Android4.0 G-Sensor工作流程

1. 简介

在了解Sensor工作流程以前，一直以为其事件是通过Event Hub来进行输送的，可是研究完Android4.0代码之后，才发现自己错了。

其主要框架如下图所示：

2.功能模块

2.1 SensorManager.java

与下层接口功能：

1) 在SensorManager函数中

(1) 调用native sensors_module_init初始化sensor list，即实例化native中的SensorManager

(2) 创建SensorThread线程

2) 在类SensorThread中

(1) 调用native sensors_create_queue创建队列

(2) 在线程中dead loop地调用native sensors_data_poll以从队列sQueue中获取事件(float[] values = new float[3];)

(3) 收到事件之后，报告sensor event给所有注册且关心此事件的listener

与上层的接口功能：

1) 在onPause时取消listener注册

2) 在onResume时注册listener

3) 把收到的事件报告给注册的listener

2.2 android_hardware_SensorManager.cpp

实现SensorManager.java中的native函数，它主要调用SenrsorManager.cpp和SensorEventQueue.cpp中的类来完成相关的工作。

2.3 SensorManager.cpp

class SensorManager :
public ASensorManager,
public Singleton<SensorManager>
{
public:
SensorManager(); //调用assertStateLocked
~SensorManager();
//调用assertStateLocked,并返回mSensorList
ssize_t getSensorList(Sensor const* const** list) const;

// 返回mSensorList中第一个类型与type一致的sensor
Sensor const* getDefaultSensor(int type);

// 调用mSensorServer->createSensorEventConnection创建一个连接(ISensorEventConnection)
// 并用此连接做为参数创建一个SensorEventQueue对象并返回
sp<SensorEventQueue> createEventQueue();

private:
// DeathRecipient interface
void sensorManagerDied();
// 调用getService获取SensorService客户端并保存在mSensorServer中
// 调用mSensorServer->getSensorList获取sensor列表，并保存在mSensors和mSensorList中
status_t assertStateLocked() const;

private:
mutable Mutex mLock;
mutable sp<ISensorServer> mSensorServer; // SensorService客户端
mutable Sensor const** mSensorList; // sensor列表
mutable Vector<Sensor> mSensors;    // sensor列表
mutable sp<IBinder::DeathRecipient> mDeathObserver;
}

class ISensorEventConnection : public IInterface
{
public:
DECLARE_META_INTERFACE(SensorEventConnection);

virtual sp<SensorChannel> getSensorChannel() const = 0;
virtual status_t enableDisable(int handle, bool enabled) = 0;
virtual status_t setEventRate(int handle, nsecs_t ns) = 0;
};

2.4 SensorService.cpp

SensorService作为一个轻量级的system service，它运行于SystemServer内，即在system_init<system_init.cpp>调用SensorService::instantiate();

SensorService主要功能如下：

1) SensorService::instantiate创建实例对象，并增加到ServiceManager中，且创建并启动线程，并执行threadLoop

2) threadLoop从sensor驱动获取原始数据，然后通过SensorEventConnection把事件发送给客户端

3) BnSensorServer的成员函数负责让客户端获取sensor列表和创建SensorEventConnection

SensorService与客户端的接口定义如下：

class ISensorServer : public IInterface
{
public:
DECLARE_META_INTERFACE(SensorServer);

virtual Vector<Sensor> getSensorList() = 0;
virtual sp<ISensorEventConnection> createSensorEventConnection() = 0;
};

SensorService定义如下：

class SensorService :
public BinderService<SensorService>, //创建SensorService对象，并增加到ServiceManager中
public BnSensorServer, // 申明了SensorService与客户端(SensorManager)间的binder接口
protected Thread // 线程辅助类，调用run创建并启动线程，然后在线程主函数内回调threadLoop函数,
// 所以在使用它时，做一个派生，并根据需要重写threadLoop即可

