Sensor HAL框架分析之三 .

让我们来看看SensorManager的代码

SensorManager框架层代码

@frameworks/base/core/java/android/hardware/SensorManager.java

public SensorManager(Looper mainLooper) {
       mMainLooper = mainLooper;    // 上面说了，这是Activity的Looper

       synchronized(sListeners) {
            if(!sSensorModuleInitialized) {
                sSensorModuleInitialized = true;
                nativeClassInit();        // 好像是调用本地方法初始化
                  sWindowManager = IWindowManager.Stub.asInterface(
                       ServiceManager.getService("window"));  // 获得Windows服务，不管它
                  if (sWindowManager != null) {
                   // if it's null we're running in the system process
                   // which won't get the rotated values
                   try {
                       sRotation = sWindowManager.watchRotation(
                                newIRotationWatcher.Stub() {
                                    public voidonRotationChanged(int rotation) {
                                       SensorManager.this.onRotationChanged(rotation);
                                    }
                                }
                        );
                   } catch (RemoteException e) {
                   }
                }

               // initialize the sensor list
               sensors_module_init();         // 初始化sensor module
               final ArrayList<Sensor> fullList = sFullSensorsList;  // SensorManager维护的Sensor列表
                 int i = 0;
               do {
                   Sensor sensor = new Sensor(); // 创建sensor对象，这个是传递给App的哦
                   //调用module的方法，获得每一个sensor设备
                   i = sensors_module_get_next_sensor(sensor, i);
                if (i>=0) {
                       //Log.d(TAG, "found sensor: " + sensor.getName() +
                       //        ", handle=" +sensor.getHandle());
                       sensor.setLegacyType(getLegacySensorType(sensor.getType()));
                       fullList.add(sensor); // 添加到SM维护的Sensor列表（嘿嘿）
                       sHandleToSensor.append(sensor.getHandle(), sensor);
                   }
                }while (i>0);

                sPool= new SensorEventPool( sFullSensorsList.size()*2 );
               sSensorThread = new SensorThread(); // 哟，创建线程了好像
            }
        }
    }

public SensorManager(Looper mainLooper) {
       mMainLooper = mainLooper;    // 上面说了，这是Activity的Looper

       synchronized(sListeners) {
            if(!sSensorModuleInitialized) {
                sSensorModuleInitialized = true;
                nativeClassInit();        // 好像是调用本地方法初始化
                  sWindowManager = IWindowManager.Stub.asInterface(
                       ServiceManager.getService("window"));  // 获得Windows服务，不管它
                  if (sWindowManager != null) {
                   // if it's null we're running in the system process
                   // which won't get the rotated values
                   try {
                       sRotation = sWindowManager.watchRotation(
                                newIRotationWatcher.Stub() {
                                    public voidonRotationChanged(int rotation) {
                                       SensorManager.this.onRotationChanged(rotation);
                                    }
                                }
                        );
                   } catch (RemoteException e) {
                   }
                }

               // initialize the sensor list
               sensors_module_init();         // 初始化sensor module
               final ArrayList<Sensor> fullList = sFullSensorsList;  // SensorManager维护的Sensor列表
                 int i = 0;
               do {
                   Sensor sensor = new Sensor(); // 创建sensor对象，这个是传递给App的哦
                   //调用module的方法，获得每一个sensor设备
                   i = sensors_module_get_next_sensor(sensor, i); 
                if (i>=0) {
                       //Log.d(TAG, "found sensor: " + sensor.getName() +
                       //        ", handle=" +sensor.getHandle());
                       sensor.setLegacyType(getLegacySensorType(sensor.getType()));
                       fullList.add(sensor); // 添加到SM维护的Sensor列表（嘿嘿）
                       sHandleToSensor.append(sensor.getHandle(), sensor);
                   }
                }while (i>0);

                sPool= new SensorEventPool( sFullSensorsList.size()*2 );
               sSensorThread = new SensorThread(); // 哟，创建线程了好像
            }
        }
    }

很明显nativeClassInit()，sensors_module_init()，sensors_module_get_next_sensor()都是本地实现的方法。

private static native void nativeClassInit();
private static native int sensors_module_init();
private static native intsensors_module_get_next_sensor(Sensor sensor, int next);

private static native void nativeClassInit();
private static native int sensors_module_init();
private static native intsensors_module_get_next_sensor(Sensor sensor, int next);

