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jni层的传感器主要是用于作为客户端来访问native的SensorService，更多的是连接的作用

入口是/frameworks/base/core/java/android/hardware/SystemSensorManager.java

看看构造函数,nativeCreate主要用于创建jni层的SensorManager，nativeClassInit用于获取jni层对java层sensor结构的映射，便于在jni层修改java层的变量，nativeGetSensorAtIndex用于填充java层的sensor结构。

    public SystemSensorManager(Context context, Looper mainLooper) {
        mMainLooper = mainLooper;
        mTargetSdkLevel = context.getApplicationInfo().targetSdkVersion;
        mContext = context;
        mNativeInstance = nativeCreate(context.getOpPackageName());

        synchronized(mLock) {
            if (!sSensorModuleInitialized) {
                sSensorModuleInitialized = true;
                nativeClassInit();
            }
        }

        // initialize the sensor list
        for (int index = 0;;++index) {
            Sensor sensor = new Sensor();
            if (!nativeGetSensorAtIndex(mNativeInstance, sensor, index)) break;
            mFullSensorsList.add(sensor);
            mHandleToSensor.append(sensor.getHandle(), sensor);
        }
    }

jni层的函数位于/frameworks/base/core/jni/android_hardware_SensorManager.cpp
注册了java层与c层的对应关系,主要注册了两个java类SystemSensorManager，BaseEventQueue。最后会在c层调用到java层BaseEventQueue的dispatchSensorEvent完成传感器数据传递。

int register_android_hardware_SensorManager(JNIEnv *env)
{
    RegisterMethodsOrDie(env, "android/hardware/SystemSensorManager",
            gSystemSensorManagerMethods, NELEM(gSystemSensorManagerMethods));

    RegisterMethodsOrDie(env, "android/hardware/SystemSensorManager$BaseEventQueue",
            gBaseEventQueueMethods, NELEM(gBaseEventQueueMethods));

    gBaseEventQueueClassInfo.clazz = FindClassOrDie(env,
            "android/hardware/SystemSensorManager$BaseEventQueue");

    gBaseEventQueueClassInfo.dispatchSensorEvent = GetMethodIDOrDie(env,
            gBaseEventQueueClassInfo.clazz, "dispatchSensorEvent", "(I[FIJ)V");

    gBaseEventQueueClassInfo.dispatchFlushCompleteEvent = GetMethodIDOrDie(env,
            gBaseEventQueueClassInfo.clazz, "dispatchFlushCompleteEvent", "(I)V");

    return 0;
}

使用CallVoidMethod回调java层的dispatchSensorEvent方法
                    if (receiverObj.get()) {
                        env->CallVoidMethod(receiverObj.get(),
                                            gBaseEventQueueClassInfo.dispatchSensorEvent,
                                            buffer[i].sensor,
                                            mScratch,
                                            status,
                                            buffer[i].timestamp);
                    }

使用registerListenerImpl来注册传感器监听,主要是构建一个SensorEventQueue，当sensor事件来到是时候，SensorEventQueue会被通知到。

    protected boolean registerListenerImpl(SensorEventListener listener, Sensor sensor,
        synchronized (mSensorListeners) {
            SensorEventQueue queue = mSensorListeners.get(listener);
            if (queue == null) {
                Looper looper = (handler != null) ? handler.getLooper() : mMainLooper;
                final String fullClassName = listener.getClass().getEnclosingClass() != null ?
                    listener.getClass().getEnclosingClass().getName() :
                    listener.getClass().getName();
                queue = new SensorEventQueue(listener, looper, this, fullClassName);
                if (!queue.addSensor(sensor, delayUs, maxBatchReportLatencyUs)) {
                    queue.dispose();
                    return false;
                }
                mSensorListeners.put(listener, queue);
                return true;
            } else {
                return queue.addSensor(sensor, delayUs, maxBatchReportLatencyUs);
            }
        }
    }

SensorEventQueue继承与BaseEventQueue，初始化是会新建一个binder connection与sensor service通信，nativeInitBaseEventQueue就是用于建立binder通信的。native层对应函数nativeInitSensorEventQueue

        BaseEventQueue(Looper looper, SystemSensorManager manager, int mode, String packageName) {
            if (packageName == null) packageName = "";
            nSensorEventQueue = nativeInitBaseEventQueue(manager.mNativeInstance,
                    new WeakReference<>(this), looper.getQueue(), mScratch,
                    packageName, mode, manager.mContext.getOpPackageName());
            mCloseGuard.open("dispose");
            mManager = manager;
        }

//native层对应函数
static jlong nativeInitSensorEventQueue(JNIEnv *env, jclass clazz, jlong sensorManager,
        jobject eventQWeak, jobject msgQ, jfloatArray scratch, jstring packageName, jint mode) {
    SensorManager* mgr = reinterpret_cast<SensorManager*>(sensorManager);
    ScopedUtfChars packageUtf(env, packageName);
    String8 clientName(packageUtf.c_str());
    sp<SensorEventQueue> queue(mgr->createEventQueue(clientName, mode));//创建binder通信

    sp<MessageQueue> messageQueue = android_os_MessageQueue_getMessageQueue(env, msgQ);
    if (messageQueue == NULL) {
        jniThrowRuntimeException(env, "MessageQueue is not initialized.");
        return 0;
    }

    sp<Receiver> receiver = new Receiver(queue, messageQueue, eventQWeak, scratch);//用于接收sensor事件
    receiver->incStrong((void*)nativeInitSensorEventQueue);
    return jlong(receiver.get());
}

SensorEventQueue被通知后，会执行dispatchSensorEvent来处理传感器事件,最后会进入onSensorChanged进入处理，这就回到了应用中的onSensorChanged处理了。注意的是，一个listener只能对应一个SensorEventQueue。

        protected void dispatchSensorEvent(int handle, float[] values, int inAccuracy,
                long timestamp) {
            final Sensor sensor = mManager.mHandleToSensor.get(handle);
            SensorEvent t = null;
            synchronized (mSensorsEvents) {
                t = mSensorsEvents.get(handle);
            }

            if (t == null) {
                // This may happen if the client has unregistered and there are pending events in
                // the queue waiting to be delivered. Ignore.
                return;
            }
            // Copy from the values array.
            System.arraycopy(values, 0, t.values, 0, t.values.length);
            t.timestamp = timestamp;
            t.accuracy = inAccuracy;
            t.sensor = sensor;

            // call onAccuracyChanged() only if the value changes
            final int accuracy = mSensorAccuracies.get(handle);
            if ((t.accuracy >= 0) && (accuracy != t.accuracy)) {
                mSensorAccuracies.put(handle, t.accuracy);
                mListener.onAccuracyChanged(t.sensor, t.accuracy);
            }
            mListener.onSensorChanged(t);
        }