Android端末でWebSocketサーバーアプリを開発する

import io.ktor.server.application.*
import io.ktor.server.engine.*
import io.ktor.server.netty.*
import io.ktor.server.routing.*
import io.ktor.server.websocket.*
import io.ktor.util.*
import io.ktor.websocket.*
import kotlinx.coroutines.channels.consumeEach
import java.time.Duration

class MyWebsocketServer {
    private val netty = embeddedServer(Netty, port = 8000) {
        install(WebSockets) {
            timeout = Duration.ofSeconds(5)
            pingPeriod = Duration.ofMinutes(1)
        }
        routing {
            webSocket("/") {
                val uniqueId = generateNonce()
                println("Connection to $uniqueId established")
                incoming.consumeEach { frame ->
                    if (frame is Frame.Text) {
                        println("Receive Message: ${frame.readText()}")
                        outgoing.send(Frame.Text("Receive Message: ${frame.readText()}"))
                    }
                }
                println("Connection to $uniqueId closed")
            }
        }
    }

    fun start() {
        netty.start(wait = true)
    }

    fun stop() {
        netty.stop()
    }
}

fun main() {
    MyWebsocketServer().start()
}

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools">

    <!-- 以下の３つのパーミッションを追加 -->
    <uses-permission android:name="android.permission.INTERNET"/>
    <uses-permission android:name="android.permission.ACCESS_WIFI_STATE" android:maxSdkVersion="30"/>
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

    <application
        android:allowBackup="true"
        android:dataExtractionRules="@xml/data_extraction_rules"

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"
        android:padding="16dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent">
        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginBottom="8dp"
            android:orientation="horizontal">

            <TextView
                android:layout_width="0dp"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:text="IPアドレス："/>

            <TextView
                android:id="@+id/ip_address_text"
                android:layout_width="0dp"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                tools:text="192.168.0.1"/>
        </LinearLayout>

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginBottom="8dp"
            android:orientation="horizontal">

            <TextView
                android:layout_width="0dp"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:text="PORT："/>

            <TextView
                android:id="@+id/ip_port_text"
                android:layout_width="0dp"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:text="8000"/>
        </LinearLayout>
    </LinearLayout>
</androidx.constraintlayout.widget.ConstraintLayout>

import android.net.ConnectivityManager
import android.net.LinkProperties
import android.net.Network
import android.os.Build
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.os.Handler
import android.os.Looper
import com.websocketserver.databinding.ActivityMainBinding
import java.net.Inet4Address
import kotlin.concurrent.thread

class MainActivity : AppCompatActivity() {
    private var mWebSocketServer: MyWebsocketServer? = null
    private lateinit var binding: ActivityMainBinding

    private var mHandler = if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
        Handler.createAsync(Looper.getMainLooper())
    } else {
        @Suppress("DEPRECATION")
        (Handler())
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        // サーバーを起動
        mWebSocketServer = MyWebsocketServer()
        thread {
            mWebSocketServer?.start()
        }

        // 端末のIPアドレスを取得
        val manager = this.applicationContext.getSystemService(CONNECTIVITY_SERVICE) as ConnectivityManager
        val networkCallback = object: ConnectivityManager.NetworkCallback() {
            override fun onLinkPropertiesChanged(network: Network, linkProperties: LinkProperties) {
                super.onLinkPropertiesChanged(network, linkProperties)
                mHandler.post {
                    binding.ipAddressText.text = linkProperties.linkAddresses.filter{
                        it.address is Inet4Address
                    }[0].toString()
                }
            }
        }
        manager.registerDefaultNetworkCallback(networkCallback)
    }

    override fun onDestroy() {
        // サーバーを終了
        thread {
            mWebSocketServer?.stop()
            mWebSocketServer = null
        }
        super.onDestroy()
    }
}