【Android】＜Kotlin＞OkHttpを使用して、ファイルをダウンロードし端末のフォルダに保存する実装方法を徹底解説！

<uses-permission android:name="android. permission. INTERNET />
<uses-permission android:name="android. permission.WRITE_EXTERNAL_STORAGE />

implementation ("com. squareup.okhttp3:0khttp:4.9.3")

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val downloadButton = findViewById<Button>(R.id.download_button)
        downloadButton.setOnClickListener() {
            val fileUrl = "https://drive.google.com/file/d/1_HESoWRVFMIIadNEF6Eu52JsYlnzBuh2/view?usp=sharing"
            val fileName = "sample.pdf"
            downloadFile(fileUrl, fileName)
        }
    }
}

private fun downloadFile(fileUrl: String, fileName: String) {
        CoroutineScope (Dispatchers. IO). launch {
            try {
                val client = OkHttpClient()
                val request = Request.Builder().url(fileUrl).build()
                // ダウンロード実行
                val response = client.newCall(request).execute()

                if (response.isSuccessful) {
                    // ダウンロードデータの取得
                    val inputStream: InputStream? = response.body?.byteStream()
                    val resolver = contentResolver
                    val contentValues = ContentValues().apply {
                        put(MediaStore.Downloads.DISPLAY_NAME, fileName)
                        put(MediaStore.Downloads.MIME_TYPE, "application/pdf")
                        put(MediaStore.Downloads.RELATIVE_PATH, Environment.DIRECTORY_DOWNLOADS)
                    }

                    val uri = resolver.insert(MediaStore.Downloads.EXTERNAL_CONTENT_URI, contentValues)
                    uri?.let { fileUri ->
                        resolver.openOutputStream(fileUri).use { outputStream ->
                            inputStream?.copyTo(outputStream!!)
                        }
                    }

                    withContext(Dispatchers.Main) {
                        Toast.makeText(applicationContext, "ダウンロード完了", Toast.LENGTH_LONG).show()
                    }
                } else {
                    withContext(Dispatchers.Main) {
                        Toast.makeText(applicationContext, "ダウンロード失敗", Toast.LENGTH_SHORT).show()
                    }
                }
            } catch(e: Exception) {
                withContext(Dispatchers.Main) {
                    Toast.makeText(applicationContext, "エラー", Toast.LENGTH_SHORT).show()
                }
            }
        }
    }