Publish an Event without PayLoad in Prism EventAggregator?

Question

Why can't we Publish Events without any PayLoad.

    _eventAggregator.GetEvent<SelectFolderEvent>().Publish(new SelectFolderEventCriteria() { });

Now, I don't need any pay load to be passed here. But the EventAggregator implementation mandates me to have an empty class to do that.

Event:

  public class SelectFolderEvent : CompositePresentationEvent<SelectFolderEventCriteria>
  {
  }

PayLoad:

  public class SelectFolderEventCriteria
  {
  }

Why has Prism not given a way to use just the Event and publish it like

    _eventAggregator.GetEvent<SelectFolderEvent>().Publish();

Is it by design and I don't understand it? Please explain. Thanks!

Answer 1

Good question, I don't see a reason for not publishing an event without a payload. There are cases where the fact that an event has been raised is all information you need and want to handle.

There are two options: As it is open source, you can take the Prism source and extract a CompositePresentation event that doesn't take a payload.

I wouldn't do that, but handle Prism as a 3rd party library and leave it as it is. It is good practice to write a Facade for a 3rd party library to fit it into your project, in this case for CompositePresentationEvent. This could look something like this:

public class EmptyPresentationEvent : EventBase
{
    /// <summary>
    /// Event which facade is for
    /// </summary>
    private readonly CompositePresentationEvent<object> _innerEvent;

    /// <summary>
    /// Dictionary which maps parameterless actions to wrapped 
    /// actions which take the ignored parameter 
    /// </summary>
    private readonly Dictionary<Action, Action<object>> _subscriberActions;

    public EmptyPresentationEvent()
    {
        _innerEvent = new CompositePresentationEvent<object>();
        _subscriberActions = new Dictionary<Action, Action<object>>();
    }

    public void Publish()
    {
        _innerEvent.Publish(null);
    }

    public void Subscribe(Action action)
    {
        Action<object> wrappedAction = o => action();
        _subscriberActions.Add(action, wrappedAction);
        _innerEvent.Subscribe(wrappedAction);
    }

    public void Unsubscribe(Action action)
    {
        if (!_subscriberActions.ContainsKey(action)) return;
        var wrappedActionToUnsubscribe = _subscriberActions[action];
        _innerEvent.Unsubscribe(wrappedActionToUnsubscribe);
        _subscriberActions.Remove(action);
    }
}

If anything is unclear, please ask.

Answer 2

Just to update the situation since this question was asked/answered, as of Prism 6.2, empty payloads are now supported in Prism PubSubEvents.

If you're using an older version, this blog shows how to create an "Empty" class that clearly indicates the intent of the payload: https://blog.davidpadbury.com/2010/01/01/empty-type-parameters/