MvvmActivities.cs

public class MainViewModel : ViewModelBase
{
  public MainViewModel()
  {
    ActivityProcessor processor = new ActivityProcessor();
    GetUnicornsCommand = processor.CreateCommand(GetUnicorns).WithContinuation(HandleGetUnicornErrors);
  }

  IEnumerable<IActivity> GetUnicorns()
  {
    yield return new BusyIndicator(true); // show that we're busy

    yield return new UnicornDownloader(); // get the unicorns!

    yield return new BusyIndicator(false); // we're no longer busy
  }

  void HandleGetUnicornErrors(ActivityContext activityContext)
  {
    foreach (var exception in activityContext.Exceptions)
    {
      if (exception.InnerException is AnimalNotFoundException)
      {
        // Log / show message / ...
        actitivityContext.Continue = true;
      }
      else
      {
        // do something with errors that we can't handle.
      }
    }
  }
}

public class ActivityCommand
{
  private readonly Func<IEnumerable<IActivity>> _execute;
  private Action<ActivityContext> _continuation;

  public void WithContinuation(Action<ActivityContext> continuation)
  {
    _continuation = continuation;
  }

  public void Execute(object parameter)
  {
    _processor.Process(_execute, _continuation, _completed);
  }
}

public class ActivityProcessor
{
  public void Process(Func<IEnumerable<IActivity>> execute, Action<ActivityContext> continuation, Action<ActivityContext> completedAction)
  {
    var activities = execute();
    foreach (var activity in activities)
    {
      try
      {
        activity.Execute();
      }
      catch (Exception e)
      {
        var activityException = new ActivityException("An error occurred in the activity", e);
        activityContext.Exceptions.Add(activityException);
        continuation(activityContext);
      }
    }
  }
}