Question

I’m trying to wrap my mind around the MVP pattern used in a C#/Winforms app. So I created a simple “notepad” like application to try to work out all the details. My goal is to create something that does the classic windows behaviors of open, save, new as well as reflecting the name of the saved file in the title bar. Also, when there are unsaved changes, the title bar should include an *.

So I created a view & a presenter that manage the application’s persistence state. One improvement I’ve considered is breaking out the text handling code so the view/presenter is truly a single-purpose entity.

Here is a screen shot for reference …

I’m including all of the relevant files below. I’m interested in feedback about whether I’ve done it the right way or if there are ways to improve.

NoteModel.cs:

public class NoteModel : INotifyPropertyChanged 
{
    public string Filename { get; set; }
    public bool IsDirty { get; set; }
    string _sText;
    public readonly string DefaultName = "Untitled.txt";

    public string TheText
    {
        get { return _sText; }
        set
        {
            _sText = value;
            PropertyHasChanged("TheText");
        }
    }

    public NoteModel()
    {
        Filename = DefaultName;
    }

    public void Save(string sFilename)
    {
        FileInfo fi = new FileInfo(sFilename);

        TextWriter tw = new StreamWriter(fi.FullName);
        tw.Write(TheText);
        tw.Close();

        Filename = fi.FullName;
        IsDirty = false;
    }

    public void Open(string sFilename)
    {
        FileInfo fi = new FileInfo(sFilename);

        TextReader tr = new StreamReader(fi.FullName);
        TheText = tr.ReadToEnd();
        tr.Close();

        Filename = fi.FullName;
        IsDirty = false;
    }

    private void PropertyHasChanged(string sPropName)
    {
        IsDirty = true;
        PropertyChanged.Invoke(this, new PropertyChangedEventArgs(sPropName));
    }


    #region INotifyPropertyChanged Members

    public event PropertyChangedEventHandler PropertyChanged;

    #endregion
}

Form2.cs:

public partial class Form2 : Form, IPersistenceStateView
{
    PersistenceStatePresenter _peristencePresenter;

    public Form2()
    {
        InitializeComponent();
    }

    #region IPersistenceStateView Members

    public string TheText
    {
        get { return this.textBox1.Text; }
        set { textBox1.Text = value; }
    }

    public void UpdateFormTitle(string sTitle)
    {
        this.Text = sTitle;
    }

    public string AskUserForSaveFilename()
    {
        SaveFileDialog dlg = new SaveFileDialog();
        DialogResult result = dlg.ShowDialog();
        if (result == DialogResult.Cancel)
            return null;
        else 
            return dlg.FileName;
    }

    public string AskUserForOpenFilename()
    {
        OpenFileDialog dlg = new OpenFileDialog();
        DialogResult result = dlg.ShowDialog();
        if (result == DialogResult.Cancel)
            return null;
        else
            return dlg.FileName;
    }

    public bool AskUserOkDiscardChanges()
    {
        DialogResult result = MessageBox.Show("You have unsaved changes. Do you want to continue without saving your changes?", "Disregard changes?", MessageBoxButtons.YesNo);

        if (result == DialogResult.Yes)
            return true;
        else
            return false;
    }

    public void NotifyUser(string sMessage)
    {
        MessageBox.Show(sMessage);
    }

    public void CloseView()
    {
        this.Dispose();
    }

    public void ClearView()
    {
        this.textBox1.Text = String.Empty;
    }

    #endregion

    private void btnSave_Click(object sender, EventArgs e)
    {
        _peristencePresenter.Save();
    }

    private void btnOpen_Click(object sender, EventArgs e)
    {
        _peristencePresenter.Open();
    }

    private void btnNew_Click(object sender, EventArgs e)
    {
        _peristencePresenter.CleanSlate();
    }

    private void Form2_Load(object sender, EventArgs e)
    {
        _peristencePresenter = new PersistenceStatePresenter(this);
    }

    private void Form2_FormClosing(object sender, FormClosingEventArgs e)
    {
        _peristencePresenter.Close();
        e.Cancel = true; // let the presenter handle the decision
    }

    private void textBox1_TextChanged(object sender, EventArgs e)
    {
        _peristencePresenter.TextModified();
    }
}

