Comment : télécharger un fichier en arrière-plan

Mise à jour : novembre 2007

Le téléchargement d'un fichier est une tâche courante, et il est souvent utile d'exécuter cette opération susceptible de prendre du temps sur un thread séparé. Utilisez le composant BackgroundWorker pour effectuer cette tâche avec très peu de code.

Exemple

L'exemple de code suivant montre comment utiliser un composant BackgroundWorker pour charger un fichier XML à partir d'une URL. Lorsque l'utilisateur clique sur le bouton Télécharger, le gestionnaire d'événements Click appelle la méthode RunWorkerAsync d'un composant BackgroundWorker pour démarrer l'opération de téléchargement. Le bouton est désactivé pendant la durée du téléchargement, et réactivé à la fin du téléchargement. Une MessageBox affiche le contenu du fichier.

Imports System
Imports System.Collections.Generic
Imports System.ComponentModel
Imports System.Drawing
Imports System.Threading
Imports System.Windows.Forms
Imports System.Xml

Public Class Form1
   Inherits Form
   Private WithEvents backgroundWorker1 As BackgroundWorker
   Private WithEvents dowloadButton As Button
   Private document As XmlDocument = Nothing


   Public Sub New()
      InitializeComponent()
    End Sub

    Private Sub dowloadButton_Click( _
    ByVal sender As Object, _
    ByVal e As EventArgs) _
    Handles dowloadButton.Click

        ' Start the download operation in the background.
        Me.backgroundWorker1.RunWorkerAsync()

        ' Disable the button for the duration of the download.
        Me.dowloadButton.Enabled = False

        ' Wait for the BackgroundWorker to finish the download.
        While Me.backgroundWorker1.IsBusy
            ' Keep UI messages moving, so the form remains 
            ' responsive during the asynchronous operation.
            Application.DoEvents()
        End While

        ' The download is done, so enable the button.
        Me.dowloadButton.Enabled = True
    End Sub

    Private Sub backgroundWorker1_DoWork( _
    ByVal sender As Object, _
    ByVal e As DoWorkEventArgs) _
    Handles backgroundWorker1.DoWork

        document = New XmlDocument()

        ' Replace this file name with a valid file name.
        document.Load("http://www.tailspintoys.com/sample.xml")

        ' Uncomment the following line to
        ' simulate a noticeable latency.
        'Thread.Sleep(5000);
    End Sub

   Private Sub backgroundWorker1_RunWorkerCompleted(sender As Object, e As RunWorkerCompletedEventArgs) Handles backgroundWorker1.RunWorkerCompleted
      If e.Error Is Nothing Then
         MessageBox.Show(document.InnerXml, "Download Complete")
      Else
         MessageBox.Show("Failed to download file", "Download failed", MessageBoxButtons.OK, MessageBoxIcon.Error)
      End If
    End Sub

    ' <summary>
    ' Required designer variable.
    ' </summary>
   Private components As System.ComponentModel.IContainer = Nothing

    ' <summary>
    ' Clean up any resources being used.
    ' </summary>
    ' <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
   Protected Overrides Sub Dispose(disposing As Boolean)
      If disposing AndAlso (components IsNot Nothing) Then
         components.Dispose()
      End If
      MyBase.Dispose(disposing)
    End Sub

   #Region "Windows Form Designer generated code"


    ' <summary>
    ' Required method for Designer support - do not modify
    ' the contents of this method with the code editor.
    ' </summary>
   Private Sub InitializeComponent()
      Me.backgroundWorker1 = New System.ComponentModel.BackgroundWorker()
      Me.dowloadButton = New System.Windows.Forms.Button()
      Me.SuspendLayout()
      ' 
      ' backgroundWorker1
      ' 
      ' 
      ' dowloadButton
      ' 
      Me.dowloadButton.Location = New System.Drawing.Point(12, 12)
      Me.dowloadButton.Name = "dowloadButton"
      Me.dowloadButton.Size = New System.Drawing.Size(75, 23)
      Me.dowloadButton.TabIndex = 0
      Me.dowloadButton.Text = "Download file"
      Me.dowloadButton.UseVisualStyleBackColor = True
      ' 
      ' Form1
      ' 
      Me.AutoScaleDimensions = New System.Drawing.SizeF(6F, 13F)
      Me.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font
      Me.ClientSize = New System.Drawing.Size(104, 54)
      Me.Controls.Add(dowloadButton)
      Me.Name = "Form1"
      Me.Text = "Form1"
      Me.ResumeLayout(False)
    End Sub

