[置顶] Android Sensor系统剖析(4.0)(下)

Author：Harish_hu@qq.com

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
@Override
public 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.java
static{     ...
        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 void
nativeClassInit(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 jint
sensors_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.cpp
static jint
sensors_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：

staticjint
sensors_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：

staticjint
sensors_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：

staticjboolean
sensors_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.

本文乃原创，转载请注明出处，谢谢。