(MVP Pattern) How to forward error messages from Presenter to View?

Question

I have a method where the user can search for a article number and if its available in the database the articlenumber gets bound to a BindingList. Now I want to let the user know if the article is not available in database. How do I do that the right way?

Just pass the message errorMessage to my interface method?

Presenter:

string errorMessage;
_view.ErrorMessage(errorMessage);

View:

public void ErrorMessage(string errorMessage)
{
      MessageBox.Show(errorMessage);
}

Would you do it the same way?

Answer 1

We bubble an event. In the presenter you register that event:

public event PresenterEventHandler Message;

And then raise it like so:

PresenterEventArgs pe = new PresenterEventArgs("Error message", Status.Error);
this.Message(this, pe);

Then in the view:

protected override void OnInit(EventArgs e)
{
    this.presenter = new MyPresenter(this, MyBusinessService.Instance);
    this.presenter.Message += new PresenterEventHandler(presenter_Message);
}

void presenter_Message(object sender, PresenterEventArgs pe)
{
    // display error message
}

You can pass different types of statuses back to the view in this way, and not just error messages. We have Success, Error, Locked, Warning, Help.

Answer 2

In the case of error messages I would call some base functionality. This way you could choose wether to update the status window on the bottom left and/or display a modal message box.

In the presenter:

_windowManager.NoItemFound(errorMessage)

In the window manager:

_statusBox.Text = errorMessage; MessageBox.Show(errorMessage);

Answer 3

We should not re-invent the wheel ....

You should simply throw an exception in your model.

Then, the view will catch the exception using a try catch block.

In the "catch", show your message box.

Answer 4

If its MVP - Passive View, then the View interface should have a property that could read:

public interface IArticleListView {
    // more stuff here...
    bool ErrorMessageVisible { set; }
    string ErrorMessage { set; }
    // more stuff here...
    int ArticleNumber { get; }
}

public interface IPresenter {
    public void Update();
}

public class ArticleListPresenter : IPresenter {
    IViewArticleList _view;

    public ArticleListPresenter(IArticleListView view) {
        this._view = view;
        // you could update the view here or with an update method,
        // completely up to you.
    }

    // Assuming you are using a fine grained presenter
    public void HandleArticleNumberSearch() {
        bool articleNotFound;
        // search for the article ...    
        if ( articleNotFound ) {
            this._view.ErrorMessageVisible = true;
            this._view.ErrorMessage = string.Format("The article #{0} was not found.", number);
        }
    }
}

public class ArticleList : Page, IArticleListView {
    ArticleListPresenter _presenter;

    public bool ErrorMessageVisible {
        set { this.lblErrorMessage.Visible = value; }
    }

    public bool ErrorMessage {
        set { this.lblErrorMessage.Text = value; }
    }

    public int ArticleNumber {
        // You have to do some sort of validation here, but I'll keep it simple
        get { return Integer.Parse(this.txtArticleNumber.Text); }
    }

    protected override void OnInit(EventArgs e) {
        this._presenter = new ArticleListPresenter(this);
    }

    protected void Page_Load(object sender, EventArgs e) {
        // this implementation keeps the state in the View, and updates it
        // in the presenter: Passive View
        if(!this.IsPostBack) {
            this._presenter.Update();
        }
    }

    protected void OnSearchArticleButtonClick(object sender, EventArgs e) {
        this._presenter.HandleArticleNumberSearch();
    }
}