Pause and resume downloads using Retrofit

Question

I used this tutorial to implement downloading files in my app: https://www.learn2crack.com/2016/05/downloading-file-using-retrofit.html

The problem is that if intern

Answer 1

This is my Kotlin implementation, inspired by Клаус Шварц:

i used Coroutines because they make the code very easy to read and use; i also used ru.gildor.coroutines:kotlin-coroutines-retrofit to add coroutine support to retrofit.

import okhttp3.OkHttpClient
import okhttp3.ResponseBody
import okhttp3.logging.HttpLoggingInterceptor
import okio.Buffer
import okio.BufferedSink
import okio.ForwardingSource
import okio.Okio
import retrofit2.Call
import retrofit2.HttpException
import retrofit2.Response
import retrofit2.Retrofit
import retrofit2.http.GET
import retrofit2.http.HeaderMap
import retrofit2.http.Streaming
import retrofit2.http.Url
import ru.gildor.coroutines.retrofit.awaitResponse
import java.io.File
import java.util.concurrent.TimeUnit
import java.util.regex.Pattern


object FileDownloader{

    private val Service by lazy { serviceBuilder().create(FileDownloaderInterface::class.java) }

    val baseUrl = "http://www.your-website-base-url.com"

    private fun serviceBuilder(): Retrofit {
        //--- OkHttp client ---//
        val okHttpClient = OkHttpClient.Builder()
                .readTimeout(60, TimeUnit.SECONDS)
                .connectTimeout(60, TimeUnit.SECONDS)

        //--- Add authentication headers ---//
        okHttpClient.addInterceptor { chain ->
            val original = chain.request()

            // Just some example headers
            val requestBuilder = original.newBuilder()
                    .addHeader("Connection","keep-alive")
                    .header("User-Agent", "downloader")

            val request = requestBuilder.build()
            chain.proceed(request)
        }

        //--- Add logging ---//

        if (BuildConfig.DEBUG) {
            // development build
            val logging = HttpLoggingInterceptor()
            logging.setLevel(HttpLoggingInterceptor.Level.BASIC)
            // NOTE: do NOT use request BODY logging or it will not work!

            okHttpClient.addInterceptor(logging)
        }


        //--- Return Retrofit class ---//
        return Retrofit.Builder()
                .client(okHttpClient.build())
                .baseUrl(baseUrl)
                .build()
    }

    suspend fun downloadOrResume(
            url:String, destination: File,
            headers:HashMap = HashMap(),
            onProgress: ((percent: Int, downloaded: Long, total: Long) -> Unit)? = null
        ){

        var startingFrom = 0L
        if(destination.exists() && destination.length()>0L){
            startingFrom = destination.length()
            headers.put("Range","bytes=${startingFrom}-")
        }
        println("Download starting from $startingFrom - headers: $headers")

        download(url,destination,headers,onProgress)
    }

    suspend fun download(
            url:String,
            destination: File,
            headers:HashMap = HashMap(),
            onProgress: ((percent: Int, downloaded: Long, total: Long) -> Unit)? = null
        ) {
        println("---------- downloadFileByUrl: getting response -------------")
        val response = Service.downloadFile(url,headers).awaitResponse()
        handleDownloadResponse(response,destination,onProgress)
    }

    fun handleDownloadResponse(
            response:Response,
            destination:File,
            onProgress: ((percent: Int, downloaded: Long, total: Long) -> Unit)?
    ) {
        println("-- downloadFileByUrl: parsing response! $response")


        var startingByte = 0L
        var endingByte = 0L
        var totalBytes = 0L


        if(!response.isSuccessful) {
            throw HttpException(response)
            //java.lang.IllegalStateException: Error downloading file: 416, Requested Range Not Satisfiable; Response Response{protocol=http/1.1, code=416, message=Requested Range Not Satisfiable, u
        }
        val contentLength = response.body()!!.contentLength()

        if (response.code() == 206) {
            println("- http 206: Continue download")
            val matcher = Pattern.compile("bytes ([0-9]*)-([0-9]*)/([0-9]*)").matcher(response.headers().get("Content-Range"))
            if (matcher.find()) {
                startingByte = matcher.group(1).toLong()
                endingByte = matcher.group(2).toLong()
                totalBytes = matcher.group(3).toLong()
            }
            println("Getting range from $startingByte to ${endingByte} of ${totalBytes} bytes" )
        } else {
            println("- new download")
            endingByte = contentLength
            totalBytes = contentLength
            if (destination.exists()) {
                println("Delete previous download!")
                destination.delete()
            }
        }


        println("Getting range from $startingByte to ${endingByte} of ${totalBytes} bytes" )
        val sink: BufferedSink
        if (startingByte > 0) {
            sink = Okio.buffer(Okio.appendingSink(destination))
        } else {
            sink = Okio.buffer(Okio.sink(destination))
        }


        var lastPercentage=-1
        var totalRead=startingByte
        sink.use {
            it.writeAll(object : ForwardingSource(response.body()!!.source()) {

                override fun read(sink: Buffer, byteCount: Long): Long {
                    //println("- Reading... $byteCount")
                    val bytesRead = super.read(sink, byteCount)

                    totalRead += bytesRead

                    val currentPercentage = (totalRead * 100 / totalBytes).toInt()
                    //println("Progress: $currentPercentage - $totalRead")
                    if (currentPercentage > lastPercentage) {
                        lastPercentage = currentPercentage
                        if(onProgress!=null){
                            onProgress(currentPercentage,totalRead,totalBytes)
                        }
                    }
                    return bytesRead
                }
            })
        }

        println("--- Download complete!")
    }

    internal interface FileDownloaderInterface{
        @Streaming
        @GET
        fun downloadFile(
                @Url fileUrl: String,
                @HeaderMap headers:Map
        ): Call
    }
}

Example usage:

    val url = "https://cdimage.debian.org/debian-cd/current/amd64/iso-cd/debian-9.4.0-amd64-xfce-CD-1.iso"
    val destination = File(context.filesDir, "debian-9.4.0-amd64-xfce-CD-1.iso")

    //Optional: you can also add custom headers
    val headers = HashMap()

    try {
        // Start or continue a download, catch download exceptions
        FileDownloader.downloadOrResume(
                url,
                destination,
                headers,
                onProgress = { progress, read, total ->
                    println(">>> Download $progress% ($read/$total b)")
                });
    }catch(e: SocketTimeoutException){
        println("Download socket TIMEOUT exception: $e")
    }catch(e: SocketException){
        println("Download socket exception: $e")
    }catch(e: HttpException){
        println("Download HTTP exception: $e")
    }

Gradle dependencies

dependencies {
    /** Retrofit 2 **/
    compile 'com.squareup.retrofit2:retrofit:2.4.0'

    // OkHttp for Retrofit request customization
    compile 'com.squareup.okhttp3:okhttp:3.10.0'

    // For http request logging
    compile 'com.squareup.okhttp3:logging-interceptor:3.10.0'

    // Retrofit Kotlin coroutines support
    compile 'ru.gildor.coroutines:kotlin-coroutines-retrofit:0.9.0'
}

Note: Kotlin coroutines must be enabled, at the moment they need to be enabled as experimental