{
friend class BinderService<SensorService>;

static const nsecs_t MINIMUM_EVENTS_PERIOD =   1000000; // 1000 Hz

SensorService();
virtual ~SensorService();

/*
在addService时，第一次构建sp强引用对象时，会调用onFirstRef函数
实现功能如下：
1) 获取SensorDevice实例
2) 调用SensorDevice.getSensorList获取sensor_t列表
3) 根据硬件sensor_t创建HardwareSensor,然后加入mSensorList(Sensor)
和mSensorMap(HardwareSensor)中
4) 根据硬件sensor_t创建对应的senosr(如GravitySensor),
然后加入mVirtualSensorList和mSensorList中
5) mUserSensorList = mSensorList;
6) run("SensorService", PRIORITY_URGENT_DISPLAY);运行线程，并执行threadLoop
*/
virtual void onFirstRef();

// Thread interface
/*
1) 调用SensorDevice.poll获取sensors_event_t事件
2) 获取已经激活的sensor列表mActiveVirtualSensors
3) 对每一个事件，执行SensorFusion.process
4) 对每一个事件，执行HardwareSensor.process(事件无变化,直接copy)
5) 调用SensorService::SensorEventConnection::sendEvents，把事件发
送给所有的listener
*/
virtual bool threadLoop();

// ISensorServer interface
// 返回mUserSensorList
virtual Vector<Sensor> getSensorList();

// 实例化SensorEventConnection并返回
virtual sp<ISensorEventConnection> createSensorEventConnection();

virtual status_t dump(int fd, const Vector<String16>& args);

//====================================================================
//============== SensorEventConnection  start ========================
class SensorEventConnection : public BnSensorEventConnection {

virtual ~SensorEventConnection();
virtual void onFirstRef();

// 返回mChannel
virtual sp<SensorChannel> getSensorChannel() const;

// 调用SensorService::enable或SensorService::disable
virtual status_t enableDisable(int handle, bool enabled);

// 调用SensorService::setEventRate
virtual status_t setEventRate(int handle, nsecs_t ns);

sp<SensorService> const mService; // 保存当前SensorService实例
sp<SensorChannel> const mChannel; // SensorChannel实例
mutable Mutex mConnectionLock;

// protected by SensorService::mLock
SortedVector<int> mSensorInfo;

public:
/*
1) 把当前service保存在mService中
2) 创建SensorChannel实例，并保存在mChannel中
(在SensorChannel::SensorChannel中创建pipe,并把收和发都设置非阻塞)
*/
SensorEventConnection(const sp<SensorService>& service);

// 调用连接中的mChannel->write (SensorChannel::write),把符合条件的事件写入pipe
status_t sendEvents(sensors_event_t const* buffer, size_t count,
sensors_event_t* scratch = NULL);

bool hasSensor(int32_t handle) const; //检查handle是否在mSensorInfo中
bool hasAnySensor() const;   //检查mSensorInfo中是否有sensor
bool addSensor(int32_t handle); //把handle增加到mSensorInfo列表中
bool removeSensor(int32_t handle); //把handle从mSensorInfo中删除
};
//============== SensorEventConnection  end ========================
//====================================================================

class SensorRecord {
SortedVector< wp<SensorEventConnection> > mConnections;
public:
SensorRecord(const sp<SensorEventConnection>& connection);
bool addConnection(const sp<SensorEventConnection>& connection);
bool removeConnection(const wp<SensorEventConnection>& connection);
size_t getNumConnections() const { return mConnections.size(); }
};

SortedVector< wp<SensorEventConnection> > getActiveConnections() const;
DefaultKeyedVector<int, SensorInterface*> getActiveVirtualSensors() const;

String8 getSensorName(int handle) const;
void recordLastValue(sensors_event_t const * buffer, size_t count);
static void sortEventBuffer(sensors_event_t* buffer, size_t count);
void registerSensor(SensorInterface* sensor);
void registerVirtualSensor(SensorInterface* sensor);