IPersistenceStateView.cs

public interface IPersistenceStateView
{
    string TheText { get; set; }

    void UpdateFormTitle(string sTitle);
    string AskUserForSaveFilename();
    string AskUserForOpenFilename();
    bool AskUserOkDiscardChanges();
    void NotifyUser(string sMessage);
    void CloseView();
    void ClearView();
}

PersistenceStatePresenter.cs

public class PersistenceStatePresenter
{
    IPersistenceStateView _view;
    NoteModel _model;

    public PersistenceStatePresenter(IPersistenceStateView view)
    {
        _view = view;

        InitializeModel();
        InitializeView();
    }

    private void InitializeModel()
    {
        _model = new NoteModel(); // could also be passed in as an argument.
        _model.PropertyChanged += new PropertyChangedEventHandler(_model_PropertyChanged);
    }

    private void InitializeView()
    {
        UpdateFormTitle();
    }

    private void _model_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
    {
        if (e.PropertyName == "TheText")
            _view.TheText = _model.TheText;

        UpdateFormTitle();
    }

    private void UpdateFormTitle()
    {
        string sTitle = _model.Filename;
        if (_model.IsDirty)
            sTitle += "*";

        _view.UpdateFormTitle(sTitle);
    }

    public void Save()
    {
        string sFilename;

        if (_model.Filename == _model.DefaultName || _model.Filename == null)
        {
            sFilename = _view.AskUserForSaveFilename();
            if (sFilename == null)
                return; // user canceled the save request.
        }
        else
            sFilename = _model.Filename;

        try
        {
            _model.Save(sFilename);
        }
        catch (Exception ex)
        {
            _view.NotifyUser("Could not save your file.");
        }

        UpdateFormTitle();
    }

    public void TextModified()
    {
        _model.TheText = _view.TheText;
    }

    public void Open()
    {
        CleanSlate();

        string sFilename = _view.AskUserForOpenFilename();

        if (sFilename == null)
            return;

        _model.Open(sFilename);
        _model.IsDirty = false;
        UpdateFormTitle();
    }

    public void Close()
    {
        bool bCanClose = true;

        if (_model.IsDirty)
            bCanClose = _view.AskUserOkDiscardChanges();

        if (bCanClose)
        {
            _view.CloseView();
        }
    }

    public void CleanSlate()
    {
        bool bCanClear = true;

        if (_model.IsDirty)
            bCanClear = _view.AskUserOkDiscardChanges();

        if (bCanClear)
        {
            _view.ClearView();
            InitializeModel();
            InitializeView();
        }
    }
}

Answer 1

The only way to get any closer to a perfect MVP passive view pattern would be to write your own MVP triads for the dialogs instead of using the WinForms dialogs. Then you could move the dialog creation logic from the view to the presenter.

This gets into the topic of communication between mvp triads, a topic which is usually glossed over when examining this pattern. What I’ve found works for me is to connect triads at their presenters.

public class PersistenceStatePresenter
{
    ...
    public Save
    {
        string sFilename;

        if (_model.Filename == _model.DefaultName || _model.Filename == null)
        {
            var openDialogPresenter = new OpenDialogPresenter();
            openDialogPresenter.Show();
            if(!openDialogPresenter.Cancel)
            {
                return; // user canceled the save request.
            }
            else
                sFilename = openDialogPresenter.FileName;

        ...

The Show() method, of course, is responsible for showing an unmentioned OpenDialogView, which would accept the users input and pass it along to the OpenDialogPresenter. In any case, it should be starting to become clear that a presenter is an elaborate middleman. Under different circumstances, you might be tempted to refactor a middleman out but here its is intentional to:

• Keep logic out of the view, where it is harder to test
• Avoid direct dependencies between the view and the model

At times I’ve also seen the model used for MVP triad communication. The benefit of this is the presenter’s don’t need to know each other exist. Its usually accomplished by setting a state in the model, which triggers an event, which another presenter then listens for. An interesting idea. One I’ve not used personally.

Here’s a few links with some of the techniques others have used to deal with triad communication:

• http://www.rickardnilsson.net/post/The-Humble-dialog-v2.aspx
• http://spin.atomicobject.com/2008/01/30/presenter-first-get-your-triads-talking/