Question

I have implemented the MVP pattern in a few WinForms applications that uses a passive view. I implement an interface containing properties and delegates in the form of Action< T > and Func< T > to wire up UI events in concrete views and call back to the presenter.

I am about to start a new project and did a little research into the pattern online including the many examples of the pattern here and notice that the all use EventHandler to notify presenters.

I don’t really understand why events would be used in this case, since I see a view as only having one presenter.

My question is, is this for the sake of remaining consistent with how the .Net framework uses events, or for some other reason that I am not seeing?

Here is a trivial example of the pattern I use:

public interface IViewAbstraction
{
    public ModelData ModelData { set; }
    public Action<ModelData> ModelDataChangedCallback { set; }
}

public class SomeWinForm : Form, IViewAbstraction
{
    private Action<ModelData> modelDataChanged;
    private ModelData model;

    public ModelData ModelData
    {
        set { /* when this property changes, update UI */ }
    }

    public Action<ModelData> ModelDataChangedCallback
    {
        set { modelDataChanged = value; }
    }

    private void OnSomeWinformsEvent(EventArgs args)
    {
        if (modelDataChanged == null) throw new Exception();

        modelDataChanged(model);
    }
}

public class Presenter
{
    private readonly IViewAbstraction view;
    private readonly IDataLayerAbstraction dataLayer;

    public Presenter(IViewAbstraction view, IDataLayerAbstraction dataLayer)
    {
        this.dataLayer = dataLayer;
        this.view = view;
        this.view.ModelDataChangedCallback = OnModelChanged;
        this.view.ModelData = dataLayer.GetData();
    }

    private void OnModelChanged(ModelData data)
    {
        // validate and save data.
    }
}

Answer 1

Your pattern is essentially the same as using events, with a key difference. Events don’t expose the underlying delegate (ModelDataChangedCallback in your example). Exposing this is bad practice as other code can clear the invocation list for example. An event will have an underlying delegate that can be added to or removed from, but never cleared from outside the scope of the class.

I don’t understand your point about not having multiple subscribers – this isn’t a reason to not use events. An event is just a way for a class to say “Hey! This thing has happened”, a 1-to-1 mapping with a Presenter object is perfectly reasonable and normal.

You also don’t end up with that rather odd looking write-only property in the view.