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

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();
}
}

Answer 5

That's what I do.
Another way I've read about would be for the model to know how to show an error (perhaps by an ErrorMessagePresenter) so the error is detached from the original presenter.
I haven't really found a use for that, for me, it always ends in the presenter and view implementing both interfaces.