【Kotlinプログラム入門】。今回はAndroidアプリケーションでMQTT受信に挑戦します！

package com.sample.samplez

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.foundation.layout.Column
import androidx.compose.material3.ExperimentalMaterial3Api
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Surface
import androidx.compose.material3.Text
import androidx.compose.material3.TopAppBar
import androidx.compose.runtime.*
import androidx.compose.ui.tooling.preview.Preview
import org.eclipse.paho.client.mqttv3.*
import org.eclipse.paho.client.mqttv3.persist.MemoryPersistence
import java.nio.charset.Charset

class MainActivity : ComponentActivity() {
    //エミュレータでlocalhostを指定する場合は「10.0.2.2」を使うべし
    private val mqttServerUri = "tcp://10.0.2.2:1883"
    private val mqttTopic = "test/Topic1"
    private lateinit var mqttClient: MqttClient
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            val messageState = remember { mutableStateOf("") }
            MqttSubscriptionApp(messageState)
            // MQTTサブスクライブ
            startMqttSubscription(messageState)
        }
    }

    private fun startMqttSubscription(messageState: MutableState) {
        try {
            mqttClient = MqttClient(mqttServerUri, MqttClient.generateClientId(), MemoryPersistence())
            val options = MqttConnectOptions()
            options.isAutomaticReconnect = true
            options.isCleanSession = false
            mqttClient.connect(options)

            mqttClient.setCallback(object : MqttCallback {
                override fun connectionLost(cause: Throwable?) {
                    // 接続が失われたときの処理
                }

                override fun messageArrived(topic: String?, message: MqttMessage?) {
                    // 新しいメッセージが到着したときの処理
                    val receivedMessage = message?.toString()
                    if (!receivedMessage.isNullOrEmpty()) {
                        // UTF-8に変換する
                        val utf8String = receivedMessage.toByteArray(Charset.forName("UTF-8"))
                        // UTF-8に変換されたバイト配列を文字列に戻す
                        val convertedString = String(utf8String, Charset.forName("UTF-8"))
                        // messageStateの値を更新する
                        messageState.value = convertedString
                    }
                }

                override fun deliveryComplete(token: IMqttDeliveryToken?) {
                    // メッセージの送信が完了したときの処理
                }
            })

            mqttClient.subscribe(mqttTopic, 0)
        } catch (e: MqttException) {
            e.printStackTrace()
        }
    }
}

@OptIn(ExperimentalMaterial3Api::class)
@Composable
fun MqttSubscriptionApp(messageState: MutableState) {
    Column {
        TopAppBar(title = { Text(text = "MQTTで受信してみよう") })
        Surface {
            Column {
                Text(text = "受信したメッセージ:")
                Text(text = messageState.value)
            }
        }
    }
}

@Preview(showBackground = true)
@Composable
fun DefaultPreview() {
    MaterialTheme {
        val messageState = remember { mutableStateOf("") }
        MqttSubscriptionApp(messageState)
    }
}