Code for the article: "Uploading a file with progress in Kotlin"

CountingRequestBody.kt

class CountingRequestBody(...) : RequestBody() {
    ...

    @Throws(IOException::class)
    override fun writeTo(sink: BufferedSink) {
        val countingSink = CountingSink(sink, this, onProgressUpdate)
        val bufferedSink = countingSink.buffer()

        requestBody.writeTo(bufferedSink)

        bufferedSink.flush()
    }
}