Android Sensor系统剖析(4.0)(下)

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    由于现在电脑上只有4.0的代码，考虑到代码差别也不大，所以下部分，就基于4.0来分析。

 3：SensorManager

    上一部分说过，开机后，system server启动时，就会初始化sensor service，也就是说，开机后她一直都在后台运行着，客户端部分，直接connect就行了。至于怎么connect，这一切都被封装到SensorManager里了。

3.1 SensorManager的创建

获取SensorManager的对象实例代码：

mSensorManager =(SensorManager)getSystemService(SENSOR_SERVICE);

调用Activity的成员函数来获取SensorManager实例，我们从Activity派生关系可以追溯到，这个函数的最终在ContextImpl实现：

//ContextImpl.java@Overridepublic ObjectgetSystemService(String name) {ServiceFetcherfetcher = SYSTEM_SERVICE_MAP.get(name);return fetcher ==null ? null : fetcher.getService(this);}

这个函数从SYSTEM_SERVICE_MAP中获取了name对应的特定对象实例，所以从SYSTEM_SERVICE_MAP的初始化，就可以看到SensorManager对象的创建：

//ContextImpl.javastatic{     ...        registerService(SENSOR_SERVICE, newServiceFetcher() {                public ObjectcreateService(ContextImpl ctx) {                    return newSensorManager(ctx.mMainThread.getHandler().getLooper());                }}); ...}

3.2 初始化并连接sensor service

初始化过程肯定是在构造函数中进行，那如何连接sensor service呢？上一部分说过，sensor service是基于c++代码编写的native binder，客户端要与其连接并交互，当然也是使用c++更方便(我只是说比较方便，当然你如果硬要用java与其建立连接并交互数据，也是可以的).

    如果一种做法可以让你更方便，我想大多数人的选择都是一样的，就是使用C++代码访问服务，然后java代码通过jni调用c++代码，这也是android系统的通用做法；接下去，我们看下SensorManager的jni函数映射：

在android jni中c++类文件的命名规则一般都是java类的package路径+类名，还有一点需要注意的是，这里jni映射函数名都是一样的，这只是这个类的设计者这么命名而已，实际上c++类中的对应函数命名是没有限制的，关于jni的详细描述，大家可查看相关资料，这里就不再赘述.

在了解了jni函数映射后，后续在java代码中如果调用了native函数，我们将直接跳转到c++代码.

SensorManager被实例化，地球人都知道构造函数先走，所以接下去看SensorManager构造函数：

 public SensorManager(Looper mainLooper) {        mMainLooper = mainLooper;        synchronized(sListeners) {            if (!sSensorModuleInitialized) {                sSensorModuleInitialized =true;                nativeClassInit();                sWindowManager =IWindowManager.Stub.asInterface(                       ServiceManager.getService("window"));                if (sWindowManager != null) {                    // if it's null we'rerunning in the system process                    // which won't get therotated 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();                final ArrayList<Sensor>fullList = sFullSensorsList;                int i = 0;                do {                    Sensor sensor = newSensor();                    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);                       sHandleToSensor.append(sensor.getHandle(), sensor);                    }                } while (i>0);                 sPool = new SensorEventPool(sFullSensorsList.size()*2 );                sSensorThread = newSensorThread();            }        }    }

先调用nativeClassInit来初始化JNI相关java类信息，对应C++代码： 

static voidnativeClassInit(JNIEnv *_env, jclass _this){    jclass sensorClass =_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");}

从代码上看出，这个函数主要是保存java类Sensor的各个filed的id值，方便后续在c++代码中利用Jni环境向jave层传递数据，这个在后续poll sensor值的时候会用到。接着调用jni函数sensors_module_init，c++代码如下:

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

函数很简单，就调用SensorManager::getInstance实例化SensorManager对象实例。注意这里是jnic++层的实现，SensorManager对象是C++层的对象实例，不要跟上面java层的搞浑了。getInstance，可以明显看出来，这是一个单例对象，继续看c++ SensorManager的构造函数：