根据之前看代码的经验可知，很可能在frameworks/base/core/对应一个jni目录下的存在其对应的本地代码：

frameworks/base/core/java/android/hardware/SensorManager.java
frameworks/base/core/jni/android_hardware_SensorManager.cpp

frameworks/base/core/java/android/hardware/SensorManager.java
frameworks/base/core/jni/android_hardware_SensorManager.cpp

果不其然，在jni存在其本地代码，让我们来看下nativeClassInit函数：

@frameworks/base/core/jni/android_hardware_SensorManager.cpp

static void
nativeClassInit (JNIEnv *_env, jclass _this)
{
   jclasssensorClass = _env->FindClass("android/hardware/Sensor");
   SensorOffsets& sensorOffsets = gSensorOffsets;
   sensorOffsets.name        =_env->GetFieldID(sensorClass, "mName",      "Ljava/lang/String;");
   sensorOffsets.vendor      =_env->GetFieldID(sensorClass, "mVendor",    "Ljava/lang/String;");
   sensorOffsets.version     =_env->GetFieldID(sensorClass, "mVersion",   "I");
   sensorOffsets.handle      =_env->GetFieldID(sensorClass, "mHandle",    "I");
   sensorOffsets.type        = _env->GetFieldID(sensorClass,"mType",      "I");
   sensorOffsets.range       =_env->GetFieldID(sensorClass, "mMaxRange",  "F");
   sensorOffsets.resolution  =_env->GetFieldID(sensorClass, "mResolution","F");
   sensorOffsets.power       =_env->GetFieldID(sensorClass, "mPower",     "F");
   sensorOffsets.minDelay    =_env->GetFieldID(sensorClass, "mMinDelay",  "I");
}

static void
nativeClassInit (JNIEnv *_env, jclass _this)
{
   jclasssensorClass = _env->FindClass("android/hardware/Sensor");
   SensorOffsets& sensorOffsets = gSensorOffsets;
   sensorOffsets.name        =_env->GetFieldID(sensorClass, "mName",      "Ljava/lang/String;");
   sensorOffsets.vendor      =_env->GetFieldID(sensorClass, "mVendor",    "Ljava/lang/String;");
   sensorOffsets.version     =_env->GetFieldID(sensorClass, "mVersion",   "I");
   sensorOffsets.handle      =_env->GetFieldID(sensorClass, "mHandle",    "I");
   sensorOffsets.type        = _env->GetFieldID(sensorClass,"mType",      "I");
   sensorOffsets.range       =_env->GetFieldID(sensorClass, "mMaxRange",  "F");
   sensorOffsets.resolution  =_env->GetFieldID(sensorClass, "mResolution","F");
   sensorOffsets.power       =_env->GetFieldID(sensorClass, "mPower",     "F");
   sensorOffsets.minDelay    =_env->GetFieldID(sensorClass, "mMinDelay",  "I");
}

其代码比较简单，将Java框架层的Sensor类中的成员保存在本地代码中的gSensorOffsets 结构体中将来使用。

         sensors_module_init()本地方法的实现：

static jint
sensors_module_ini(JNIEnv *env, jclass clazz)
{
   SensorManager::getInstance();
    return 0;
}

static jint
sensors_module_ini(JNIEnv *env, jclass clazz)
{
   SensorManager::getInstance();
    return 0;
}

         在本地代码中调用了SensorManager的getInstance方法，这又是一个典型的单例模式获得类的对象，注意这儿的SensorManager是本地的类，而不是Java层的SensorManager类。

本地SensorManager的定义

@frameworks/base/include/gui/SensorManager.h

class SensorManager :
    publicASensorManager,
    publicSingleton<SensorManager>
{
public:
   SensorManager();
   ~SensorManager();

    ssize_tgetSensorList(Sensor const* const** list) const;

    Sensor const*getDefaultSensor(int type);
   sp<SensorEventQueue> createEventQueue();
private:
    //DeathRecipient interface
    voidsensorManagerDied();
    status_tassertStateLocked() const;
private:
    mutable MutexmLock;
    mutablesp<ISensorServer> mSensorServer;
    mutableSensor const** mSensorList;
    mutableVector<Sensor> mSensors;
    mutablesp<IBinder::DeathRecipient> mDeathObserver;
};

class SensorManager :
    publicASensorManager,
    publicSingleton<SensorManager>
{
public:
   SensorManager();
   ~SensorManager();

    ssize_tgetSensorList(Sensor const* const** list) const;

