Question

in a classic Passive-MVP pattern, how can i avoid a reference of the presenter in my view completely & still inject the presenter instance which needs the view instance as a parameter.

with asp.net as an example:

• my implemented views (web project) should not have a reference to the Presenters. (Neither IPresenter nor the concrete ones)
• when the view instantiates, (basically my web page), the presenter should be instantiated with the current view’s reference.
• i am using unity as my ioc container.

right now what i do in the web page’s code behind is this:

public partial class SomePage : MyBasePage, ISomeView
{
    private readonly ISomePresenter presenter;

    public SomePage()
    {
        this.presenter = ResolveSomeWay(this);
    }
}

for this i have a reference of the ‘Presenter Contracts DLL’ in my view implementation. is there a way to avoid this reference completely & still hook up the presenter with the view instance, when the view instantiates?

i just care about the presenter instantiation, since the presenter’s constructor can set the passed parameter-view-instance to its View Property & it subscribes to the view’s events, for any future communication.

thanks folks for your time.

Answer 1

You could “publish” a new View was instantiated to a Message Bus, to which a Presenter factory could “bind” the instantiated View to a Presenter. Although the View would be agnostic of the presenter, it would not be of the Message Bus.

public partial class SomePage : MyBasePage, ISomeView
{
    // Alternative #1
    public SomePage(IMessageBus messageBus)
    {
        // You publish a message saying that a handler for ISomeView is to handle the
        // message.
        messageBus.Publish<ISomeView>(this);
    }
    // Alternative #2
    public SomePage()
    {
        Resolver.Resolve<IMessageBus>().Publish<ISomeView>(this);
    }
}

// This could be somewhere else in your application (this could be a separate DLL), but 
// I will use the Global.asax.cs here, for simplicity
public void Application_Start()
{
    Container.Resolve<IMessageBus>()
             .Subscribe<ISomeView>(someView => 
                 {
                    var presenter = new SomePresenter(someView);
                 });
}

public interface ISomeView {
    event Action<ISomeView> SomeEvent;
}

public class SomePresenter
{
    public SomePresenter(ISomeView view) {
        // if you attach the presenter to the View event, 
        // the Garbage Collector won't finalize the Presenter until
        // the View is finalized too
        view.SomeEvent += SomeEventHandler;
    }

    private void SomeEventHandler(ISomeView view) { 
       // handle the event
    }
}