GzipRequestInterceptor.kt

// The following code is referenced from the sources below. I encourage you to review + debug it yourself to get a better understanding.
// https://github.com/square/okhttp/issues/350#issuecomment-123105641
// https://github.com/square/okhttp/blob/f4ff4f4a8dce5f44596115f9564280e41d845f98/samples/guide/src/main/java/okhttp3/recipes/RequestBodyCompression.java#L73

private class GzipRequestInterceptor : Interceptor {

    override fun intercept(chain: Interceptor.Chain): Response {
        val originalRequest = chain.request()
        val body = originalRequest.body
        if (body == null || originalRequest.header("Content-Encoding") != null
        ) {
            return chain.proceed(originalRequest)
        }

        val compressedRequest = originalRequest.newBuilder()
            .header("Content-Encoding", "gzip")
            .method(originalRequest.method, gzip(body))
            .build()
        return chain.proceed(compressedRequest)
    }

    private fun gzip(body: RequestBody): RequestBody {
        return object : RequestBody() {
            override fun contentType(): MediaType? {
                return body.contentType()
            }

            override fun contentLength(): Long {
                return -1 // We don't know the compressed length in advance!
            }

            override fun writeTo(sink: BufferedSink) {
                val gzipSink = GzipSink(sink).buffer()
                body.writeTo(gzipSink)
                gzipSink.close()
            }
        }
    }
}