L'interface sera constituée des éléments suivants :
- TextBox pour inscrire son Pseudo
- TextBox pour saisir les messages
- ScrollViewer pour afficher les messages

Un aperçu de l'interface :

On utilisera l'objet WebClient pour la communication HTTP

Ecriture d'un message :
Client SL -> WebClient -> POST "data" -> write_msg.php : écriture dans un dossier "/msg" d'un fichier référencé temporellement (timestamped) sur le serveur
Retour HTTP : 0

Relever un message :
Client SL -> WebClient -> read_msg.php : lecture de tous les fichiers "timestamped"
Retour HTTP : fichier si non encore lu ou nothing

Le Script write_msg.php :

<?php
if (isset($_POST["user"]))
{
    list($usec, $sec) = explode(" ", microtime());
    $ts = $sec.substr($usec,2,4);
    // On crée le fichier message dans le dossier msg
    $fichier = fopen('msg/'.$ts, 'w+');
    fwrite($fichier, stripslashes($_POST['data']));
    fclose($fichier);
    echo "0";
    $dossier = opendir('msg');
    while ($fichier = readdir($dossier))
    {
        if (!preg_match("#^[.]{1,2}[a-z0-9 -.]*#i",$fichier))
        {
            // On en profite pour purger les msg plus vieux que 15 secondes
            if ($fichier < ($ts - 150000))
            {
                unlink('msg/'.$fichier);
            }
        }
    }
    closedir($dossier);
}
?>

Le script read_msg.php :

<?php
session_start();
// ouvrir le dosssier msg
$dossier = opendir('msg');
 
while ($fichier = readdir($dossier))
{
    if (!preg_match("#^[.]{1,2}[a-z0-9 -.]*#i",$fichier))    
    {
        $files_array[] = $fichier;      
    }
}
closedir($dossier);
 
sort($files_array);
 
foreach ($files_array as $key => $val)
{
   if ($_SESSION['last_msg_ts'] < $val)
   {
        // Lit le message non lu
        readfile('msg/'.$val,'r');
        // Indique le dernier message lu
        $_SESSION['last_msg_ts'] = $val;
        exit;       
   }
}
?>

Les données sont envoyées de manière asynchrone avec une requête POST.
Suite à l'appui du bouton Send :
- Stop à la réception des messages
- Transmission au script write_msg.php les paramètres "user" et "data".
- Attente du retour serveur de la valeur "0", puis relance de la réception des messages.
Coté serveur :
- écriture du fichier référencé temporellement par rapport à l'heure du serveur
- vérification des vieux messages présents dans le dossier '/msg'.
- S'il sont plus vieux de 15 secondes on les supprime.

Private Sub SendMsg(ByVal Data As String)
 
        ReadMsgTmr.Stop()
 
        Try
            Dim Client As New WebClient()
            Client.Headers("Content-Type") = "application/x-www-form-urlencoded"
            AddHandler Client.UploadStringCompleted, AddressOf SendMsgCompleted
            Client.UploadStringAsync(New Uri(String.Format("{0}/write_msg.php",
            BASEPATH)),    "Post", "user=" & UsrTbx.Text & "&data=" & Data)
 
        Catch ex As Exception
 
        End Try
 
 
    End Sub
 
    Private Sub SendMsgCompleted(ByVal sender As Object, _
                                 ByVal e As UploadStringCompletedEventArgs)
 
        Try
            Dim Msg As String = e.Result
            Select Case Msg
                Case Nothing
                    ReadMsgTmr.Interval = System.TimeSpan.FromMilliseconds(1)
                    ReadMsgTmr.Start()
                    Exit Sub
                Case "0"
                    ReadMsgTmr.Interval = System.TimeSpan.FromMilliseconds(1)
                    ReadMsgTmr.Start()
            End Select
        Catch ex As Exception
 
        End Try
 
    End Sub

Le relevé des messages est fait de façon périodique selon la logique suivante :
Mode "Idle" :
- Toute les 1,5 seconde
Mode "Rafale" :
- Toute les 0,15 seconde
Mode "After Writing" :
- Aussitot après avoir posté un message

Nota : on aurait pu imaginer un "LongIdle" de 3 secondes pour les périodes d'inactivité ... longue.

