Wait for user to choose from the backend

Question

I'm coding the backend portion of a software and at some point I need my user to choose some things, it can be a convoluted process, the user can even cancel the selection at any point.

From the back end I'd like to do something like:

private async void StartAction()
{
    //some code
    var SelectedItem = await UI.RequestUserToChooseItem();
    // some final code using the selected item
}

Here I don't know how to handle cancellation, but I can send null and assume that if the SelectedItem is null it was canceled.

But what about the UI portion of it? How do I handle it to return the call when the thing is selected by the user?

I need to perform some steps here: (this is pseudocode, I don't even know where to start)

public List<Item> RequestUserToChooseItem()
{
    PrepareItemsInList();
    ShowSelectionPanel();
    List<Items> SelectedItemsFromPanel = WaitForUserToChose(); //???????
    return SelectedItemsFromPanel;

}

And then we have the cancel button:

private void CancelButtonClicked(object sender, EventArgs e)
{
    CancelWaitedSelectionProcessAndReturnNull(); //????
}

Answer 1

Here is an asynchronous method that waits asynchronously for the first click on one or more buttons, and returns the clicked button:

public static Task<Button> OnClickAsync(params Button[] buttons)
{
    var tcs = new TaskCompletionSource<Button>();
    foreach (var button in buttons) button.Click += OnClick;
    return tcs.Task;

    void OnClick(object sender, RoutedEventArgs e)
    {
        foreach (var button in buttons) button.Click -= OnClick;
        tcs.SetResult((Button)sender);
    }
}

It could be used like this:

public async Task<List<Item>> RequestUserToChooseItemAsync()
{
    PrepareItemsInList();
    ShowSelectionPanel();
    var selectedButton = await OnClickAsync(btnOK, btnCancel);
    if (selectedButton == btnCancel) return null;
    return SelectedItemsFromPanel;
}

This method should be called exclusively on the UI thread, not on background threads, because it interacts with UI elements.