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（1）Gsensor的注册和监听

App -->I2C过程：App通过SensorManager.getSystemServer调用到SystemSensorManager，SystemSensorManager通过jni调用到SensorManager.cpp，后通过binder调用到SensorService。SensorService通过SystemServer启动，后调用到hal kernel。

I2C -->App过程：通过JNI调用到SystemSensorManager中的SensorEventQueue.dispatchSensorEvent，然后通过App向SensorEventQueue注册的mListener，来回调数据到App的onSensorChanged()方法。

（网图）

//frameworks/base/core/java/android/hardware/SensorManager.javapublic boolean registerListener(SensorEventListener listener, Sensor sensor,int samplingPeriodUs) {return registerListener(listener, sensor, samplingPeriodUs, null);}public boolean registerListener(SensorEventListener listener, Sensor sensor,int samplingPeriodUs, int maxReportLatencyUs) {int delay = getDelay(samplingPeriodUs);return registerListenerImpl(listener, sensor, delay, null, maxReportLatencyUs, 0);}public boolean registerListener(SensorEventListener listener, Sensor sensor,int samplingPeriodUs, Handler handler) {int delay = getDelay(samplingPeriodUs);return registerListenerImpl(listener, sensor, delay, handler, 0, 0);}public boolean registerListener(SensorEventListener listener, Sensor sensor,int samplingPeriodUs, int maxReportLatencyUs, Handler handler) {int delayUs = getDelay(samplingPeriodUs);return registerListenerImpl(listener, sensor, delayUs, handler, maxReportLatencyUs, 0);}/** @hide */
protected abstract boolean registerListenerImpl(SensorEventListener listener, Sensor sensor,int delayUs, Handler handler, int maxReportLatencyUs, int reservedFlags);

registerListenerImpl的具体实现如下：

//frameworks/base/core/java/android/hardware/SystemSensorManager.javapublic class SystemSensorManager extends SensorManager {protected boolean registerListenerImpl(SensorEventListener listener, Sensor sensor,int delayUs, Handler handler, int maxBatchReportLatencyUs, int reservedFlags) {//...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();//这里创建SensorEventQueue，并调用addSensor进而调用addSensorEvent函数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);}}}
}

接下来我们看一下SensorEventQueue队列

static final class SensorEventQueue extends BaseEventQueue {private final SensorEventListener mListener;private final SparseArray<SensorEvent> mSensorsEvents = new SparseArray<SensorEvent>();public SensorEventQueue(SensorEventListener listener, Looper looper,SystemSensorManager manager, String packageName) {super(looper, manager, OPERATING_MODE_NORMAL, packageName);//App传过来的监听器进行赋值mListener = listener;}@Overridepublic void addSensorEvent(Sensor sensor) {SensorEvent t = new SensorEvent(Sensor.getMaxLengthValuesArray(sensor,mManager.mTargetSdkLevel));synchronized (mSensorsEvents) {mSensorsEvents.put(sensor.getHandle(), t);}}@Overridepublic void removeSensorEvent(Sensor sensor) {synchronized (mSensorsEvents) {mSensorsEvents.delete(sensor.getHandle());}}// Called from native code.@SuppressWarnings("unused")@Override//在addSensorEvent put事件后，通过mSensorsEvents.get获取事件，然后通过mListener分发，这里的mListener就是App传过来的监听接口protected void dispatchSensorEvent(int handle, float[] values, int inAccuracy,long timestamp) {final Sensor sensor = mManager.mHandleToSensor.get(handle);if (sensor == null) {// sensor disconnectedreturn;}SensorEvent t = null;synchronized (mSensorsEvents) {t = mSensorsEvents.get(handle);}//...//这里会回调注册过的监听器的onSensorChangedmListener.onSensorChanged(t);}}

后续文章所提及的屏幕旋转方向等都和这里事件分发相关。

（2）应用注册屏幕旋转事件监听

应用注册使用，通过enable和disable来控制注册和取消注册

private class OrientationEventListenerImpl extends OrientationEventListener {public OrientationEventListenerImpl(Context context) {super(context);}@Overridepublic void onOrientationChanged(int orientation) {if (orientation == ORIENTATION_UNKNOWN) {return;}//...}}mOrientationListener.enable();
mOrientationListener.disable();

如下为系统代码

//frameworks/base/core/java/android/view/OrientationEventListener.javapublic abstract class OrientationEventListener {public OrientationEventListener(Context context) {this(context, SensorManager.SENSOR_DELAY_NORMAL);}public OrientationEventListener(Context context, int rate) {mSensorManager = (SensorManager)context.getSystemService(Context.SENSOR_SERVICE);mRate = rate;mSensor = mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);if (mSensor != null) {// Create listener only if sensors do existmSensorEventListener = new SensorEventListenerImpl();}}public void enable() {if (mSensor == null) {Log.w(TAG, "Cannot detect sensors. Not enabled");return;}if (mEnabled == false) {if (localLOGV) Log.d(TAG, "OrientationEventListener enabled");mSensorManager.registerListener(mSensorEventListener, mSensor, mRate);mEnabled = true;}}public void disable() {if (mSensor == null) {Log.w(TAG, "Cannot detect sensors. Invalid disable");return;}if (mEnabled == true) {if (localLOGV) Log.d(TAG, "OrientationEventListener disabled");mSensorManager.unregisterListener(mSensorEventListener);mEnabled = false;}}class SensorEventListenerImpl implements SensorEventListener {private static final int _DATA_X = 0;private static final int _DATA_Y = 1;private static final int _DATA_Z = 2;public void onSensorChanged(SensorEvent event) {float[] values = event.values;int orientation = ORIENTATION_UNKNOWN;float X = -values[_DATA_X];float Y = -values[_DATA_Y];float Z = -values[_DATA_Z];        float magnitude = X*X + Y*Y;// Don't trust the angle if the magnitude is small compared to the y valueif (magnitude * 4 >= Z*Z) {float OneEightyOverPi = 57.29577957855f;float angle = (float)Math.atan2(-Y, X) * OneEightyOverPi;orientation = 90 - (int)Math.round(angle);// normalize to 0 - 359 rangewhile (orientation >= 360) {orientation -= 360;} while (orientation < 0) {orientation += 360;}}if (mOldListener != null) {mOldListener.onSensorChanged(Sensor.TYPE_ACCELEROMETER, event.values);}if (orientation != mOrientation) {mOrientation = orientation;//回调函数onOrientationChanged(orientation);}}//回调函数public void onAccuracyChanged(Sensor sensor, int accuracy) {}}
}