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

构造函数就调了assertStateLocked，继续看这个函数：

status_tSensorManager::assertStateLocked() const {    if (mSensorServer == NULL) {        // try for one second        const String16name("sensorservice");        for (int i=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;        }         class DeathObserver : publicIBinder::DeathRecipient {            SensorManager& mSensorManger;            virtual void binderDied(constwp<IBinder>& who) {                LOGW("sensorservice died[%p]", who.unsafe_get());                mSensorManger.sensorManagerDied();            }        public:            DeathObserver(SensorManager&mgr) : mSensorManger(mgr) { }        };         mDeathObserver = newDeathObserver(*const_cast<SensorManager *>(this));        mSensorServer->asBinder()->linkToDeath(mDeathObserver);         mSensors =mSensorServer->getSensorList();        size_t count = mSensors.size();        mSensorList = (Sensorconst**)malloc(count * sizeof(Sensor*));        for (size_t i=0 ; i<count ; i++) {            mSensorList[i] = mSensors.array() +i;        }    }     return NO_ERROR;}

这个函数通过getService拿到sensorservice的proxy binder，这样就建立了与sensorservice的数据连接，然后调用getsensorlist从sensorservice获取sensor list并保存。

ok，到这里，java层的jni函数sensors_module_init()就走完了，我们已经与sensor service建立连接，并已经取得了sensor list，但是这些数据目前是存于c++层的，我们要通过jni将数据拿到java层，所以在java层SensorManager构造函数调用sensors_module_init()后，调用sensors_module_get_next_sensor获取sensor数据并保存。

下面是jni函数sensors_module_get_next_sensor的c++实现：

//android_hardware_SensorManager.cppstatic jintsensors_module_get_next_sensor(JNIEnv*env, jobject clazz, jobject sensor, jint next){    SensorManager&mgr(SensorManager::getInstance());     Sensor const* const* sensorList;    size_t count =mgr.getSensorList(&sensorList);    if (size_t(next) >= count)        return -1;       Sensor const* const list =sensorList[next];    const SensorOffsets&sensorOffsets(gSensorOffsets);    jstring name =env->NewStringUTF(list->getName().string());    jstring vendor =env->NewStringUTF(list->getVendor().string());    env->SetObjectField(sensor,sensorOffsets.name,      name);    env->SetObjectField(sensor,sensorOffsets.vendor,    vendor);    env->SetIntField(sensor,sensorOffsets.version,      1);    env->SetIntField(sensor,sensorOffsets.handle,       list->getHandle());    env->SetIntField(sensor,sensorOffsets.type,        list->getType());    env->SetFloatField(sensor,sensorOffsets.range,     list->getMaxValue());    env->SetFloatField(sensor,sensorOffsets.resolution, list->getResolution());    env->SetFloatField(sensor,sensorOffsets.power,     list->getPowerUsage());    env->SetIntField(sensor,sensorOffsets.minDelay,    list->getMinDelay());       next++;    return size_t(next) < count ? next : 0;}

在这个函数将对应的c++层保存的sensor数据传给jobjectsensor。

java层SensorManager构造函数最后创建SensorEventPool和sSensorThread, 这两个对象干嘛用的？看名字就知道啦，一个是事件池，sensor 事件很频繁，如果对每一个事件都创建一个新对象，开销太大，弄一个事件池肯定是最好的选择；另外一个是sensor 线程，负责读取sensor 数据.

