Retrofit extensions to save file to Disk.

RetrofitExtensions.kt

import android.content.Context
import io.reactivex.Observable
import okhttp3.ResponseBody
import okio.Okio
import retrofit2.Response
import java.io.File

private val <T> Response<T>.attachmentFileName
    get() = headers()["Content-Disposition"]
        ?.removePrefix("attachment; filename=")

fun Observable<Response<ResponseBody>>.saveToDisk(context: Context, fileName: String? = null):
    Observable<File?> {
    return map { response ->
        val body = response.body() ?: return@map null
        val attachmentFileName = fileName ?: response.attachmentFileName ?: return@map null
        val destinationFile = File("${context.filesDir.path}/$attachmentFileName")

        val bufferedSink = Okio.buffer(Okio.sink(destinationFile))
        bufferedSink.writeAll(body.source())
        bufferedSink.close()

        destinationFile
    }
}

Sample usage:

// API Interface:
interface UserApi {
    @GET(Routes.photoUrl)
    fun getPhoto(): Observable<Response<ResponseBody>>
}

// In your Activity:
userApi
    .getPhoto()
    .saveToDisk(context)
	.subscribe()