question: public void start ( ) { if ( ( mProxSensor == null ) || mIsActive ) { return ; } mIsActive = true ; mShouldDropEvents = true ; mSensorManager . registerListener ( mListener , mProxSensor , SensorManager . SENSOR_DELAY_UI ) ; LogUtils . log ( this , Log . VERBOSE , "Proximity sensor registered at %d." , System . currentTimeMillis ( ) ) ; mHandler . postDelayed ( mFilterRunnable , REGISTRATION_EVENT_FILTER_TIMEOUT ) ; }
target:  Starts listening for sensor events.