// constants
Vector<Sensor> mSensorList;  // Sensor列表
Vector<Sensor> mUserSensorList; //与mSensorList一样
DefaultKeyedVector<int, SensorInterface*> mSensorMap; //其成员为HardwareSensor
Vector<SensorInterface *> mVirtualSensorList; //其成员为HardwareSensor
status_t mInitCheck;

// protected by mLock
mutable Mutex mLock;
DefaultKeyedVector<int, SensorRecord*> mActiveSensors; //成员为SensorRecord
DefaultKeyedVector<int, SensorInterface*> mActiveVirtualSensors; //成员为HardwareSensor
SortedVector< wp<SensorEventConnection> > mActiveConnections;

// The size of this vector is constant, only the items are mutable
KeyedVector<int32_t, sensors_event_t> mLastEventSeen;

public:
static char const* getServiceName() { return "sensorservice"; }

void cleanupConnection(SensorEventConnection* connection);

/*
1) 调用HardwareSensor::activate，即SensorDevice::activate
2) 然后创建SensorRecord并增加到列表mActiveSensors
3) 把此HardwareSensor增加到连接的mSensorInfo
4) 把此连接增加到mActiveConnections中
*/
status_t enable(const sp<SensorEventConnection>& connection, int handle);

/*
1) 把此sensor从连接的mSensorInfo中删除
2) 把此连接从mActiveConnections中删除
3) 调用HardwareSensor::activate，即SensorDevice::activate
*/
status_t disable(const sp<SensorEventConnection>& connection, int handle);
/*
1)调用HardwareSensor::setDelay，即SensorDevice::setDelay
*/
status_t setEventRate(const sp<SensorEventConnection>& connection, int handle, nsecs_t ns);
}

2.5 SensorDevice.cpp

SensorDevice封装了对SensorHAL层代码的调用，主要包含以下功能：

1) 获取sensor列表(getSensorList)

2) 获取sensor事件(poll)

3) Enable或Disable sensor (activate)

4) 设置delay时间

class SensorDevice : public Singleton<SensorDevice> {

friend class Singleton<SensorDevice>;

struct sensors_poll_device_t* mSensorDevice; // sensor设备

struct sensors_module_t* mSensorModule;

mutable Mutex mLock; // protect mActivationCount[].rates
// fixed-size array after construction
struct Info {
Info() : delay(0) { }
KeyedVector<void*, nsecs_t> rates;
nsecs_t delay;
status_t setDelayForIdent(void* ident, int64_t ns);
nsecs_t selectDelay();
};
DefaultKeyedVector<int, Info> mActivationCount;

/*
1) 调用hw_get_module(SENSORS_HARDWARE_MODULE_ID,..)获取sensors_module_t，
并保存在mSensorModule中
2) 调用mSensorModule->common->methods->open,以返回sensors_poll_device_t,
并保存在mSensorDevice中
3) 调用mSensorModule->get_sensors_list所有可访问的sensor_t
4) 调用mSensorDevice->activate激活所有的sensor
*/
SensorDevice();
public:
// 调用mSensorModule->get_sensors_list实现
ssize_t getSensorList(sensor_t const** list);

status_t initCheck() const;

// 调用mSensorDevice->poll实现
ssize_t poll(sensors_event_t* buffer, size_t count);

// 调用mSensorDevice->activate实现
status_t activate(void* ident, int handle, int enabled);

// 调用mSensorDevice->setDelay实现
status_t setDelay(void* ident, int handle, int64_t ns);
void dump(String8& result, char* buffer, size_t SIZE);
};

2.6 Sensor HAL

定义：/hardware/libhardware/include/hardware/sensors.h

实现：/hardware/mychip/sensor/st/sensors.c

2.6.1 struct sensors_poll_device_t 定义

struct sensors_poll_device_t {
struct hw_device_t common;