3.3 sensor数据读取

    继续来看下应用层获取sensor数据的代码：

public classSensorActivity extends Activity, implements SensorEventListener {      private final SensorManagermSensorManager;      private final Sensor mAccelerometer;      public SensorActivity() {          //获取对应服务          mSensorManager =(SensorManager)getSystemService(SENSOR_SERVICE);          //获取指定sensor对象          mAccelerometer =mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);      }      protected void onResume() {          super.onResume();          //注册listener用于数据回调          mSensorManager.registerListener(this,mAccelerometer, SensorManager.SENSOR_DELAY_NORMAL);      }      protected void onPause() {          super.onPause();         mSensorManager.unregisterListener(this);      }      public void onAccuracyChanged(Sensorsensor, int accuracy) {      }      public void onSensorChanged(SensorEventevent) {      }  } 

现在看这代码就很清楚了

1：(SensorManager)getSystemService(SENSOR_SERVICE)获取SensorManager对象，做了我们上面所介绍的初始化工作

2：mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER)，获取指定sensor对象，根据初始化获取的Sensor List。

3：mSensorManager.registerListener(this,mAccelerometer, SensorManager.SENSOR_DELAY_NORMAL);注册listener获取sensor数据

还记得上一部分说的sensor client与sensor service建立active connection来传递数据吗？service端创建connection是由client端也就是由应用端发起的; 上面1,2都是初始化工作，那真正发起的代码，肯定就是registerlistener了，下面根据代码详细分析：

    public booleanregisterListener(SensorEventListener listener, Sensor sensor, int rate) {        return registerListener(listener,sensor, rate, null);    }

直接调用重载函数

public booleanregisterListener(SensorEventListener listener, Sensor sensor, int rate,            Handler handler) {        if (listener == null || sensor == null){            return false;        }        boolean result = true;        int delay = -1;        switch (rate) {            case SENSOR_DELAY_FASTEST:                delay = 0;                break;            case SENSOR_DELAY_GAME:                delay = 20000;                break;            case SENSOR_DELAY_UI:                delay = 66667;                break;            case SENSOR_DELAY_NORMAL:                delay = 200000;                break;            default:                delay = rate;                break;        }         synchronized (sListeners) {            // look for this listener in our list            ListenerDelegate l = null;            for (ListenerDelegate i :sListeners) {                if (i.getListener() ==listener) {                    l = i;                    break;                }           }             // if we don't find it, add it tothe list            if (l == null) {                l = newListenerDelegate(listener, sensor, handler);                sListeners.add(l);                // if the list is not empty,start our main thread                if (!sListeners.isEmpty()) {                    if(sSensorThread.startLocked()) {                        if(!enableSensorLocked(sensor, delay)) {                            // oops. there was an error                           sListeners.remove(l);                            result = false;                        }                    } else {                        // there was an error,remove the listener                        sListeners.remove(l);                        result = false;                    }                } else {                    // weird, we couldn't addthe listener                    result = false;                }            } else {                l.addSensor(sensor);                if (!enableSensorLocked(sensor,delay)) {                    // oops. there was an error                    l.removeSensor(sensor);                    result = false;                }            }        }         return result;｝

这个函数使用出现了两个新的变量，分别是sListeners和sSensorThread，对应的类型分别是ListenerDelegate和SensorThread，ListenerDelegate主要是对SensorEventListener做封装，从而使一个listener可以对应多个sensor，SensorThread则负责从sensor service读取sensor数据；该函数先判断lstener对应的ListenerDelegate是否已经创建，如果未创建，新建并将其添加入sListeners，然后查看Sensor Thread是否已经启动，如果没有启动，调用sSensorThread.startLocked()启动线程，接下去调用enableSensorLocked到service端enable对应的sensor.

先来看startlocked：

 boolean startLocked() {            try {                if (mThread == null) {                    mSensorsReady = false;                    SensorThreadRunnablerunnable = new SensorThreadRunnable();                    Thread thread = newThread(runnable, SensorThread.class.getName());                    thread.start();                    synchronized (runnable) {                        while (mSensorsReady ==false) {                            runnable.wait();                        }                    }                    mThread = thread;                }            } catch (InterruptedException e) {            }            return mThread == null ? false :true;        }

