Framework基础:传感器服务的通信流程（binder与socket）

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传感器服务，是通过binder进行业务控制，使用socket进行传感器感应数据传输。

客户端是/frameworks/native/libs/gui/SensorManager.cpp
服务端是/frameworks/native/services/sensorservice/SensorService.cpp

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我们走一下使能sensor上电的过程吧！

/frameworks/base/core/java/android/hardware/SystemSensorManager.java
首先进入enableSensor，其中nSensorEventQueue是jni层创建SensorEventQueue的返回值，标识一个queue。

        private int enableSensor(
                Sensor sensor, int rateUs, int maxBatchReportLatencyUs) {
            if (nSensorEventQueue == 0) throw new NullPointerException();
            if (sensor == null) throw new NullPointerException();
            return nativeEnableSensor(nSensorEventQueue, sensor.getHandle(), rateUs,
                    maxBatchReportLatencyUs);
        }

调用到jni接口nativeEnableSensor

static jint nativeEnableSensor(JNIEnv *env, jclass clazz, jlong eventQ, jint handle, jint rate_us,
                               jint maxBatchReportLatency) {
    sp receiver(reinterpret_cast(eventQ));
    return receiver->getSensorEventQueue()->enableSensor(handle, rate_us, maxBatchReportLatency,
                                                         0);
}

进入receiver的getSensorEventQueue()->enableSensor，看看receiver的定义，他是在构建native层的SensorEventQueue是创建的，里面保存着队列SensorEventQueue

class Receiver : public LooperCallback {
    sp mSensorQueue;
    sp mMessageQueue;
    jobject mReceiverWeakGlobal;
    jfloatArray mScratch;
public:
    Receiver(const sp& sensorQueue,
            const sp& messageQueue,
            jobject receiverWeak, jfloatArray scratch) {
        JNIEnv* env = AndroidRuntime::getJNIEnv();
        mSensorQueue = sensorQueue;
        mMessageQueue = messageQueue;
        mReceiverWeakGlobal = env->NewGlobalRef(receiverWeak);
        mScratch = (jfloatArray)env->NewGlobalRef(scratch);
    }
    ~Receiver() {
        JNIEnv* env = AndroidRuntime::getJNIEnv();
        env->DeleteGlobalRef(mReceiverWeakGlobal);
        env->DeleteGlobalRef(mScratch);
    }
    sp getSensorEventQueue() const {
        return mSensorQueue;
    }

/frameworks/native/libs/gui/SensorEventQueue.cpp
进而进入SensorEventQueue的enableSensor方法，他是通过服务端与客户端的连接SensorEventConnection来发送数据给服务端SensorService。

status_t SensorEventQueue::enableSensor(Sensor const* sensor) const {
    return mSensorEventConnection->enableDisable(sensor->getHandle(), true, 0, 0, false);
}

SensorEventConnection也是基于binder架构的，客户端的SensorEventConnection其实是个代理，不是真正的SensorEventConnection，真正的SensorEventConnection在服务器端呢。
/frameworks/native/services/sensorservice/SensorService.cpp

sp SensorService::createSensorEventConnection(const String8& packageName,
        int requestedMode, const String16& opPackageName) {
    uid_t uid = IPCThreadState::self()->getCallingUid();
    sp result(new SensorEventConnection(this, uid, packageName,
            requestedMode == DATA_INJECTION, opPackageName)); 
   //创建一个SensorEventConnection
    return result;
}

//SensorEventConnection的构造函数
SensorService::SensorEventConnection::SensorEventConnection(
        const sp& service, uid_t uid, String8 packageName, bool isDataInjectionMode,
        const String16& opPackageName)
    : mService(service), mUid(uid), mWakeLockRefCount(0), mHasLooperCallbacks(false),
      mDead(false), mDataInjectionMode(isDataInjectionMode), mEventCache(NULL),
      mCacheSize(0), mMaxCacheSize(0), mPackageName(packageName), mOpPackageName(opPackageName) {
    mChannel = new BitTube(mService->mSocketBufferSize);
#if DEBUG_CONNECTIONS
    mEventsReceived = mEventsSentFromCache = mEventsSent = 0;
    mTotalAcksNeeded = mTotalAcksReceived = 0;
#endif
}

接下来会利用这个代理的SensorEventConnection构造一个队列SensorEventQueue，直接传入代理SensorEventConnection来构造队列。

sp SensorManager::createEventQueue(String8 packageName, int mode) {
   .......
    sp queue;
    queue = new SensorEventQueue(connection);
    return queue;
}

真正的传感器测量数据传输不在SensorEventConnection，而是利用一个管道BitTube，他用socket实现，便于大量数据传输，每个SensorEventConnection都配套一个BitTube，BitTube在SensorEventConnection的构造函数中构建了。那这个东西在服务器端构建了，如何传给客户端呢？是在SensorEventQueue的onFirstRef()函数中产生的,利用代理的SensorEventConnection来远程调用getSensorChannel，最后生成本地的管道BitTube。

void SensorEventQueue::onFirstRef()
{
    mSensorChannel = mSensorEventConnection->getSensorChannel();
}

/frameworks/native/libs/gui/ISensorEventConnection.cpp
在客户端调用transact，发送GET_SENSOR_CHANNEL

    //客户端调用transact
    virtual sp getSensorChannel() const
    {
        Parcel data, reply;
        data.writeInterfaceToken(ISensorEventConnection::getInterfaceDescriptor());
        remote()->transact(GET_SENSOR_CHANNEL, data, &reply);
        return new BitTube(reply);
    }

        //服务器端处理消息
        case GET_SENSOR_CHANNEL: {
            CHECK_INTERFACE(ISensorEventConnection, data, reply);
            sp channel(getSensorChannel());
            channel->writeToParcel(reply);
            return NO_ERROR;
        }

最后，服务器和客户端就利用这个管道BitTube进行通信了。

但是，然并卵，使能上电并不需要传输传感器数据,最后会在服务器端SensorService上了下电而已！！

status_t SensorService::enable(const sp& connection,
        int handle, nsecs_t samplingPeriodNs, nsecs_t maxBatchReportLatencyNs, int reservedFlags,
        const String16& opPackageName)
{
    ......
    BatteryService::enableSensor(connection->getUid(), handle);
    return err;
    .....
}

参考:
Service与Android系统设计（6）--- Native Service
Android BitTube进程间数据传递
Android6.0 Sensor架构和问题分析