ImageFileThread.kt

class ImageFileThread(
    private val mainHandler: Handler,
    private val onAvailableToSendMessage: Handler.() -> Unit
) : Thread() {
    
    private var handler: Handler? = null
    
    override fun run() {
        Looper.prepare()
        handler = Handler(checkNotNull(Looper.myLooper())) { msg: Message -> 
            // get data from received message
            val imageUrl = msg.data.getString("image_url")                                                            

            // do some work here
            val imageFilePath = getImageFileFromUrl(imageUrl).absolutePath

            // create message object and put the result from work
            val result = Message().apply { data = bundleOf("image_file" to imageFilePath) }
                                                            
            // send result to main thread
            mainHandler.sendMessage(result)
            true
        }
        handler?.onAvailableToSendMessage()
        Looper.loop()
    }
    
    private fun getImageFileFromUrl(url: String?): File { ... }
}