如果线程未创建，创建SensorThreadRunnable，然后初始化线程并启动，线程启动后SensorThreadRunnable.run会被执行：

private class SensorThreadRunnable implementsRunnable {            SensorThreadRunnable() {            }             private boolean open() {                // NOTE: this cannotsynchronize on sListeners, since                // it's held in the main threadat least until we                // return from here.                sQueue = sensors_create_queue();                return true;            }             public void run() {                //Log.d(TAG, "enteringmain sensor thread");                final float[] values = newfloat[3];                final int[] status = newint[1];                final long timestamp[] = newlong[1];               Process.setThreadPriority(Process.THREAD_PRIORITY_URGENT_DISPLAY);                 if (!open()) {                    return;                }                 synchronized (this) {                    // we've open the driver,we're ready to open the sensors                    mSensorsReady = true;                    this.notify();                }                 while (true) {                    // wait for an event                    final int sensor =sensors_data_poll(sQueue, values, status, timestamp);                     int accuracy = status[0];                    synchronized (sListeners) {                        if (sensor == -1 ||sListeners.isEmpty()) {                            // we lost theconnection to the event stream. this happens                            // when the lastlistener is removed or if there is an error                            if (sensor == -1&& !sListeners.isEmpty()) {                                // log awarning in case of abnormal termination                                Log.e(TAG,"_sensors_data_poll() failed, we bail out: sensors=" + sensor);                            }                            // we have no morelisteners or polling failed, terminate the thread                           sensors_destroy_queue(sQueue);                            sQueue = 0;                            mThread = null;                            break;                        }                        final SensorsensorObject = sHandleToSensor.get(sensor);                        if (sensorObject !=null) {                            // report thesensor event to all listeners that                            // care about it.                            final int size =sListeners.size();                            for (int i=0 ;i<size ; i++) {                               ListenerDelegate listener = sListeners.get(i);                                if(listener.hasSensor(sensorObject)) {                                    // this isasynchronous (okay to call                                    // withsListeners lock held).                                   listener.onSensorChangedLocked(sensorObject,                                           values, timestamp, accuracy);                                }                            }                        }                    }                }                //Log.d(TAG, "exiting mainsensor thread");            }        }    }

run执行时，先调用open，open函数很简单，就调用sensors_create_queue()来创建数据队列，显然这个队列是用于sensor数据传输的，sensors_create_queue()是jni函数，接下去看其对应c++部分代码：

staticjintsensors_create_queue(JNIEnv*env, jclass clazz){    SensorManager&mgr(SensorManager::getInstance());    sp<SensorEventQueue>queue(mgr.createEventQueue());    queue->incStrong(clazz);    returnreinterpret_cast<int>(queue.get());}

调用SensorManager.createEventQueue来创建队列：

sp<SensorEventQueue>SensorManager::createEventQueue(){    sp<SensorEventQueue> queue;    Mutex::Autolock _l(mLock);    while(assertStateLocked() == NO_ERROR) {        sp<ISensorEventConnection>connection =               mSensorServer->createSensorEventConnection();        if (connection == NULL) {            // SensorService just died.            LOGE("createEventQueue:connection is NULL. SensorService died.");            continue;        }        queue = newSensorEventQueue(connection);        break;    }    return queue;}

调用mSensorServer->createSensorEventConnection()与server端建立连接，接着将获取的connection对象创建SensorEventQueue对象并返回。

sensors_create_queue函数接着调用queue.get()获取队列的指针，并返回给java层

回过头来继续看java调用sensors_create_queue的open函数：

 private boolean open() { // NOTE: this cannot synchronize onsListeners, since // it's held in the main thread at least untilwe // return from here.//将返回的SensorEventQueue指针保存到sQueue里 sQueue = sensors_create_queue(); return true; }

将c++创建的SensorEventQueue对象地址保存到java层的一个变量里，这在android里面是很常见也很好用的方法

在open结束后，SensorThreadRunnable.run接下去调用sensors_data_poll来抓去sensor数据

staticnative int sensors_data_poll(int queue, float[] values, int[] status, long[]timestamp);

