使用 Retrofit 与协程下载 PDF 文件

1. Overview

When working with documents stored in the cloud, we sometimes need to download them to perform additional processing. The process of downloading can be automated with a program that can take the URL as a parameter to get these documents and store them later in the file system of our choice.

In this tutorial, we’ll see how to download PDF files with Kotlin using Coroutines and the Retrofit library.

2. Implementation

Using Retrofit and Kotlin Coroutines is an efficient approach when we want to perform HTTP requests to get resources stored in the cloud. In the next flowchart, we can see the process we’ll apply:

PDF Download Process

Let’s start defining the first component of the process.

3. Retrofit Service Interface

First, we need to define a service interface for Retrofit that will expose a function for downloading files:

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

The @GET annotation specifies the HTTP method we want to execute, and the @Url parameter allows us to dynamically pass the URL of the file we need to download.

4. Retrofit Instance

Now we need to create and setup the Retrofit instance to perform the HTTP requests:

val retrofit = Retrofit.Builder()
  .baseUrl("https://example.com/")
  .build()

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

Since we’ll pass the @Url parameter dynamically, the baseUrl will not be used in the request. However, this is a required parameter in Retrofit configuration and for that reason, we need to define a valid URL as a placeholder.

5. Download the PDF

Now that our Retrofit service is ready, we can execute the download process in a suspend function. We need the Dispatchers.IO to ensure that the operation runs on a separate thread:

suspend fun downloadPdfWithRetrofit(url: String, outputFile: File) {
    // retrofit service setup

    withContext(Dispatchers.IO) {
        val responseBody = service.downloadFile(url)

        // Save the file to disk
        responseBody.byteStream().use { inputStream ->
            FileOutputStream(outputFile).use { outputStream ->
                inputStream.copyTo(outputStream)
            }
        }

        println("File downloaded successfully to ${outputFile.absolutePath}")
    }
}

6. Execute the Coroutine

Now we can execute our function in a runBlocking block. This block provides the support to run coroutines functions synchronously. In the next example, we can see how to pass the URL parameter to the function and download the PDF file:

fun main() {
    val url = "https://www.w3.org/WAI/ER/tests/xhtml/testfiles/resources/pdf/dummy.pdf"
    val outputFile = File("sample.pdf")

    runBlocking {
        try {
            downloadPdfWithRetrofit(url, outputFile)
        } catch (e: Exception) {
            println("Failed to download PDF: ${e.message}")
        }
    }
}

7. Conclusion

In this tutorial, we’ve explored an efficient approach to downloading PDF files using Retrofit and the support of Kotlin Coroutines.

Persistence

REST

Security