Progress Reporting from an Async method in a Windows 8 Store App

Question

In a Windows 8 Style app, I have the following code which is retrieving a list of files in a custom FileService. This is being fired in the Constructor of a ViewModel.

private async void Construct()
{
   Files = new ObservableCollection<FileViewModel>();
   IList _files = await _fileRepository.GetFiles();
   foreach (File file in _files)
   {
      Files.Add(new FileViewModel(file));
   }
}

It works perfectly, but what I am struggling to figure out is how I would perform Progress reporting on this to inform the user that something is happening while the files are being loaded.

Looking at it, I would like to have a bool IsBusy property which I could bind to something like a ProgressRing on the View. I get that I could set to this to true when the process starts but how can I get a Completed callback from operation where I could then set this property to False?

Many thanks!

Answer 1

First off, I think you shouldn't use async void methods (unless they are event handlers, of course). "Asynchronous constructors" are best implemented as a static async factory method or with asynchronous lazy initialization (explained on my blog). Both of these approaches are better than async void .

Now, on to progress reporting. The Task-based Asynchronous Programming document describes the recommended way of reporting progress, and my AsyncEx library includes an IProgress<T> implementation called PropertyProgress specifically for treating progress updates as ViewModel observable properties.

However, it sounds like you just need a "busy indicator", not a full "progress report". This is quite a bit simpler:

private async Task Construct()
{
  Files = new ObservableCollection<FileViewModel>();
  IsBusy = true;
  IList _files = await _fileRepository.GetFiles();
  IsBusy = false;
  foreach (File file in _files)
  {
    Files.Add(new FileViewModel(file));
  }
}

In this example, I'm assuming that the IsBusy setter will raise INotifyPropertyChanged.PropertyChanged appropriately.