   #End Region
End Class


Public Class Program

    ' <summary>
    ' The main entry point for the application.
    ' </summary>
    <STAThread()> _
    Shared Sub Main()
        Application.EnableVisualStyles()
        Application.Run(New Form1())
    End Sub
End Class

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Drawing;
using System.Threading;
using System.Windows.Forms;
using System.Xml;

public class Form1 : Form
{
    private BackgroundWorker backgroundWorker1;
    private Button dowloadButton;
    private XmlDocument document = null;

    public Form1()
    {
        InitializeComponent();
    }

    private void dowloadButton_Click(object sender, EventArgs e)
    {
        // Start the download operation in the background.
        this.backgroundWorker1.RunWorkerAsync();

        // Disable the button for the duration of the download.
        this.dowloadButton.Enabled = false;

        // Wait for the BackgroundWorker to finish the download.
        while (this.backgroundWorker1.IsBusy)
        {
            // Keep UI messages moving, so the form remains 
            // responsive during the asynchronous operation.
            Application.DoEvents();
        }

        // The download is done, so enable the button.
        this.dowloadButton.Enabled = true;
    }

    private void backgroundWorker1_DoWork(
        object sender, 
        DoWorkEventArgs e)
    {
        document = new XmlDocument();

        // Replace this file name with a valid file name.
        document.Load(@"http://www.tailspintoys.com/sample.xml");

        // Uncomment the following line to
        // simulate a noticeable latency.
        //Thread.Sleep(5000);
    }

    private void backgroundWorker1_RunWorkerCompleted(
        object sender, 
        RunWorkerCompletedEventArgs e)
    {
        if (e.Error == null)
        {
            MessageBox.Show(document.InnerXml, "Download Complete");
        }
        else
        {
            MessageBox.Show(
                "Failed to download file", 
                "Download failed", 
                MessageBoxButtons.OK, 
                MessageBoxIcon.Error );
        }
    }

    /// <summary>
    /// Required designer variable.
    /// </summary>
    private System.ComponentModel.IContainer components = null;

    /// <summary>
    /// Clean up any resources being used.
    /// </summary>
    /// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
    protected override void Dispose(bool disposing)
    {
        if (disposing && (components != null))
        {
            components.Dispose();
        }
        base.Dispose(disposing);
    }

    #region Windows Form Designer generated code

    /// <summary>
    /// Required method for Designer support - do not modify
    /// the contents of this method with the code editor.
    /// </summary>
    private void InitializeComponent()
    {
        this.backgroundWorker1 = new System.ComponentModel.BackgroundWorker();
        this.dowloadButton = new System.Windows.Forms.Button();
        this.SuspendLayout();
        // 
        // backgroundWorker1
        // 
        this.backgroundWorker1.DoWork += new System.ComponentModel.DoWorkEventHandler(this.backgroundWorker1_DoWork);
        this.backgroundWorker1.RunWorkerCompleted += new System.ComponentModel.RunWorkerCompletedEventHandler(this.backgroundWorker1_RunWorkerCompleted);
        // 
        // dowloadButton
        // 
        this.dowloadButton.Location = new System.Drawing.Point(12, 12);
        this.dowloadButton.Name = "dowloadButton";
        this.dowloadButton.Size = new System.Drawing.Size(75, 23);
        this.dowloadButton.TabIndex = 0;
        this.dowloadButton.Text = "Download file";
        this.dowloadButton.UseVisualStyleBackColor = true;
        this.dowloadButton.Click += new System.EventHandler(this.dowloadButton_Click);
        // 
        // Form1
        // 
        this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
        this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
        this.ClientSize = new System.Drawing.Size(104, 54);
        this.Controls.Add(this.dowloadButton);
        this.Name = "Form1";
        this.Text = "Form1";
        this.ResumeLayout(false);

    }

    #endregion
}

static class Program
{
    /// <summary>
    /// The main entry point for the application.
    /// </summary>
    [STAThread]
    static void Main()
    {
        Application.EnableVisualStyles();
        Application.Run(new Form1());
    }
}

Téléchargement du fichier

Le fichier est téléchargé sur le thread de travail du composant BackgroundWorker, qui exécute le gestionnaire d'événements DoWork. Ce thread démarre lorsque votre code appelle la méthode RunWorkerAsync.