这个函数的第一个参数就是之前保存的c++层SensorEventQueue对象指针，看对应c++实现：

staticjintsensors_data_poll(JNIEnv*env, jclass clazz, jint nativeQueue,        jfloatArray values, jintArray status,jlongArray timestamp){   //强制类型转换    sp<SensorEventQueue> queue(reinterpret_cast<SensorEventQueue*>(nativeQueue));    if (queue == 0) return -1;    status_t res;ASensorEventevent;//从队列中读取数据    res = queue->read(&event, 1);    if (res == -EAGAIN) {        res = queue->waitForEvent();        if (res != NO_ERROR)            return -1;        res = queue->read(&event, 1);    }    if (res < 0)        return -1;    jint accuracy = event.vector.status;    env->SetFloatArrayRegion(values, 0, 3,event.vector.v);    env->SetIntArrayRegion(status, 0, 1,&accuracy);    env->SetLongArrayRegion(timestamp, 0, 1,&event.timestamp);    return event.sensor;}

先将java层传过来的对象地址强制类型转换成SensorEventQueue，然后调用 queue->read(&event, 1)读取sensor数据

ssize_tSensorEventQueue::read(ASensorEvent* events, size_t numEvents){    ssize_t size =mSensorChannel->read(events, numEvents*sizeof(events[0]));    LOGE_IF(size<0 && size!=-EAGAIN,            "SensorChannel::read error(%s)", strerror(-size));    if (size >= 0) {        if (size % sizeof(events[0])) {            // partial read!!! should neverhappen.            LOGE("SensorEventQueue partialread (event-size=%u, read=%d)",                    sizeof(events[0]),int(size));            return -EINVAL;        }        // returns number of events read        size /= sizeof(events[0]);    }returnsize;}

关于数据的具体传输，上一部分已经详细介绍，这里就不再描述

Jni部分sensors_data_poll在获取到sensor数据并返回到java层，SensorThreadRunnable.run在得到sensor数据后，通过下面代码将数据通过listener回调

finalint size = sListeners.size();for(int i=0 ; i<size ; i++) {   ListenerDelegate listener = sListeners.get(i);   if (listener.hasSensor(sensorObject)) {   // this is asynchronous (okay to call   // with sListeners lock held).   istener.onSensorChangedLocked(sensorObject,values,timestamp, accuracy);}}

就这样，通过registerlistener注册的listener就可以获取到想要的sensor数据，这样就可以了吗？还不行，上面只是说数据流是这么走的，SensorEventQueue::read现在读不到数据的，因为在sensor service那边，sensor还是inactive的，所以registerListener 在sSensorThread.startLocked()成功后，再调用enableSensorLocked来active指定sensor：

      private booleanenableSensorLocked(Sensor sensor, int delay) {        boolean result = false;        for (ListenerDelegate i : sListeners) {            if (i.hasSensor(sensor)) {                String name = sensor.getName();                int handle =sensor.getHandle();                result = sensors_enable_sensor(sQueue,name, handle, delay);                break;            }        }        return result;    }

sensors_enable_sensor，又一个jni函数，直接看对应c++函数：

staticjbooleansensors_enable_sensor(JNIEnv*env, jclass clazz,        jint nativeQueue, jstring name, jintsensor, jint delay){    sp<SensorEventQueue>queue(reinterpret_cast<SensorEventQueue *>(nativeQueue));    if (queue == 0) return JNI_FALSE;    status_t res;    if(delay >= 0) {        res = queue->enableSensor(sensor,delay);    } else {        res = queue->disableSensor(sensor);    }    return res == NO_ERROR ? true : false;}

继续看queue->enableSensor

  status_tSensorEventQueue::enableSensor(int32_t handle, int32_t us) const {    status_t err =mSensorEventConnection->enableDisable(handle, true);    if (err == NO_ERROR) {       mSensorEventConnection->setEventRate(handle, us2ns(us));    }    return err;}

调用mSensorEventConnection->enableDisable(handle,true)将对应的sensor激活。

 Ok，激活后，SensorThreadRunnable.run中sensors_data_poll就可以拿到数据，并回调给注册的listener.

 

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