Question

Somehow I cannot believe that I am the first one to run into that problem (and I don’t want to believe that I am the only one stupid enough not to see a solution directly), but my search-fu was not strong enough.

I regularly run into a situation, when I need to do a few time-consuming steps one after the other. The workflow looks like

var data = DataGetter.GetData();
var processedData = DataProcessor.Process(data);
var userDecision = DialogService.AskUserAbout(processedData);
// ...

I don’t want to block the UI during each step, so every method does return immediately, and raises an event once it has finished. Now hilarity ensues, since the above code block mutates into

DataGetter.Finished += (data) =>
    {
        DataProcessor.Finished += (processedData) =>
        {
            DialogService.Finished(userDecision) =>
                {
                    // ....
                }
                DialogService.AskUserAbout(processedData);
            }
        DataProcessor.Process(data);
    };
DataGetter.GetData();

This reads too much like Continuation-passing style for my taste, and there has to be a better way to structure this code. But how?

Answer 1

The correct way would be to design your components in a synchronous way and execute the complete chain in a background thread.