Kotlin中级——callbackFlow

callbackFlow

将监听外部（其他线程）的不定时回调转为Flow，并在Flow取消时同步取消监听

如下，是一个温度监听类，每隔1秒发送数据

复制代码

    interface TemperatureListener {
        fun onTemperatureChanged(value: Double)
    }

    class TemperatureSensor {
        private var listener: TemperatureListener? = null
        private var senderJob: Job? = null

        fun register(listener: TemperatureListener) {
            this.listener = listener
            println("传感器：开始监听")
            senderJob = CoroutineScope(Dispatchers.Default).launch {
                var temperature = 25.0
                while (isActive) {
                    delay(1_000)
                    listener.onTemperatureChanged(temperature)
                    temperature += 0.5
                }
            }
        }

        fun unregister() {
            senderJob?.cancel()
            senderJob = null
            listener = null
            println("传感器：停止监听")
        }
    }

若普通调用，需要自行注册和取消注册

复制代码

    @Test
    fun runCallbackExample() = runBlocking {
        println("------------------------------------------------------------------------------------------------------------------------------------------------------")

        val sensor = TemperatureSensor()
        val listener = object : TemperatureListener {
            override fun onTemperatureChanged(value: Double) {
                println("普通回调收到温度：$value")
            }
        }

        sensor.register(listener)
        delay(2_100)
        sensor.unregister()

        println("------------------------------------------------------------------------------------------------------------------------------------------------------")
    }

传感器：开始监听
普通回调收到温度：25.0
普通回调收到温度：25.5
传感器：停止监听

如下新增扩展函数，将监听过程转为Flow，可以流式处理，并绑定相关生命周期

复制代码

    fun TemperatureSensor.temperatureFlow(): Flow<Double> = callbackFlow {
        val listener = object : TemperatureListener {
            override fun onTemperatureChanged(value: Double) {
                trySend(value)
            }
        }

        register(listener)

        // collect 被取消时，注销传感器监听
        awaitClose {
            unregister()
        }
    }

    @Test
    fun runCallbackFlowExample() = runBlocking {
        println("------------------------------------------------------------------------------------------------------------------------------------------------------")

        val sensor = TemperatureSensor()
        val collectJob = launch {
            sensor.temperatureFlow().collect { temperature ->
                println("收到温度：$temperature")
            }
        }

        // 收集两次每隔 1 秒发送的温度
        delay(2_100)
        collectJob.cancelAndJoin()

        // 取消 collect 后，awaitClose 会调用 sensor.unregister()
        println("------------------------------------------------------------------------------------------------------------------------------------------------------")
    }

传感器：开始监听
收到温度：25.0
收到温度：25.5
传感器：停止监听