    Sensor const*getDefaultSensor(int type);
   sp<SensorEventQueue> createEventQueue();
private:
    //DeathRecipient interface
    voidsensorManagerDied();
    status_tassertStateLocked() const;
private:
    mutable MutexmLock;
    mutablesp<ISensorServer> mSensorServer;
    mutableSensor const** mSensorList;
    mutableVector<Sensor> mSensors;
    mutablesp<IBinder::DeathRecipient> mDeathObserver;
};

注意SensorManager又继承了ASensorManager和泛型类Singleton<SensorManager>，而SensorManager类定义里没有getInstance所以其定义肯定是在ASensorManager或Singleton中。

@frameworks/base/include/utils/Singleton.h

template <typename TYPE>
class ANDROID_API Singleton
{
public:

    staticTYPE& getInstance() {
       Mutex::Autolock _l(sLock);
        TYPE*instance = sInstance;
        if(instance == 0) {
           instance = new TYPE();
           sInstance = instance;
        }

        return*instance;
    }

    static boolhasInstance() {
       Mutex::Autolock _l(sLock);
        returnsInstance != 0;
    }

protected:
    ~Singleton(){ };
    Singleton() {};

private:
   Singleton(const Singleton&);
   Singleton& operator = (const Singleton&);
    static MutexsLock;
    static TYPE*sInstance;
};
//---------------------------------------------------------------------------
}; // namespace android

template <typename TYPE>
class ANDROID_API Singleton
{
public:

    staticTYPE& getInstance() {
       Mutex::Autolock _l(sLock);
        TYPE*instance = sInstance;
        if(instance == 0) {
           instance = new TYPE();
           sInstance = instance;
        }

        return*instance;
    }

    static boolhasInstance() {
       Mutex::Autolock _l(sLock);
        returnsInstance != 0;
    }

protected:
    ~Singleton(){ };
    Singleton() {};

private:
   Singleton(const Singleton&);
   Singleton& operator = (const Singleton&);
    static MutexsLock;
    static TYPE*sInstance;
};
//---------------------------------------------------------------------------
}; // namespace android

第一次调用getInstance方法时，创建泛型对象即：SensorManager，随后再调用该方法时返回第一次创建的泛型对象。

1)    本地SensorManager的创建

本地SensorManager是一个单例模式，其构造方法相对比较简单，它的主要工作交给了assertStateLocked方法：

@frameworks/base/libs/gui/SensorManager.cpp

SensorManager::SensorManager()
    :mSensorList(0)
{
    // okay we'renot locked here, but it's not needed during construction
   assertStateLocked();
}

status_t SensorManager::assertStateLocked() const {
    if(mSensorServer == NULL) {
        // try for one second
        constString16 name("sensorservice");
        for (inti=0 ; i<4 ; i++) {
            status_t err = getService(name,&mSensorServer);
            if(err == NAME_NOT_FOUND) {
               usleep(250000);
               continue;
            }

            if(err != NO_ERROR) {
               return err;
            }
            break;
        }

        classDeathObserver : public IBinder::DeathRecipient {
           SensorManager& mSensorManger;
           virtual void binderDied(const wp<IBinder>& who) {
               LOGW("sensorservice died [%p]", who.unsafe_get());
               mSensorManger.sensorManagerDied();
            }
        public:
           DeathObserver(SensorManager& mgr) : mSensorManger(mgr) { }
        };

       mDeathObserver = new DeathObserver(*const_cast<SensorManager*>(this));
        mSensorServer->asBinder()->linkToDeath(mDeathObserver);

        mSensors= mSensorServer->getSensorList();
       size_tcount = mSensors.size();
       mSensorList = (Sensor const**)malloc(count * sizeof(Sensor*));
        for(size_t i=0 ; i<count ; i++) {
           mSensorList[i] = mSensors.array() + i;
        }
    }

    returnNO_ERROR;
}

SensorManager::SensorManager()      
    :mSensorList(0)
{
    // okay we'renot locked here, but it's not needed during construction
   assertStateLocked();
}

