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How to download files using Retrofit in Kotlin Android

How to download files using Retrofit in Kotlin Android.

Here's a detailed step-by-step tutorial on how to download files using Retrofit in Kotlin Android.

Step 1: Add Retrofit and OkHttp Dependencies​

First, you need to add the Retrofit and OkHttp dependencies to your project's build.gradle file.

dependencies {
    implementation 'com.squareup.retrofit2:retrofit:2.9.0'
    implementation 'com.squareup.okhttp3:okhttp:4.9.0'
}

Step 2: Create a Retrofit Service Interface​

Next, create a service interface that defines the API endpoints for downloading files. This interface will use Retrofit annotations to specify the HTTP method, URL, and other details.

interface FileDownloadService {
    @GET
    @Streaming
    suspend fun downloadFile(@Url url: String): ResponseBody
}

In this example, we use the @GET annotation to specify the HTTP GET method, and @Streaming to indicate that we want to download a large file. The @Url annotation is used to dynamically pass the file URL.

Step 3: Create a Retrofit Instance​

Now, create a Retrofit instance with the base URL of the API.

val retrofit = Retrofit.Builder()
    .baseUrl(BASE_URL)
    .client(OkHttpClient())
    .build()

val service = retrofit.create(FileDownloadService::class.java)

Replace BASE_URL with the base URL of the API you're working with. The OkHttpClient is used as the HTTP client for making network requests.

Step 4: Download the File​

To download a file, you can simply call the downloadFile function on the Retrofit service interface.

val url = "https://example.com/file.txt"
val response = service.downloadFile(url)

In this example, we pass the URL of the file we want to download. The downloadFile function will return a ResponseBody object, which represents the raw file data.

Step 5: Save the File​

To save the downloaded file, you can use the response.body()?.byteStream() method to get an InputStream from the ResponseBody. You can then use this stream to write the file data to a local file.

val inputStream = response.body()?.byteStream()
val outputFile = File(context.filesDir, "file.txt")

inputStream?.let {
    outputFile.outputStream().use { fileOut ->
        inputStream.copyTo(fileOut)
    }
}

In this example, we create a File object to represent the output file location. We then use the copyTo extension function to copy the file data from the input stream to the output file.

Step 6: Handle Errors and Progress​

You can also handle errors and track the download progress by adding appropriate code. For example, you can handle network errors by wrapping the download code in a try-catch block.

try {
    val response = service.downloadFile(url)
    // Save the file
} catch (e: Exception) {
    // Handle the error
}

To track the download progress, you can use the response.body()?.contentLength() method to get the total file size, and then monitor the number of bytes read from the input stream.

val contentLength = response.body()?.contentLength()
val inputStream = response.body()?.byteStream()
val outputFile = File(context.filesDir, "file.txt")

inputStream?.let {
    outputFile.outputStream().use { fileOut ->
        val buffer = ByteArray(4096)
        var bytesRead = inputStream.read(buffer)
        var totalBytesRead = bytesRead

        while (bytesRead != -1) {
            fileOut.write(buffer, 0, bytesRead)
            bytesRead = inputStream.read(buffer)
            totalBytesRead += bytesRead

            // Calculate progress percentage
            val progress = (totalBytesRead.toDouble() / contentLength!!) * 100

            // Update UI with progress
        }
    }
}

In this example, we use a buffer to read the file data in chunks, and update the progress based on the total bytes read.

That's it! You now have a step-by-step tutorial on how to download files using Retrofit in Kotlin Android. You can customize the code as per your requirements and handle additional functionality as needed.