// Activate/deactivate one sensor.
int (*activate)(struct sensors_poll_device_t *dev,
int handle, int enabled);

// Set the delay between sensor events in nanoseconds for a given sensor.
int (*setDelay)(struct sensors_poll_device_t *dev,
int handle, int64_t ns);

// Returns an array of sensor data.
int (*poll)(struct sensors_poll_device_t *dev,
sensors_event_t* data, int count);
};

2.6.2 struct sensors_module_t 定义

struct sensors_module_t {
struct hw_module_t common;

/**
* Enumerate all available sensors. The list is returned in "list".
* @return number of sensors in the list
*/
int (*get_sensors_list)(struct sensors_module_t* module,
struct sensor_t const** list);
};

2.6.3 struct sensor_t 定义

struct sensor_t {
/* name of this sensors */
const char*     name;
/* vendor of the hardware part */
const char*     vendor;
/* version of the hardware part + driver. The value of this field
* must increase when the driver is updated in a way that changes the
* output of this sensor. This is important for fused sensors when the
* fusion algorithm is updated.
*/
int             version;
/* handle that identifies this sensors. This handle is used to activate
* and deactivate this sensor. The value of the handle must be 8 bits
* in this version of the API.
*/
int             handle;
/* this sensor's type. */
int             type;
/* maximaum range of this sensor's value in SI units */
float           maxRange;
/* smallest difference between two values reported by this sensor */
float           resolution;
/* rough estimate of this sensor's power consumption in mA */
float           power;
/* minimum delay allowed between events in microseconds. A value of zero
* means that this sensor doesn't report events at a constant rate, but
* rather only when a new data is available */
int32_t         minDelay;
/* reserved fields, must be zero */
void*           reserved[8];
};

2.6.4 struct sensors_event_t 定义

typedef struct {
union {
float v[3];
struct {
float x;
float y;
float z;
};
struct {
float azimuth;
float pitch;
float roll;
};
};
int8_t status;
uint8_t reserved[3];
} sensors_vec_t;

/**
* Union of the various types of sensor data
* that can be returned.
*/
typedef struct sensors_event_t {
/* must be sizeof(struct sensors_event_t) */
int32_t version;

/* sensor identifier */
int32_t sensor;

/* sensor type */
int32_t type;

/* reserved */
int32_t reserved0;

/* time is in nanosecond */
int64_t timestamp;

union {
float           data[16];

/* acceleration values are in meter per second per second (m/s^2) */
sensors_vec_t   acceleration;

/* magnetic vector values are in micro-Tesla (uT) */
sensors_vec_t   magnetic;

/* orientation values are in degrees */
sensors_vec_t   orientation;

/* gyroscope values are in rad/s */
sensors_vec_t   gyro;

/* temperature is in degrees centigrade (Celsius) */
float           temperature;

/* distance in centimeters */
float           distance;

/* light in SI lux units */
float           light;

/* pressure in hectopascal (hPa) */
float           pressure;

/* relative humidity in percent */
float           relative_humidity;
};
uint32_t        reserved1[4];
} sensors_event_t;

2.6.5 struct sensors_module_t 实现

#include <hardware/sensors.h>
#include "nusensors.h"

/*
* the AK8973 has a 8-bit ADC but the firmware seems to average 16 samples,
* or at least makes its calibration on 12-bits values. This increases the
* resolution by 4 bits.
*/
static const struct sensor_t sSensorList[] = {
{ "MMA8452Q 3-axis Accelerometer",
"Freescale Semiconductor",
1, SENSORS_HANDLE_BASE+ID_A,
SENSOR_TYPE_ACCELEROMETER, 4.0f*9.81f, (4.0f*9.81f)/256.0f, 0.2f, 0, { } },
{ "AK8975 3-axis Magnetic field sensor",
"Asahi Kasei",
1, SENSORS_HANDLE_BASE+ID_M,
SENSOR_TYPE_MAGNETIC_FIELD, 2000.0f, 1.0f/16.0f, 6.8f, 0, { } },
{ "AK8975 Orientation sensor",
"Asahi Kasei",
1, SENSORS_HANDLE_BASE+ID_O,
SENSOR_TYPE_ORIENTATION, 360.0f, 1.0f, 7.0f, 0, { } },