status_t SensorManager::assertStateLocked() const {
    if(mSensorServer == NULL) {
        // try for one second
        constString16 name("sensorservice");
        for (inti=0 ; i<4 ; i++) {
            status_t err = getService(name,&mSensorServer);
            if(err == NAME_NOT_FOUND) {
               usleep(250000);
               continue;
            }

            if(err != NO_ERROR) {
               return err;
            }
            break;
        }

        classDeathObserver : public IBinder::DeathRecipient {
           SensorManager& mSensorManger;
           virtual void binderDied(const wp<IBinder>& who) {
               LOGW("sensorservice died [%p]", who.unsafe_get());
               mSensorManger.sensorManagerDied();
            }
        public:
           DeathObserver(SensorManager& mgr) : mSensorManger(mgr) { }
        };

       mDeathObserver = new DeathObserver(*const_cast<SensorManager*>(this));
        mSensorServer->asBinder()->linkToDeath(mDeathObserver);

        mSensors= mSensorServer->getSensorList();
       size_tcount = mSensors.size();
       mSensorList = (Sensor const**)malloc(count * sizeof(Sensor*));
        for(size_t i=0 ; i<count ; i++) {
           mSensorList[i] = mSensors.array() + i;
        }
    }

    returnNO_ERROR;
}

在assertStateLocked方法里，先通过getService获得SensorService对象，然后注册了对SensorService的死亡监听器，SensorManager与SensorService不求同年同月同日，只求同年同月同日死。拜完了兄弟之后，调用getSensorList得到所有传感器的对象，存放到mSensorList中，保存在本地空间里。

2)    本地SensorManager中列表的获取

在上面函数调用中首先调用getService来获得SensorService服务，然后执行mSensorServer->getSensorList来获得服务提供的传感器列表：

Vector<Sensor> SensorService::getSensorList()
{
    return mUserSensorList;
}

Vector<Sensor> SensorService::getSensorList()
{
    return mUserSensorList;
}

大家要注意啊，上面的getSensorList函数只是返回了mUserSensorList，而这个变量是在什么时候初始化的呢？

根据2.1节可知，SensorService在本地被初始化时，构造函数里并没有对mUserSensorList进行初始化，而SensorService里有一个onFirstRef方法，这个方法当SensorService第一次被强引用时被自动调用。那SensorService第一次被强引用是在什么时候呢？

在SensorManager::assertStateLocked方法里调用getService获得SensorService保存到mSensorServer成员变量中。

mSensorServer的定义在frameworks/base/include/gui/SensorManager.h中：

class SensorManager :
    public ASensorManager,
    public Singleton<SensorManager>
{
 mutable sp<ISensorServer>mSensorServer;
 mutable Sensorconst** mSensorList;
 mutable Vector<Sensor> mSensors;
};

class SensorManager :
    public ASensorManager,
    public Singleton<SensorManager>
{
 mutable sp<ISensorServer>mSensorServer;
 mutable Sensorconst** mSensorList;
 mutable Vector<Sensor> mSensors;
};

可以看出mSensroServer为强引用类型。所以在创建本地中的SensorManager类对象时，自动强引用SensorService，自动调用onFirstRef方法：

@frameworks/base/services/sensorservice/SensorService.cpp的onFirstRef简化方法如下：

void SensorService::onFirstRef()
{
   LOGD("nuSensorService starting...");
   SensorDevice& dev(SensorDevice::getInstance());                //创建SensorDevice对象dev


    if(dev.initCheck() == NO_ERROR) {
        sensor_tconst* list;
        ssize_tcount = dev.getSensorList(&list);                          //获得传感器设备列表
        if (count> 0) {
            …
            for(ssize_t i=0 ; i<count ; i++) {
                registerSensor( new HardwareSensor(list[i]) );  // 注册在本地获得的传感器
                           …
                      }
            constSensorFusion& fusion(SensorFusion::getInstance());

            if(hasGyro) {            // 如果有陀螺仪设备，则先注册和陀螺仪有关的虚拟传感器设备
                registerVirtualSensor( newRotationVectorSensor() );     // 虚拟旋转传感器
               registerVirtualSensor( new GravitySensor(list, count) ); // 虚拟重力传感器
               registerVirtualSensor( new LinearAccelerationSensor(list, count) ); // 虚拟加速器

