【Kotlinプログラム入門】。今回はAndroidアプリケーションでWEB APIを利用するプログラムに挑戦します！

package com.sample.webapisample

import kotlinx.serialization.Serializable

@Serializable
data class Item(
    val id: Int,
    val name: String,
    val age: Int,
    val field1: String,
    val field2: String,
    val field3: String
)

package com.sample.webapisample

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.foundation.layout.*
import androidx.compose.foundation.lazy.LazyColumn
import androidx.compose.foundation.lazy.items
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Surface
import androidx.compose.material3.Text
import androidx.compose.runtime.*
import androidx.compose.ui.Modifier
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.tooling.preview.Preview
import androidx.compose.ui.unit.dp
import androidx.compose.ui.unit.sp
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.withContext
import kotlinx.serialization.decodeFromString
import kotlinx.serialization.json.Json
import java.io.BufferedReader
import java.io.DataOutputStream
import java.io.InputStreamReader
import java.net.HttpURLConnection
import java.net.URL

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            MaterialTheme {
                DataFetchingScreen()
            }
        }
    }
}

@Composable
fun DataFetchingScreen() {
    var items by remember { mutableStateOf(emptyList()) }

    LaunchedEffect(Unit) {
        val response = fetchDataFromApi()
        items = response ?: emptyList()
    }

    Surface(color = Color.Blue) {
        LazyColumn {
            items(items) { item ->
                Column(
                    modifier = Modifier
                        .fillMaxWidth()
                        .padding(16.dp)
                ) {
                    Text(text = item.id.toString(), fontSize = 24.sp, color = Color.White)
                    Spacer(modifier = Modifier.height(8.dp))
                    Text(text = item.name, fontSize = 24.sp, color = Color.White)
                }
            }
        }
    }
}

@Preview(showBackground = true)
@Composable
fun DefaultPreview() {
    MaterialTheme {
        DataFetchingScreen()
    }
}

// APIエンドポイント (localhostの場合は10.0.2.2を指定する)
private const val API_URL = "http://10.0.2.2:8080/sampleApi/api/sample"

private suspend fun fetchDataFromApi(): List? = withContext(Dispatchers.IO) {
    return@withContext try {
        val url = URL(API_URL)
        val connection = url.openConnection() as HttpURLConnection
        connection.requestMethod = "POST"
        connection.setRequestProperty("Content-Type", "application/json")
        connection.doOutput = true
        // リクエストボディにパラメータを追加
        val requestBody = """
            {
                "code": "Sample"
            }
        """.trimIndent()

        val dataOutputStream = DataOutputStream(connection.outputStream)
        dataOutputStream.writeBytes(requestBody)
        dataOutputStream.flush()
        dataOutputStream.close()

        val responseCode = connection.responseCode
        if (responseCode == HttpURLConnection.HTTP_OK) {
            val inputStream = connection.inputStream
            val reader = BufferedReader(InputStreamReader(inputStream))
            val responseData = StringBuilder()
            var line: String?
            while (reader.readLine().also { line = it } != null) {
                responseData.append(line)
            }
            reader.close()
            val jsonString = responseData.toString()
            // JSONをデシリアライズしてItemクラスのリストに変換
            val items = Json.decodeFromString>(jsonString)
            //リターン
            items
        } else {
            //リターン
            null
        }
    } catch (e: Exception) {
        e.printStackTrace()
        null
    }
}