new Intent(LocationManager.PROVIDERS_CHANGED_ACTION));
          }
     }

 private void updateProviderListenersLocked(String provider, boolean enabled) {
        int listeners = 0;

         LocationProviderInterface p = mProvidersByName.get(provider);
         if (p == null) {
              return;
         }

         ArrayList<Receiver> deadReceivers = null;
        
         ArrayList<UpdateRecord> records = mRecordsByProvider.get(provider);
         if (records != null) {
              final int N = records.size();
              for (int i=0; i<N; i++) {
                  UpdateRecord record = records.get(i);
                  // Sends a notification message to the receiver
                  if (!record.mReceiver.callProviderEnabledLocked(provider, enabled)) {
                      if (deadReceivers == null) {
                           deadReceivers = new ArrayList<Receiver>();
                      }
                      deadReceivers.add(record.mReceiver);
                  }
                  listeners++;
             }
         }

         if (deadReceivers != null) {
              for (int i=deadReceivers.size()-1; i>=0; i--) {
                  removeUpdatesLocked(deadReceivers.get(i));
              }
         }
        
         if (enabled) {
              p.enable();
              if (listeners > 0) {
                   p.setMinTime(getMinTimeLocked(provider), mTmpWorkSource);
                   p.enableLocationTracking(true);
              }
         } else {
              p.enableLocationTracking(false);
              p.disable();
         }
     }

可知是在 updateProviderListenersLocked函数中，通过 LocationProviderInterface p调用enable或者disable来开关位置服务。这里会调用到 LocationProviderInterface的子类GpsLocationProvider中的enable或者disable中：
 
 GpsLocationProvider.java

 public void enable() {
          synchronized (mHandler) {
             sendMessage(ENABLE, 1, null);
          }
       }

handlenable就是ENABLE的消息处理函数：

 private void handleEnable() {
          if (DEBUG) Log.d(TAG, "handleEnable");
          if (mEnabled) return;
          mEnabled = native_init();
        
          Intent intent = new Intent(LocationManager.GPS_SETTING_ENABLED_CHANGE_ACTION);
          intent.putExtra(LocationManager.EXTRA_GPS_ENABLED, mEnabled);
          mContext.sendBroadcast(intent);

          if (mEnabled) {
              mSupportsXtra = native_supports_xtra();
              if (mSuplServerHost != null) {
                   native_set_agps_server(AGPS_TYPE_SUPL, mSuplServerHost, mSuplServerPort);
              }
              if (mC2KServerHost != null) {
                   native_set_agps_server(AGPS_TYPE_C2K, mC2KServerHost, mC2KServerPort);
              }
          } else {
              Log.w(TAG, "Failed to enable location provider");
          }
     }

可见这里就开始与native层通信了。
 native_init对应jni中的android_location_GpsLocationProvider_init函数，在该函数中调用了
 sGpsInterface->init(&sGpsCallbacks)，
 而 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,
      request_utc_time_callback,
 };

 这是将jni的一些函数作为参数传递到native c 空间中去，这样在native c中如果有可用数据将通过回调的方式调用到jni中的函数。简单的看一下 location_callback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__);
 }

其中有定义：
 static jmethodID method_reportLocation;并且：
 method_reportLocation = env->GetMethodID(clazz, "reportLocation","(IDDDFFFJ)V"); 
 可见jni中的callback函数其实又回调掉了java空间（Framework）中的  reportLocation函数，这便是整个GPS框架的数据流向结构了。所有的数据都是通过回调的方式通知上层：
 nativeC通过回调通知JNI，JNI通过回调通知Framework。这应该是gps整个框架最重要的部分吧，理解了数据流向，其他的应该都简单了。
 我们的native c中并没有实现 native_set_agps_server的功能，所以使用agps的数据服务器不在上层指定，而是在c文件中直接指定的。 native_set_agps_server只是一个空函数。

 上面介绍了 sGpsInterface->init(&sGpsCallbacks)中sGpsCallbacks的作用，但是init函数调用还未进行剖析：
  sGpsInterface定义如下：
 static const GpsInterface* sGpsInterface = NULL;
 它是在android_location_GpsLocationProvider_class_init_native函数初始化的，该函数在GpsLocationProvider中通过static方式预加载：

 err = hw_get_module(GPS_HARDWARE_MODULE_ID, (hw_module_t const**)&module);
     if (err == 0) {
         hw_device_t* device;
 //通过open调用，在native层分配了一个 gps_device_t的空间，并传递回来。
         err = module->methods->open(module, GPS_HARDWARE_MODULE_ID, &device);
         if (err == 0) {
  //这个强制转换是因为 gps_device_t中第一个item是一个 hw_device_t。
  //并且在native中确实是分配的gps_device_t空间
            gps_device_t* gps_device = (gps_device_t *)device;
            sGpsInterface = gps_device->get_gps_interface(gps_device);
         }
     }

 

可见这个 sGpsInterface是从hwModule中获取的。来看看get_gps_interface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));    
      if (dev == NULL) { 
          LOGE("gps device can not malloc memery!"); 
          return -ENOMEM; 
     } 
    
     memset(dev, 0, sizeof(*dev));    
     dev->common.tag = HARDWARE_DEVICE_TAG;    
     dev->common.version = 0;    
     dev->common.module = (struct hw_module_t*)module;  
     dev->common.close = close_gps; 
 //注意一个有参 一个无参   
     dev->get_gps_interface = gps_get_hardware_interface;    
     *device = (struct hw_device_t *)dev;    
    
     return 0; 
 }

 const GpsInterface *gps_get_hardware_interface() 
 { 
      return &ubloxGpsInterface; 
 }

 /*gps interface struct*/ 
 static const GpsInterface  ubloxGpsInterface = 
 { 
      .size = sizeof(GpsInterface), 
      .init = ublox_gps_init, 
      .start = ublox_gps_start, 
      .stop = ublox_gps_stop, 
      .cleanup = ublox_gps_cleanup, 
      .inject_location = ublox_gps_inject_location, 
      .delete_aiding_data = ublox_gps_delete_aiding_data, 
      .set_position_mode = ublox_gps_set_position_mode, 
      .get_extension = ublox_gps_get_extension, 
 };

 static int ublox_gps_init(GpsCallbacks* callbacks) 
 { 
      UbloxGpsData *gps_data = &ublox_gps_data; 

      //ublox gps support MS-based A-GPS, fix in the gps terminal 
      callbacks->set_capabilities_cb(GPS_CAPABILITY_SCHEDULING | GPS_CAPABILITY_MSB);


      pthread_mutex_init(&gps_data->deferred_action_mutex, NULL); 
      pthread_cond_init(&gps_data->deferred_action_cond, NULL); 
      agps_state->used_proxy = 0; 

      gps_data->gps_callbacks = *callbacks; 
      gps_data->thread_start = 0; 

      LOGD("gps finishes initialization"); 
   
      return 0; 
 }

 所以 sGpsInterface实际上指向了native层的 ubloxGpsInterface。调用其init函数，将JNI空间的回调函数传递到了native空间。
应用层通过调用

 Settings.Secure.setLocationProviderEnabled(
                        resolver,
                        LocationManager.GPS_PROVIDER,
                        desiredState);

来enable或者Disable位置服务。

url:http://greatverve.cnblogs.com/archive/2012/01/31/android-gps.html