{ "ST 3-axis Gyroscope sensor",
"STMicroelectronics",
1, SENSORS_HANDLE_BASE+ID_GY,
SENSOR_TYPE_GYROSCOPE, RANGE_GYRO, CONVERT_GYRO, 6.1f, 1190, { } },

{ "AL3006Proximity sensor",
"Dyna Image Corporation",
1, SENSORS_HANDLE_BASE+ID_P,
SENSOR_TYPE_PROXIMITY,
PROXIMITY_THRESHOLD_CM, PROXIMITY_THRESHOLD_CM,
0.5f, 0, { } },

{ "AL3006 light sensor",
"Dyna Image Corporation",
1, SENSORS_HANDLE_BASE+ID_L,
SENSOR_TYPE_LIGHT, 10240.0f, 1.0f, 0.5f, 0, { } },

};

static int open_sensors(const struct hw_module_t* module, const char* name,
struct hw_device_t** device);

static int sensors__get_sensors_list(struct sensors_module_t* module,
struct sensor_t const** list)
{
*list = sSensorList;
return ARRAY_SIZE(sSensorList);
}

static struct hw_module_methods_t sensors_module_methods = {
.open = open_sensors
};

const struct sensors_module_t HAL_MODULE_INFO_SYM = {
.common = {
.tag = HARDWARE_MODULE_TAG,
.version_major = 1,
.version_minor = 0,
.id = SENSORS_HARDWARE_MODULE_ID,
.name = "MMA8451Q & AK8973A & gyro Sensors Module",
.author = "The Android Project",
.methods = &sensors_module_methods,
},
.get_sensors_list = sensors__get_sensors_list
};

static int open_sensors(const struct hw_module_t* module, const char* name,
struct hw_device_t** device)
{
return init_nusensors(module, device); //待后面讲解
}

2.6.6 struct sensors_poll_device_t 实现

实现代码位于：/hardware/mychip/sensor/st/nusensors.cpp

从上面的代码中可以看出，当调用init_nusensors时，它将返回sensors_poll_device_t，然后就可以调用sensors_poll_device_t 的以下方法进行相关操作：

1) activate

2) setDelay

3) poll

6.1) struct sensors_poll_context_t 定义

struct sensors_poll_context_t {
struct sensors_poll_device_t device; // must be first

sensors_poll_context_t();
~sensors_poll_context_t();
int activate(int handle, int enabled);
int setDelay(int handle, int64_t ns);
int pollEvents(sensors_event_t* data, int count);

private:
enum {
light           = 0,
proximity       = 1,
mma             = 2,
akm             = 3,
gyro            = 4,
numSensorDrivers,
numFds,
};

static const size_t wake = numFds - 1;
static const char WAKE_MESSAGE = 'W';
struct pollfd mPollFds[numFds];
int mWritePipeFd;
SensorBase* mSensors[numSensorDrivers];

int handleToDriver(int handle) const {
switch (handle) {
case ID_A:
return mma;
case ID_M:
case ID_O:
return akm;
case ID_P:
return proximity;
case ID_L:
return light;
case ID_GY:
return gyro;
}
return -EINVAL;
}
}

6.2) init_nusensors 实现

int init_nusensors(hw_module_t const* module, hw_device_t** device)
{
int status = -EINVAL;

sensors_poll_context_t *dev = new sensors_poll_context_t();
memset(&dev->device, 0, sizeof(sensors_poll_device_t));

dev->device.common.tag = HARDWARE_DEVICE_TAG;
dev->device.common.version  = 0;
dev->device.common.module   = const_cast<hw_module_t*>(module);
dev->device.common.close    = poll__close;
dev->device.activate        = poll__activate;
dev->device.setDelay        = poll__setDelay;
dev->device.poll            = poll__poll;

