Android9.0源码学习-Sensor Framework

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在之前的学习中， Android Sensor概述介绍了常用传感器的功能，测量值的含义，测量原理等等。 Android Sensor应用介绍了app如何得到Sensor的数值。那么接下来就应该思考应用层是如何得到硬件测量的数值，传感器的数值又是如何一步一步上传至应用层的。

FileList

Framework(/frameworks/base/core/java/android/hardware/)

• SensorManager.java：SDK接口，封装了Sensor相关的API，提供给APP层使用。
• SystemSensorManager.java：继承于SensorManager，SensorManager的实现类，负责与系统Sensor的联系。
• Sensor.java：代表一个Sensor类
• SensorEvent.java：代表一个SensorEvent类
• SensorEventListener.java：用于接收传感器事件的Interface

JNI(/framework/base/core/jni)

• android_hardware_SensorManager.cpp：负责Java层和Native层的通信

Native Framework

• Client(/framework/native/lib/sensor)
  • SensorManager.cpp：SensorManager的Native层，负责与SensorService的通信
  • SensorEventQueue.cpp：消息队列
  • Sensor.cpp：Sensor的native层
  • ISensorServer.cpp：SensorServer在客户端接口
  • ISensorEventConnection.cpp：SensorEventConnection在客户端接口
  • BitTube.cpp：单向字节管道，提供进程间单向数据通信功能。
• Server(/framework/native/services/sensorservice)
  • SensorDevice.cpp：负责管理和维护系统中所有的Sensor，封装了Sensor的使能，配置，数据读取等功能。
  • SensorInterface.cpp：服务端Sensor接口
  • SensorService.cpp：是整个Android Sensor Framework最核心的模块，它实现了主要的Sensor控制流和数据流，完成Sensor的参数配置，数据分发
  • SensorEventConnection.cpp：Sensor的数据传输通道，当Client开始监听某一个Sensor时，一个对应的SensorEventConnection将会被创建，Server端在接收到Sensor数据后，通过写入到SensorEventConnection传递给Client端。

Questions

在Framework的学习中，我们需要搞清楚以下几个问题？

• APP层的控制指令是怎样一层一层下发到Framework层和HAL层？
• sensor采集到的数据又是怎样一层一层传到APP层？
• Client端和Server端又是怎样交互的，数据是如何通讯的？
• 当系统第一次创建SensorService时，SensorService做了哪些初始化的工作？

SensorService初始化

SystemServer的main函数，会调用startSensorService()，从而创建第一个SensorService实例。当SensorService第一个实例创建时，其onFirstRef()接口会被调用。

onFirstRef()做了这么以下几个工作：

• 创建SensorDevice的实例（SensorDevice& dev(SensorDevice::getInstance());），并且是单例类
  • connectHidlService()
    • mSensors = ISensors::getService()
      • Sensors::Sensors()
        • hw_get_module()加载Sensor HAL的动态库
        • sensors_open_1()执行Sensor HAL的初始化
  • 初始化mSensorList（Vector<sensor_t> mSensorList）
  • mSensors->activate()
• 通过dev获取SensorList（list）
• registerSensor
• 创建Looper（mLooper = new Looper(false)），用于enable sensor后，进行数据的接收
• 创建SensorEventAckReceiver（mAckReceiver = new SensorEventAckReceiver(this)）：用于在dispatch wake up sensor event给上层后，接收上层返回的确认ACK。
  

registerListener流程分析

APP

下面的代码是在Android官网提供的APP获取light sensor数据的代码段，这里在获取SensorManager和Sensor后，会调用sensorManager.registerListener(this, mLight, SensorManager.SENSOR_DELAY_NORMAL);注册。

public class SensorActivity extends Activity implements SensorEventListener {private SensorManager sensorManager;private Sensor mLight;@Overridepublic final void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.main);sensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE);mLight = sensorManager.getDefaultSensor(Sensor.TYPE_LIGHT);}@Overridepublic final void onAccuracyChanged(Sensor sensor, int accuracy) {// Do something here if sensor accuracy changes.}@Overridepublic final void onSensorChanged(SensorEvent event) {// The light sensor returns a single value.// Many sensors return 3 values, one for each axis.float lux = event.values[0];// Do something with this sensor value.}@Overrideprotected void onResume() {super.onResume();sensorManager.registerListener(this, mLight, SensorManager.SENSOR_DELAY_NORMAL);}@Overrideprotected void onPause() {super.onPause();sensorManager.unregisterListener(this);}
}

Framework

Java

在java层的调用顺序为
SensorManager.java

• registerListener()
  • registerListenerImpl()

SystemSensorManager.java

• registerListenerImpl()

registerListenerImpl()会维护一个mSensorListeners的HashMap（HashMap<SensorEventListener, SensorEventQueue> mSensorListeners）。

• queue.addSensor()（SensorEventQueue queue）
  • mActiveSensors.put(handle, true)（handle = snesor.getHandle()）
  • enableSensor()
    • nativeEnableSensor()到这里会通过JNI去call到Native层

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

    @Overrideprotected boolean registerListenerImpl(SensorEventListener listener, Sensor sensor,int delayUs, Handler handler, int maxBatchReportLatencyUs, int reservedFlags) {if (listener == null || sensor == null) {Log.e(TAG, "sensor or listener is null");return false;}// Trigger Sensors should use the requestTriggerSensor call.if (sensor.getReportingMode() == Sensor.REPORTING_MODE_ONE_SHOT) {Log.e(TAG, "Trigger Sensors should use the requestTriggerSensor.");return false;}if (maxBatchReportLatencyUs < 0 || delayUs < 0) {Log.e(TAG, "maxBatchReportLatencyUs and delayUs should be non-negative");return false;}if (mSensorListeners.size() >= MAX_LISTENER_COUNT) {throw new IllegalStateException("register failed, "+ "the sensor listeners size has exceeded the maximum limit "+ MAX_LISTENER_COUNT);}// Invariants to preserve:// - one Looper per SensorEventListener// - one Looper per SensorEventQueue// We map SensorEventListener to a SensorEventQueue, which holds the loopersynchronized (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);}}}

Native

Client

通过JNI，java层的enableSensor()会call到SensorEventQueue.cpp的enableSensor()，到这里就要去通过mSensorEventConnection（const sp<ISensorEventConnection>）去call到Server端了。

/frameworks/native/libs/sensor/SensorEventQueue.cpp

status_t SensorEventQueue::enableSensor(Sensor const* sensor, int32_t samplingPeriodUs) const {return mSensorEventConnection->enableDisable(sensor->getHandle(), true,us2ns(samplingPeriodUs), 0, 0);
}

/frameworks/native/libs/sensor/ISensorEventConnection.cpp

    virtual status_t enableDisable(int handle, bool enabled, nsecs_t samplingPeriodNs,nsecs_t maxBatchReportLatencyNs, int reservedFlags){Parcel data, reply;data.writeInterfaceToken(ISensorEventConnection::getInterfaceDescriptor());data.writeInt32(handle);data.writeInt32(enabled);data.writeInt64(samplingPeriodNs);data.writeInt64(maxBatchReportLatencyNs);data.writeInt32(reservedFlags);remote()->transact(ENABLE_DISABLE, data, &reply);return reply.readInt32();}

Server

/frameworks/native/services/sensorservice/SensorEventConnection.cpp

status_t SensorService::SensorEventConnection::enableDisable(int handle, bool enabled, nsecs_t samplingPeriodNs, nsecs_t maxBatchReportLatencyNs,int reservedFlags)
{status_t err;if (enabled) {err = mService->enable

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