                // these are optional
               registerVirtualSensor( new OrientationSensor() ); // 虚拟方向传感器
               registerVirtualSensor( new CorrectedGyroSensor(list, count) );        // 真正陀螺仪

               // virtual debugging sensors...
                char value[PROPERTY_VALUE_MAX];
               property_get("debug.sensors", value, "0");
               if (atoi(value)) {
                   registerVirtualSensor( new GyroDriftSensor() );      // 虚拟陀螺测漂传感器
                }
            }

            // build the sensor list returned tousers
            mUserSensorList = mSensorList;
            if(hasGyro &&
                   (virtualSensorsNeeds & (1<<SENSOR_TYPE_ROTATION_VECTOR))) {
               // if we have the fancy sensor fusion, and it's not provided by the
               // HAL, use our own (fused) orientation sensor by removing the
               // HAL supplied one form the user list.
               if (orientationIndex >= 0) {
                   mUserSensorList.removeItemsAt(orientationIndex);
                }
            }

            run("SensorService",PRIORITY_URGENT_DISPLAY);
           mInitCheck = NO_ERROR;
        }
    }
}

void SensorService::onFirstRef()
{   
   LOGD("nuSensorService starting...");
   SensorDevice& dev(SensorDevice::getInstance());                //创建SensorDevice对象dev
    

    if(dev.initCheck() == NO_ERROR) {
        sensor_tconst* list;
        ssize_tcount = dev.getSensorList(&list);                          //获得传感器设备列表
        if (count> 0) {
            … 
            for(ssize_t i=0 ; i<count ; i++) {
                registerSensor( new HardwareSensor(list[i]) );  // 注册在本地获得的传感器
                           …
                      }
            constSensorFusion& fusion(SensorFusion::getInstance());
                     
            if(hasGyro) {            // 如果有陀螺仪设备，则先注册和陀螺仪有关的虚拟传感器设备
                registerVirtualSensor( newRotationVectorSensor() );     // 虚拟旋转传感器
               registerVirtualSensor( new GravitySensor(list, count) ); // 虚拟重力传感器
               registerVirtualSensor( new LinearAccelerationSensor(list, count) ); // 虚拟加速器

                // these are optional
               registerVirtualSensor( new OrientationSensor() ); // 虚拟方向传感器
               registerVirtualSensor( new CorrectedGyroSensor(list, count) );        // 真正陀螺仪

               // virtual debugging sensors...
                char value[PROPERTY_VALUE_MAX];
               property_get("debug.sensors", value, "0");
               if (atoi(value)) {
                   registerVirtualSensor( new GyroDriftSensor() );      // 虚拟陀螺测漂传感器
                }
            }
                    
            // build the sensor list returned tousers
            mUserSensorList = mSensorList;
            if(hasGyro &&
                   (virtualSensorsNeeds & (1<<SENSOR_TYPE_ROTATION_VECTOR))) {
               // if we have the fancy sensor fusion, and it's not provided by the
               // HAL, use our own (fused) orientation sensor by removing the
               // HAL supplied one form the user list.
               if (orientationIndex >= 0) {
                   mUserSensorList.removeItemsAt(orientationIndex);
                }
            }

            run("SensorService",PRIORITY_URGENT_DISPLAY);
           mInitCheck = NO_ERROR;
        }
    }
}

上面代码首先通过SensorDevice::getInstance()创建对象dev，调用dev.getSensorList(&list)获得传感器列表，将取出的sensor_t类型list传感器列表，塑造了HardwareSensor对象，传递给了registerSensor方法，通过registerSensor注册传感器，然后通过单例模型创建了SensorFusion对象，创建并注册了一系列的虚拟传感器，疑惑，极大的疑惑，怎么传感器还有虚拟的？？其实你注意看这几个传感器最前面的条件，if(hasGyro)，表示如果存在陀螺仪的话，会创建这些虚拟设备，再看这些虚拟设备：旋转，重力，加速器，方向等，这些设备都对应一个物理硬件：陀螺仪，所以这些逻辑上存在，物理上不存在的设备叫虚拟设备。在初始化了虚拟设备后，将mSensorList传感器列表赋值给mUserSensorList，mSensorList是由registerSensor初始化的，mUserSensorList是要提交给Java框架层的传感器列表，最后通过run方法运行了SensorService线程，我们先来看下registerSensor的代码：

