getting value from callback kotlin

I think your problem stems from the fact that you have a list of domain models with complex behaviors in your adapter where it should be really dumb view models that just represent the current state of each list item.

onBindViewHolder is only there to slap the current state of the item at the given position to the matching view at the same position. It should not be holding on to callbacks or trying to update itself.

You could try wrapping your domain model in an object that maintains the current state of progress and then notifies the owning adapter that it's time to refresh.

For example:

This is the item that will be in the adapter. It will wrap the data model and expose the current state for binding to the view.

class ListItem(val url : String) {
    val currentProgress = MutableLiveData<Int>(0)
    val dItem = DItem(url, object : callback {
        fun onProgress(int progress) {
            currentProgress.postValue(progress)
        }
        // ...
        // Store file or error from other functions as well
    }
 }

Then your activity creates a list of these objects instead of DItem, observes the state change, and notifies the adapter that that index needs to be refreshed when it changes:

class DownloadActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        val list = mutableListOf<ListItem>()
        list.add(ListItem("url").apply {
            currentProgress.observe(this@DownloadActivity) {
                // Simple implementation just refreshes the whole list when one progress changes
                // Exercise for the reader to pass along the specific index to mark as change
                adapter.notifyDataChanged()
            }
        })

Then your ViewHolder only cares about delegating the button click event and setting the view to the current state:

override fun onBindViewHolder(holder: MyViewhOdler, position: Int) {
    val listItem = list.get(position)
    holder.buttonThatTriggersDownload.setOnClickListener {
        // Set click listener to trigger download
    }
    holder.viewThatShowsDownloadProgress.value = listItem.currentProgress.value
}

If I understand it correctly, you want to get the "progress" value in "onBindViewHolder()" but you don't know how to do it.

One of the first things that I thought about, not modifying much of the code but may also be a dirty solution, is:

You actually define your object callback later, so the constructor will not accept "callback" anymore, instead it will be a var that you will initialize later.

class DItem(
    var url: String? = null,
    var fcallback: ((File, Error, Int) -> Unit)
) {
    lateinit var callback: FileDownloadManager.Listener
}

At this point, in "onBindViewHolder()" you can now define your callback and get the value in that place:

fun onBindViewHolder() {
    println("onBindViewHolder")
    list[0].callback = object : FileDownloadManager.Listener {
        override fun onSuccess(file: File) {
        }
        override fun onFailure(error: Error) {
        }
        override fun onProgress(progress: Int) {
            print("progress: $progress")
        }
    }
}

I hope I did understand the issue correctly and I really want to stress the fact that this is not necessarily a clean solution but one of the fastest to implement and with the least amount of code refactoring.