SensorManager unregisterListener失效：通话结束后无法停止距离传感器

问题 #

我希望在接听电话时读取距离传感器的数据：处于OFFHOOK状态时可正常读取，但通话结束进入IDLE状态时，调用proximitySensorListener.stopSensors()无法停止传感器。若在通话中启动传感器监听后通过延迟调用stopSensors可停止传感器，但这并非理想方案，请问为何直接调用无法生效？

相关代码如下：

class CallStateReceiver : BroadcastReceiver() {

    override fun onReceive(context: Context?, intent: Intent?) {

        var proximitySensorListener = ProximitySensorListener(context!!)
        if (context == null || intent == null) {
            return
        }

        if (intent.action == TelephonyManager.ACTION_PHONE_STATE_CHANGED) {
            when (intent.getStringExtra(TelephonyManager.EXTRA_STATE)) {
                TelephonyManager.EXTRA_STATE_RINGING -> {
                    Toast.makeText(context, "Incoming Call", Toast.LENGTH_SHORT).show()
                }

                TelephonyManager.EXTRA_STATE_OFFHOOK -> {
                    Toast.makeText(context, "Call in progress", Toast.LENGTH_SHORT).show()
                    proximitySensorListener.startSensors()
                }

                TelephonyManager.EXTRA_STATE_IDLE -> {
                    Toast.makeText(context, "Call ended", Toast.LENGTH_SHORT).show()
                    proximitySensorListener.stopSensors()
                }
            }
        }
    }
}

class ProximitySensorListener(private val context: Context) : SensorEventListener {
    private var sensorManager: SensorManager =
        context.getSystemService(Context.SENSOR_SERVICE) as SensorManager

    val proximitySensor = sensorManager.getDefaultSensor(Sensor.TYPE_PROXIMITY)

    fun startSensors() {
        if (proximitySensor != null) {
            sensorManager.registerListener(this, proximitySensor, SensorManager.SENSOR_DELAY_NORMAL)
        }
    }

    fun stopSensors() {
        sensorManager.unregisterListener(this, proximitySensor)
    }

    override fun onAccuracyChanged(sensor: Sensor?, accuracy: Int) {
    }

    override fun onSensorChanged(event: SensorEvent?) {
        if (event?.sensor?.type == Sensor.TYPE_PROXIMITY) {
            val distance = event.values[0]
            if (distance < 5) {
                println("Off")
            } else {
                println("On")
            }
        }
    }
}

原因分析 #

核心问题在于每次onReceive被调用时，你都会新建一个ProximitySensorListener实例：

• 当电话进入OFFHOOK状态时，新建的Listener实例调用startSensors()完成了传感器注册；
• 当通话结束进入IDLE状态时，又新建了一个全新的Listener实例，调用它的stopSensors()时，实际上是尝试注销这个新实例的监听——但这个实例根本没注册过传感器，自然无法停止之前那个正在运行的Listener。

延迟调用能生效的原因是：延迟期间当前onReceive对应的进程还未被系统回收，此时调用的是同一个Listener实例的stopSensors()，但这只是巧合，并非可靠方案。

解决方案 #

要解决这个问题，必须保证启动和停止传感器时使用同一个ProximitySensorListener实例，可以通过以下方式实现：

方案1：将Listener设为单例 #

修改ProximitySensorListener为单例模式，确保全局只有一个实例：

class ProximitySensorListener private constructor(private val context: Context) : SensorEventListener {
    private var sensorManager: SensorManager =
        context.getSystemService(Context.SENSOR_SERVICE) as SensorManager

    val proximitySensor = sensorManager.getDefaultSensor(Sensor.TYPE_PROXIMITY)

    companion object {
        @Volatile
        private var INSTANCE: ProximitySensorListener? = null

        fun getInstance(context: Context): ProximitySensorListener {
            return INSTANCE ?: synchronized(this) {
                INSTANCE ?: ProximitySensorListener(context.applicationContext).also { INSTANCE = it }
            }
        }
    }

    fun startSensors() {
        if (proximitySensor != null) {
            sensorManager.registerListener(this, proximitySensor, SensorManager.SENSOR_DELAY_NORMAL)
        }
    }

    fun stopSensors() {
        sensorManager.unregisterListener(this, proximitySensor)
    }

    override fun onAccuracyChanged(sensor: Sensor?, accuracy: Int) {}

    override fun onSensorChanged(event: SensorEvent?) {
        if (event?.sensor?.type == Sensor.TYPE_PROXIMITY) {
            val distance = event.values[0]
            if (distance < 5) {
                println("Off")
            } else {
                println("On")
            }
        }
    }
}

然后在CallStateReceiver中获取单例实例：

override fun onReceive(context: Context?, intent: Intent?) {
    if (context == null || intent == null) {
        return
    }
    val proximitySensorListener = ProximitySensorListener.getInstance(context)
    // 后续逻辑不变
}

方案2：动态注册BroadcastReceiver并持有Listener引用 #

如果是动态注册CallStateReceiver，可以在Receiver类中初始化一次Listener，而不是每次onReceive都新建：

class CallStateReceiver : BroadcastReceiver() {
    private lateinit var proximitySensorListener: ProximitySensorListener

    fun init(context: Context) {
        proximitySensorListener = ProximitySensorListener(context)
    }

    override fun onReceive(context: Context?, intent: Intent?) {
        if (context == null || intent == null) {
            return
        }
        when (intent.getStringExtra(TelephonyManager.EXTRA_STATE)) {
            TelephonyManager.EXTRA_STATE_RINGING -> {
                Toast.makeText(context, "Incoming Call", Toast.LENGTH_SHORT).show()
            }

            TelephonyManager.EXTRA_STATE_OFFHOOK -> {
                Toast.makeText(context, "Call in progress", Toast.LENGTH_SHORT).show()
                proximitySensorListener.startSensors()
            }

            TelephonyManager.EXTRA_STATE_IDLE -> {
                Toast.makeText(context, "Call ended", Toast.LENGTH_SHORT).show()
                proximitySensorListener.stopSensors()
            }
        }
    }
}

动态注册时先调用init()方法初始化Listener：

val receiver = CallStateReceiver()
receiver.init(this)
registerReceiver(receiver, IntentFilter(TelephonyManager.ACTION_PHONE_STATE_CHANGED))

内容的提问来源于stack exchange，提问作者NullPointerException