void SensorService::registerSensor(SensorInterface* s)
{
    sensors_event_t event;
    memset(&event,0, sizeof(event));

    const Sensorsensor(s->getSensor());
    // add to thesensor list (returned to clients)
    mSensorList.add(sensor);
    // add to ourhandle->SensorInterface mapping
   mSensorMap.add(sensor.getHandle(), s);
    // create anentry in the mLastEventSeen array
   mLastEventSeen.add(sensor.getHandle(), event);
}

void SensorService::registerSensor(SensorInterface* s)
{
    sensors_event_t event;
    memset(&event,0, sizeof(event));

    const Sensorsensor(s->getSensor());
    // add to thesensor list (returned to clients)
    mSensorList.add(sensor);
    // add to ourhandle->SensorInterface mapping
   mSensorMap.add(sensor.getHandle(), s);
    // create anentry in the mLastEventSeen array
   mLastEventSeen.add(sensor.getHandle(), event);
}

通过分析上面代码可知，将传入的HardwareSensor对象塑造了Sensor，添加到mSensorList向量表里，然后将HardwareSensor对象添加到mSensroMap键值对里，将新建的传感器事件数据封装对象event添加到mLastEventSeen键值对中。

我们通过下面的时序图来看下Sensor列表的获取过程。

1)    SensorService监听线程及传感器事件的捕获

让我们再来看看SensorService线程，还记得前面SensorService的父类中有一个Thread类，当调用run方法时会创建线程并调用threadLoop方法。

bool SensorService::threadLoop()
{
   LOGD("nuSensorService thread starting...");

    const size_tnumEventMax = 16 * (1 + mVirtualSensorList.size());
   sensors_event_t buffer[numEventMax];
   sensors_event_t scratch[numEventMax];
   SensorDevice& device(SensorDevice::getInstance());
    const size_tvcount = mVirtualSensorList.size();

    ssize_tcount;
    do {
             // 调用SensorDevice的poll方法看样子要多路监听了
        count = device.poll(buffer,numEventMax);
        if(count<0) {
           LOGE("sensor poll failed (%s)", strerror(-count));
           break;
        }
              // 记录poll返回的每一个传感器中的最后一个数据信息到mLastEventSeen中
        recordLastValue(buffer, count);

        // handlevirtual sensors 处理虚拟传感器数据
        if (count&& vcount) {
           sensors_event_t const * const event = buffer;
                      // 获得虚拟传感器列表
            constDefaultKeyedVector<int, SensorInterface*> virtualSensors(
                   getActiveVirtualSensors());
            constsize_t activeVirtualSensorCount = virtualSensors.size();  // 虚拟传感器个数
            if(activeVirtualSensorCount) {
               size_t k = 0;
               SensorFusion& fusion(SensorFusion::getInstance());
               if (fusion.isEnabled()) {
                   for (size_t i=0 ; i<size_t(count) ; i++) {
                       fusion.process(event[i]);              //处理虚拟传感器设备事件
                    }
                }
               for (size_t i=0 ; i<size_t(count) ; i++) {
                   for (size_t j=0 ; j<activeVirtualSensorCount ; j++) {
                       sensors_event_t out;
                       if (virtualSensors.valueAt(j)->process(&out, event[i])) {
                            buffer[count + k] =out;
                            k++;
                       }
                   }
                }
               if (k) {
                   // record the last synthesized values
                   recordLastValue(&buffer[count], k);
                   count += k;
                   // sort the buffer by time-stamps
                   sortEventBuffer(buffer, count);
                }
            }
        }

        // sendour events to clients...
             // 获得传感器连接对象列表
        constSortedVector< wp<SensorEventConnection> > activeConnections(
               getActiveConnections());
        size_tnumConnections = activeConnections.size();
        for(size_t i=0 ; i<numConnections ; i++) {
           sp<SensorEventConnection> connection(
                   activeConnections[i].promote());
            if(connection != 0) {
                               // 向指定的传感器连接客户端发送传感器数据信息
                connection->sendEvents(buffer, count, scratch);
            }
        }
    } while (count>= 0 || Thread::exitPending());   // 传感器循环监听线程