*device = &dev->device.common;
status = 0;
return status;
}

由以上代码可见，sensors_poll_device_t的activate、setDelay和poll的实现函数分别为：

(1) poll__activate

(2) poll__setDelay

(3) poll__poll

下面讲解以上三个关键函数的实现

6.3) struct sensors_poll_context_t 的实现

sensors_poll_context_t::sensors_poll_context_t()
{
mSensors[light] = new LightSensor();
mPollFds[light].fd = mSensors[light]->getFd();
mPollFds[light].events = POLLIN;
mPollFds[light].revents = 0;

mSensors[proximity] = new ProximitySensor();
mPollFds[proximity].fd = mSensors[proximity]->getFd();
mPollFds[proximity].events = POLLIN;
mPollFds[proximity].revents = 0;

mSensors[mma] = new MmaSensor();  //下面MmmaSensor为例进行分析
mPollFds[mma].fd = mSensors[mma]->getFd();
mPollFds[mma].events = POLLIN;
mPollFds[mma].revents = 0;

mSensors[akm] = new AkmSensor();
mPollFds[akm].fd = mSensors[akm]->getFd();
mPollFds[akm].events = POLLIN;
mPollFds[akm].revents = 0;

mSensors[gyro] = new GyroSensor();
mPollFds[gyro].fd = mSensors[gyro]->getFd();
mPollFds[gyro].events = POLLIN;
mPollFds[gyro].revents = 0;

int wakeFds[2];
int result = pipe(wakeFds);
LOGE_IF(result<0, "error creating wake pipe (%s)", strerror(errno));
fcntl(wakeFds[0], F_SETFL, O_NONBLOCK);
fcntl(wakeFds[1], F_SETFL, O_NONBLOCK);
mWritePipeFd = wakeFds[1];

mPollFds[wake].fd = wakeFds[0];
mPollFds[wake].events = POLLIN;
mPollFds[wake].revents = 0;
}

sensors_poll_context_t::~sensors_poll_context_t() {
for (int i=0 ; i<numSensorDrivers ; i++) {
delete mSensors[i];
}
close(mPollFds[wake].fd);
close(mWritePipeFd);
}

int sensors_poll_context_t::activate(int handle, int enabled) {
int index = handleToDriver(handle);
if (index < 0) return index;
int err =  mSensors[index]->enable(handle, enabled);
if (enabled && !err) {
const char wakeMessage(WAKE_MESSAGE);
int result = write(mWritePipeFd, &wakeMessage, 1);
LOGE_IF(result<0, "error sending wake message (%s)", strerror(errno));
}
return err;
}

int sensors_poll_context_t::setDelay(int handle, int64_t ns) {

int index = handleToDriver(handle);
if (index < 0) return index;
return mSensors[index]->setDelay(handle, ns);
}

int sensors_poll_context_t::pollEvents(sensors_event_t* data, int count)
{
int nbEvents = 0;
int n = 0;

do {
// see if we have some leftover from the last poll()
for (int i=0 ; count && i<numSensorDrivers ; i++) {
SensorBase* const sensor(mSensors[i]);
if ((mPollFds[i].revents & POLLIN) || (sensor->hasPendingEvents())) {
int nb = sensor->readEvents(data, count);	// num of evens received.
D("nb = %d.", nb);
if (nb < count) {
// no more data for this sensor
mPollFds[i].revents = 0;
}
count -= nb;
nbEvents += nb;
data += nb;
}
}

