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。