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

CountingSink.kt

typealias CountingRequestListener = (bytesWritten: Long, contentLength: Long) -> Unit

class CountingSink(
    sink: Sink,
    private val requestBody: RequestBody,
    private val onProgressUpdate: CountingRequestListener
) : ForwardingSink(sink) {
    private var bytesWritten = 0L

    override fun write(source: Buffer, byteCount: Long) {
        super.write(source, byteCount)

        bytesWritten += byteCount
        onProgressUpdate(bytesWritten, requestBody.contentLength())
    }
}