if (count) {
// we still have some room, so try to see if we can get
// some events immediately or just wait if we don't have
// anything to return
n = poll(mPollFds, numFds, nbEvents ? 0 : -1);
if (n<0) {
LOGE("poll() failed (%s)", strerror(errno));
return -errno;
}
if (mPollFds[wake].revents & POLLIN) {
char msg;
int result = read(mPollFds[wake].fd, &msg, 1);
LOGE_IF(result<0, "error reading from wake pipe (%s)", strerror(errno));
LOGE_IF(msg != WAKE_MESSAGE, "unknown message on wake queue (0x%02x)", int(msg));
mPollFds[wake].revents = 0;
}
}
// if we have events and space, go read them
} while (n && count);

return nbEvents;
}

/*****************************************************************************/

static int poll__close(struct hw_device_t *dev)
{
sensors_poll_context_t *ctx = (sensors_poll_context_t *)dev;
if (ctx) {
delete ctx;
}
return 0;
}

static int poll__activate(struct sensors_poll_device_t *dev,
int handle, int enabled) {
sensors_poll_context_t *ctx = (sensors_poll_context_t *)dev;
return ctx->activate(handle, enabled);
}

static int poll__setDelay(struct sensors_poll_device_t *dev,
int handle, int64_t ns) {
sensors_poll_context_t *ctx = (sensors_poll_context_t *)dev;
return ctx->setDelay(handle, ns);
}

static int poll__poll(struct sensors_poll_device_t *dev,
sensors_event_t* data, int count) {
sensors_poll_context_t *ctx = (sensors_poll_context_t *)dev;
return ctx->pollEvents(data, count);
}

下面MmaSensor为例进行分析。

2.7 MmaSensor.cpp

1) SensorBase的实现(SensorBase.cpp)

class SensorBase {
protected:
const char* dev_name; // "/dev/mma8452_daemon"
const char* data_name; // "gsensor"
int         dev_fd; // 打开设备"/dev/mma8452_daemon"的fd

// 打开事件"/dev/input/eventx"的fd,其驱动的名字为"gsensor"
int         data_fd;

// 打开与"gsensor"对应的事件"/dev/input/eventx"
static int openInput(const char* inputName);

//通过clock_gettime获取当前时间
static int64_t getTimestamp();

static int64_t timevalToNano(timeval const& t) {
return t.tv_sec*1000000000LL + t.tv_usec*1000;
}

int open_device(); //打开设备"dev/mma8452_daemon"
int close_device(); //关闭设备"dev/mma8452_daemon"

public:
// 调用openInput
SensorBase(
const char* dev_name,
const char* data_name);

virtual ~SensorBase();

virtual int readEvents(sensors_event_t* data, int count) = 0;
virtual bool hasPendingEvents() const;
virtual int getFd() const;  //返回data_fd
virtual int setDelay(int32_t handle, int64_t ns);
virtual int enable(int32_t handle, int enabled) = 0;
};

2) MmaSensor的实现

class MmaSensor : public SensorBase {
public:
/*
1) 设置dev_name为 "/dev/mma8452_daemon"
2) 设置data_name为 "gsensor"
3) open设备 "/dev/mma8452_daemon"
*/
MmaSensor();
virtual ~MmaSensor();

enum {
Accelerometer   = 0,
numSensors
};

// 调用ioctl(MMA_IOCTL_APP_SET_RATE)
virtual int setDelay(int32_t handle, int64_t ns);

/*
1) Activate: ioctl(MMA_IOCTL_START)
2) Deactivate: ioctl(MMA_IOCTL_CLOSE)
*/
virtual int enable(int32_t handle, int enabled);

/*
1) 从data_fd read input_event
2) 调用processEvent对事件进行处理
3) 把事件通过data返回
*/
virtual int readEvents(sensors_event_t* data, int count);

void processEvent(int code, int value);

private:
int update_delay();
uint32_t mEnabled;
uint32_t mPendingMask;
InputEventCircularReader mInputReader;
sensors_event_t mPendingEvents[numSensors];
uint64_t mDelays[numSensors];
};