    Private Sub ReadMsgTmr_Tick(ByVal sender As Object, _
                                ByVal e As System.EventArgs) Handles ReadMsgTmr.Tick
 
        ReadMsgTmr.Stop()
 
        Try
 
            Dim Client = New WebClient
            AddHandler Client.DownloadStringCompleted, AddressOf ReadMsgComplete
            Client.DownloadStringAsync(New Uri(Config.BASEPATH & "/read_msg.php", UriKind.Absolute), 0)
 
            Client = Nothing
 
        Catch ex As Exception
            'HtmlPage.Window.Alert(ex.Message)
        End Try
 
    End Sub
 
    Private Sub ReadMsgComplete(ByVal sender As Object, _
                                ByVal e As DownloadStringCompletedEventArgs)
        Try
            ' Si il y a un message
            Dim Msg As String = e.Result
            If Msg <> "" Then DisplayTextinChatBox(Msg)
                ' raffale
                ReadMsgTmr.Interval = System.TimeSpan.FromMilliseconds(150)
            Else
                ' idle
                ReadMsgTmr.Interval = System.TimeSpan.FromMilliseconds(1500)
            End If
 
        Catch ex As Exception
 
        End Try
        ReadMsgTmr.Start()
 
    End Sub

Un simple copier coller des fichiers via ftp, on change le CHMOD du dossier "msg" à 777 et voilà :)

Le code VB complet :

Imports System.IO.IsolatedStorage
Imports System.Windows.Threading
 
Partial Public Class MainPage
    Inherits UserControl
    Private AppSetting As IsolatedStorageSettings = IsolatedStorageSettings.ApplicationSettings
    Private WithEvents ReadMsgTmr As New DispatcherTimer
    Private Smoke As Boolean
    Private BASEPATH As String = "" ' "http://www.monsite.fr/MiniChat" 'Test path
    Private Client As New WebClient
 
    Public Sub New()
        InitializeComponent()
        AddHandler Client.DownloadStringCompleted, AddressOf ReadMsgComplete
        ReadMsgTmr.Interval = TimeSpan.FromMilliseconds(1500)
        If AppSetting.Keys.Count > 0 Then
            For Each Key In AppSetting.Keys
                Select Case Key.ToString
                    Case "UserName"
                        If AppSetting("UserName").ToString <> "" Then
                            UsrTbx.Text = AppSetting("UserName").ToString
                        End If
                End Select
            Next
        End If
        MsgTbx.Focus()
        ReadMsgTmr.Start()
 
        'Dim Src As System.Uri = Application.Current.Host.Source
        If BASEPATH = "" Then
            Dim uri As String = Application.Current.Host.Source.AbsoluteUri
            uri = uri.Substring(0, uri.IndexOf("/ChatSL2phpVB.xap"))
            BASEPATH = uri ' : SendMsg(uri)
            'supprimez la ligne ci dessous pour compiler une version déployable
            BASEPATH = "http://www.furukoo.fr/MiniChat"
        End If
 
    End Sub
 
    Public Sub DisplayTextinChatBox(ByVal Msg As String)
 
        If Msg Is Nothing Then Exit Sub
        '' Crée un TextBlock
        Dim Tbl As New TextBox
        With Tbl
            .Text = Msg
            .TextWrapping = TextWrapping.Wrap
            .IsReadOnly = True
            .Style = DirectCast(Me.Resources("ChatStyle"), Windows.Style)
            If Smoke Then
                .Background = New SolidColorBrush(Color.FromArgb(255, 248, 248, 248))
                Smoke = False
            Else
                Smoke = True
            End If
        End With
 