Private Sub backgroundWorker1_DoWork( _
ByVal sender As Object, _
ByVal e As DoWorkEventArgs) _
Handles backgroundWorker1.DoWork

    document = New XmlDocument()

    ' Replace this file name with a valid file name.
    document.Load("http://www.tailspintoys.com/sample.xml")

    ' Uncomment the following line to
    ' simulate a noticeable latency.
    'Thread.Sleep(5000);
End Sub

private void backgroundWorker1_DoWork(
    object sender, 
    DoWorkEventArgs e)
{
    document = new XmlDocument();

    // Replace this file name with a valid file name.
    document.Load(@"http://www.tailspintoys.com/sample.xml");

    // Uncomment the following line to
    // simulate a noticeable latency.
    //Thread.Sleep(5000);
}

Attente de la fin d'un composant BackgroundWorker

Le gestionnaire d'événements dowloadButton_Click montre comment attendre qu'un composant BackgroundWorker ait terminé sa tâche asynchrone. Utilisez la propriété IsBusy pour déterminer si le thread BackgroundWorker est toujours en cours d'exécution. Si votre code se trouve sur le thread d'interface utilisateur principal, comme c'est le cas avec le gestionnaire d'événements Click, veillez à appeler la méthode Application.DoEvents pour que l'interface utilisateur demeure réactive.

Private Sub dowloadButton_Click( _
ByVal sender As Object, _
ByVal e As EventArgs) _
Handles dowloadButton.Click

    ' Start the download operation in the background.
    Me.backgroundWorker1.RunWorkerAsync()

    ' Disable the button for the duration of the download.
    Me.dowloadButton.Enabled = False

    ' Wait for the BackgroundWorker to finish the download.
    While Me.backgroundWorker1.IsBusy
        ' Keep UI messages moving, so the form remains 
        ' responsive during the asynchronous operation.
        Application.DoEvents()
    End While

    ' The download is done, so enable the button.
    Me.dowloadButton.Enabled = True
End Sub

private void dowloadButton_Click(object sender, EventArgs e)
{
    // Start the download operation in the background.
    this.backgroundWorker1.RunWorkerAsync();

    // Disable the button for the duration of the download.
    this.dowloadButton.Enabled = false;

    // Wait for the BackgroundWorker to finish the download.
    while (this.backgroundWorker1.IsBusy)
    {
        // Keep UI messages moving, so the form remains 
        // responsive during the asynchronous operation.
        Application.DoEvents();
    }

    // The download is done, so enable the button.
    this.dowloadButton.Enabled = true;
}

Affichage du résultat

La méthode backgroundWorker1_RunWorkerCompleted gère l'événement RunWorkerCompleted et est appelée lorsque l'opération en arrière-plan est terminée. Elle vérifie d'abord la propriété AsyncCompletedEventArgs.Error, et si elle est null, affiche le contenu du fichier.

Private Sub backgroundWorker1_RunWorkerCompleted(sender As Object, e As RunWorkerCompletedEventArgs) Handles backgroundWorker1.RunWorkerCompleted
   If e.Error Is Nothing Then
      MessageBox.Show(document.InnerXml, "Download Complete")
   Else
      MessageBox.Show("Failed to download file", "Download failed", MessageBoxButtons.OK, MessageBoxIcon.Error)
   End If
 End Sub

private void backgroundWorker1_RunWorkerCompleted(
    object sender, 
    RunWorkerCompletedEventArgs e)
{
    if (e.Error == null)
    {
        MessageBox.Show(document.InnerXml, "Download Complete");
    }
    else
    {
        MessageBox.Show(
            "Failed to download file", 
            "Download failed", 
            MessageBoxButtons.OK, 
            MessageBoxIcon.Error );
    }
}

Compilation du code

Cet exemple nécessite les actions ou les éléments suivants :

Références aux assemblys System.Drawing, System.Windows.Forms et System.Xml.

Pour plus d'informations sur la génération de cet exemple à partir de la ligne de commande pour Visual Basic ou Visual C#, consultez Génération à partir de la ligne de commande (Visual Basic) ou Génération à partir de la ligne de commande avec csc.exe. Vous pouvez aussi générer cet exemple dans Visual Studio en collant le code dans un nouveau projet.

Programmation fiable

Vérifiez toujours la propriété AsyncCompletedEventArgs.Error de votre gestionnaire d'événements RunWorkerCompleted avant d'essayer d'accéder à la propriété RunWorkerCompletedEventArgs.Result ou à tout autre objet ayant pu être affecté par le gestionnaire d'événements DoWork.

Voir aussi

Tâches

Comment : exécuter une opération en arrière-plan

Comment : implémenter un formulaire qui utilise une opération d'arrière-plan

Référence

BackgroundWorker

Last updated on 2008-08-21