AsyncPresentationEvent: how to know what Task to return?

KirkKuykendall1

Hi -

How do I figure out what to return in a Task when handling an AsyncPresentationEvent?

For example this sample shows Task.FromResult(0) being returned, how was this (0) determined?

protected void subEditEvents()
{
  //subscribe to editcompleted
  var eceToken = EditCompletedEvent.Subscribe(onEce);
}

protected Task onEce(EditCompletedEventArgs args)
{
  //show number of edits
  Console.WriteLine("Creates: " + args.Creates.Values.Sum(list => list.Count).ToString());
  Console.WriteLine("Modifies: " + args.Modifies.Values.Sum(list => list.Count).ToString());
  Console.WriteLine("Deletes: " + args.Deletes.Values.Sum(list => list.Count).ToString());
  return Task.FromResult(0);
}

 

Thanks, Kirk

 

Wolf

If you hover over the EditCompletedEvent.Subscribe method in Visual Studio (or use the Pro SDK API reference) you find the following parameters for the Subscribe method:

 

public static SubscriptionToken Subscribe( 
   Func<EditCompletedEventArgs,Task> action,
   bool keepSubscriberAlive
)

 

This shows that the first parameter is a delegate function that has an input parameter of the type 'EditCompletedEventArgs' and returns a class of the type System.Threading.Tasks.Task  .  The Task class is derived from the 'Task <TResult> class' .  Both variations of the Task class represent a single operation that usually executes asynchronously, however, whereas 'Task<TResult>' returns a value (of the type TResult), 'Task' doesn't return a value.  Consequently your delegate above 'onEce' has to return an instance of the Task class and the way to do this is to use the 'Task.FromResult<TResult>(TResult)' method and since TResult is not required you can specify either null or 0 as the parameter.  FromResult returns an instance of the task class that you need as your return parameter.