        With MsgSpl.Children
            '' Limite le nombre de messages dans la ListBox
            If .Count > 50 Then .RemoveAt(50)
            .Insert(0, Tbl)
        End With
 
        ' Déplace l'ascenseur vers le haut
        MsgSvr.ScrollToVerticalOffset(0)
 
    End Sub
 
    Private Sub SendMsg(ByVal Data As String)
 
        ReadMsgTmr.Stop()
 
        Try
            Dim Client As New WebClient()
            Client.Headers("Content-Type") = "application/x-www-form-urlencoded"
            AddHandler Client.UploadStringCompleted, AddressOf SendMsgCompleted
            Client.UploadStringAsync(New Uri(String.Format("{0}/write_msg.php", BASEPATH)), _
                                     "Post", _
                                     "user=" & UsrTbx.Text & "&data=" & Data)
 
        Catch ex As Exception
 
        End Try
 
    End Sub
 
    Private Sub SendMsgCompleted(ByVal sender As Object, _
                                 ByVal e As UploadStringCompletedEventArgs)
 
        Try
            Dim Msg As String = e.Result
            Select Case Msg
                Case Nothing
                    ReadMsgTmr.Interval = System.TimeSpan.FromMilliseconds(1)
                    ReadMsgTmr.Start()
                    Exit Sub
                Case "0"
                    ReadMsgTmr.Interval = System.TimeSpan.FromMilliseconds(1)
                    ReadMsgTmr.Start()
            End Select
        Catch ex As Exception
 
        End Try
 
    End Sub
 
    Private Sub ReadMsgTmr_Tick(ByVal sender As Object, _
                                ByVal e As System.EventArgs) Handles ReadMsgTmr.Tick
 
        ReadMsgTmr.Stop()
 
        Try
 
            Client.DownloadStringAsync(New Uri(BASEPATH & "/read_msg.php", UriKind.Absolute), 0)
 
        Catch ex As Exception
            'HtmlPage.Window.Alert(ex.Message)
        End Try
 
    End Sub
 
    Private Sub ReadMsgComplete(ByVal sender As Object, _
                                ByVal e As DownloadStringCompletedEventArgs)
        Try
            ' Si il y a un message
            Dim Msg As String = e.Result
            If Msg <> "" Then
                DisplayTextinChatBox(Msg)
                ' raffale
                ReadMsgTmr.Interval = System.TimeSpan.FromMilliseconds(150)
            Else
                ' idle
                ReadMsgTmr.Interval = System.TimeSpan.FromMilliseconds(1500)
            End If
 
        Catch ex As Exception
 
        End Try
        ReadMsgTmr.Start()
 
    End Sub
 
    Private Sub MsgTbx_GotFocus(ByVal sender As Object, _
                                ByVal e As System.Windows.RoutedEventArgs) Handles MsgTbx.GotFocus
        MsgTbx.Text = ""
        MsgTbx.Focus()
 
    End Sub
 
    Private Sub MsgTbx_KeyUp(ByVal sender As System.Object, _
                             ByVal e As System.Windows.Input.KeyEventArgs) Handles MsgTbx.KeyUp
        Select Case e.Key
            Case Key.Enter
                If MsgTbx.Text = "" Then MsgTbx.Focus() : Exit Sub
                SendMsg(UsrTbx.Text & " : " & MsgTbx.Text)
                MsgTbx.Text = ""
                MsgTbx.Focus()
            Case Else
                Exit Sub
        End Select
    End Sub
 
    Private Sub UsrTbx_TextChanged(ByVal sender As Object, _
                                   ByVal e As System.Windows.Controls.TextChangedEventArgs) _
                                   Handles UsrTbx.TextChanged
 
        AppSetting("UserName") = UsrTbx.Text
        AppSetting.Save()
 
    End Sub
 
End Class

Vous pouvez tester ce Mini Chat ici

Les sources disponible ici