   LOGW("Exiting SensorService::threadLoop => aborting...");
    abort();
    return false;
}

bool SensorService::threadLoop()
{
   LOGD("nuSensorService thread starting...");

    const size_tnumEventMax = 16 * (1 + mVirtualSensorList.size());
   sensors_event_t buffer[numEventMax];
   sensors_event_t scratch[numEventMax];
   SensorDevice& device(SensorDevice::getInstance());
    const size_tvcount = mVirtualSensorList.size();

    ssize_tcount;
    do {
             // 调用SensorDevice的poll方法看样子要多路监听了
        count = device.poll(buffer,numEventMax);     
        if(count<0) {
           LOGE("sensor poll failed (%s)", strerror(-count));
           break;
        }
              // 记录poll返回的每一个传感器中的最后一个数据信息到mLastEventSeen中
        recordLastValue(buffer, count);

        // handlevirtual sensors 处理虚拟传感器数据
        if (count&& vcount) {
           sensors_event_t const * const event = buffer;
                      // 获得虚拟传感器列表
            constDefaultKeyedVector<int, SensorInterface*> virtualSensors(
                   getActiveVirtualSensors());
            constsize_t activeVirtualSensorCount = virtualSensors.size();  // 虚拟传感器个数
            if(activeVirtualSensorCount) {
               size_t k = 0;
               SensorFusion& fusion(SensorFusion::getInstance());
               if (fusion.isEnabled()) {
                   for (size_t i=0 ; i<size_t(count) ; i++) {
                       fusion.process(event[i]);              //处理虚拟传感器设备事件
                    }
                }
               for (size_t i=0 ; i<size_t(count) ; i++) {
                   for (size_t j=0 ; j<activeVirtualSensorCount ; j++) {
                       sensors_event_t out;
                       if (virtualSensors.valueAt(j)->process(&out, event[i])) {
                            buffer[count + k] =out;
                            k++;
                       }
                   }
                }
               if (k) {
                   // record the last synthesized values
                   recordLastValue(&buffer[count], k);
                   count += k;
                   // sort the buffer by time-stamps
                   sortEventBuffer(buffer, count);
                }
            }
        }

        // sendour events to clients... 
             // 获得传感器连接对象列表
        constSortedVector< wp<SensorEventConnection> > activeConnections(
               getActiveConnections());
        size_tnumConnections = activeConnections.size();
        for(size_t i=0 ; i<numConnections ; i++) {
           sp<SensorEventConnection> connection(
                   activeConnections[i].promote());
            if(connection != 0) {
                               // 向指定的传感器连接客户端发送传感器数据信息
                connection->sendEvents(buffer, count, scratch);
            }
        }
    } while (count>= 0 || Thread::exitPending());   // 传感器循环监听线程

   LOGW("Exiting SensorService::threadLoop => aborting...");
    abort();
    return false;
}

我们看到device.poll方法，阻塞在了SensorDevice的poll方法上，它肯定就是读取Sensor硬件上的数据了，将传感器数据保存在buff中，然后调用recordLastValue方法，只保存同一类型传感器的最新数据（最后采集的一组数据）到键值对象mLastEventSeen里对应传感器的值域中。如果传感器设备是虚拟设备则调用SensorFusion.Process（）方法对虚拟设备数据进行处理。SensorFusion关联一个SensorDevice，它是虚拟传感器设备的一个加工类，负责虚拟传感器数据的计算、处理、设备激活、设置延迟、获得功耗信息等操作。

让我们来回顾下整个过程吧。

1. SensorManager对象创建并调用assertStateLocked方法

2. 在assertStateLocked方法中调用getService，获得SensorService服务

3. 当SensorService第一次强引用时，自动调用OnFirstRef方法

4．获得SensorDevice单例对象

6. 调用SensorDevice.getSensorList方法sensor_t列表保存在SensorService中

8. 调用registerSensor注册传感器，添加到mSensorList列表中

9. 启动SensorService线程，准备监听所有注册的传感器设备

12. 多路监听注册的传感器设备，当有传感器事件时，返回sensor_event_t封装的事件信息

16. 记录产生传感器事件的设备信息

17. 调用getActiveConnections获得所有的活动的客户端SensorEventConnection类对象

19.向客户端发送传感器事件信息