Procedura: scaricare file in background
Il download di file è un'attività comune. Spesso è utile eseguire questa operazione potenzialmente dispendiosa in termini di tempo in un thread separato. Utilizzare il componente BackgroundWorker per eseguire questa attività con una quantità di codice molto ridotta.
Esempio
Nell'esempio di codice riportato di seguito è illustrato l'utilizzo di un componente BackgroundWorker per caricare un file XML da un URL. Quando l'utente sceglie il pulsante Download, il gestore eventi Click chiama il metodo RunWorkerAsync di un componente BackgroundWorker per avviare l'operazione di download. Il pulsante è disabilitato per la durata del download e quindi abilitato una volta che il download è completato. La classe MessageBox consente di visualizzare il contenuto del file.
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 downloadButton As Button
Private WithEvents progressBar1 As ProgressBar
Private WithEvents backgroundWorker1 As BackgroundWorker
Private document As XmlDocument = Nothing
Public Sub New()
InitializeComponent()
Me.backgroundWorker1 = New System.ComponentModel.BackgroundWorker()
End Sub
Private Sub downloadButton_Click( _
ByVal sender As Object, _
ByVal e As EventArgs) _
Handles downloadButton.Click
' Start the download operation in the background.
Me.backgroundWorker1.RunWorkerAsync()
' Disable the button for the duration of the download.
Me.downloadButton.Enabled = False
' Once you have started the background thread you
' can exit the handler and the application will
' wait until the RunWorkerCompleted event is raised.
' If you want to do something else in the main thread,
' such as update a progress bar, you can do so in a loop
' while checking IsBusy to see if the background task is
' still running.
While Me.backgroundWorker1.IsBusy
progressBar1.Increment(1)
' Keep UI messages moving, so the form remains
' responsive during the asynchronous operation.
Application.DoEvents()
End While
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( _
ByVal sender As Object, _
ByVal e As RunWorkerCompletedEventArgs) _
Handles backgroundWorker1.RunWorkerCompleted
' Set progress bar to 100% in case it isn't already there.
progressBar1.Value = 100
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
' Enable the download button and reset the progress bar.
Me.downloadButton.Enabled = True
progressBar1.Value = 0
End Sub
#Region "Windows Form Designer generated code"
' <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(ByVal disposing As Boolean)
If disposing AndAlso (components IsNot Nothing) Then
components.Dispose()
End If
MyBase.Dispose(disposing)
End Sub
' <summary>
' Required method for Designer support - do not modify
' the contents of this method with the code editor.
' </summary>
Private Sub InitializeComponent()
Me.downloadButton = New System.Windows.Forms.Button
Me.progressBar1 = New System.Windows.Forms.ProgressBar
Me.SuspendLayout()
'
'downloadButton
'
Me.downloadButton.Location = New System.Drawing.Point(12, 12)
Me.downloadButton.Name = "downloadButton"
Me.downloadButton.Size = New System.Drawing.Size(100, 23)
Me.downloadButton.TabIndex = 0
Me.downloadButton.Text = "Download file"
Me.downloadButton.UseVisualStyleBackColor = True
'
'progressBar1
'
Me.progressBar1.Location = New System.Drawing.Point(12, 50)
Me.progressBar1.Name = "progressBar1"
Me.progressBar1.Size = New System.Drawing.Size(100, 26)
Me.progressBar1.TabIndex = 1
'
'Form1
'
Me.AutoScaleDimensions = New System.Drawing.SizeF(6.0!, 13.0!)
Me.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font
Me.ClientSize = New System.Drawing.Size(136, 104)
Me.Controls.Add(Me.downloadButton)
Me.Controls.Add(Me.progressBar1)
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 downloadButton;
private ProgressBar progressBar1;
private XmlDocument document = null;
public Form1()
{
InitializeComponent();
// Instantiate BackgroundWorker and attach handlers to its
// DowWork and RunWorkerCompleted events.
backgroundWorker1 = new System.ComponentModel.BackgroundWorker();
backgroundWorker1.DoWork += new System.ComponentModel.DoWorkEventHandler(this.backgroundWorker1_DoWork);
backgroundWorker1.RunWorkerCompleted += new System.ComponentModel.RunWorkerCompletedEventHandler(this.backgroundWorker1_RunWorkerCompleted);
}
private void downloadButton_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.downloadButton.Enabled = false;
// Once you have started the background thread you
// can exit the handler and the application will
// wait until the RunWorkerCompleted event is raised.
// Or if you want to do something else in the main thread,
// such as update a progress bar, you can do so in a loop
// while checking IsBusy to see if the background task is
// still running.
while (this.backgroundWorker1.IsBusy)
{
progressBar1.Increment(1);
// Keep UI messages moving, so the form remains
// responsive during the asynchronous operation.
Application.DoEvents();
}
}
private void backgroundWorker1_DoWork(
object sender,
DoWorkEventArgs e)
{
document = new XmlDocument();
// Uncomment the following line to
// simulate a noticeable latency.
//Thread.Sleep(5000);
// Replace this file name with a valid file name.
document.Load(@"http://www.tailspintoys.com/sample.xml");
}
private void backgroundWorker1_RunWorkerCompleted(
object sender,
RunWorkerCompletedEventArgs e)
{
// Set progress bar to 100% in case it's not already there.
progressBar1.Value = 100;
if (e.Error == null)
{
MessageBox.Show(document.InnerXml, "Download Complete");
}
else
{
MessageBox.Show(
"Failed to download file",
"Download failed",
MessageBoxButtons.OK,
MessageBoxIcon.Error);
}
// Enable the download button and reset the progress bar.
this.downloadButton.Enabled = true;
progressBar1.Value = 0;
}
#region Windows Form Designer generated code
/// <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);
}
/// <summary>
/// Required method for Designer support
/// </summary>
private void InitializeComponent()
{
this.downloadButton = new System.Windows.Forms.Button();
this.progressBar1 = new System.Windows.Forms.ProgressBar();
this.SuspendLayout();
//
// downloadButton
//
this.downloadButton.Location = new System.Drawing.Point(12, 12);
this.downloadButton.Name = "downloadButton";
this.downloadButton.Size = new System.Drawing.Size(100, 23);
this.downloadButton.TabIndex = 0;
this.downloadButton.Text = "Download file";
this.downloadButton.UseVisualStyleBackColor = true;
this.downloadButton.Click += new System.EventHandler(this.downloadButton_Click);
//
// progressBar1
//
this.progressBar1.Location = new System.Drawing.Point(12, 50);
this.progressBar1.Name = "progressBar1";
this.progressBar1.Size = new System.Drawing.Size(100, 26);
this.progressBar1.TabIndex = 1;
//
// Form1
//
this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
this.ClientSize = new System.Drawing.Size(133, 104);
this.Controls.Add(this.progressBar1);
this.Controls.Add(this.downloadButton);
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());
}
}
Download di file
Il download dei file viene eseguito nel thread di lavoro del componente BackgroundWorker, che esegue il gestore eventi DoWork. Questo thread inizia nel momento in cui il codice chiama il metodo 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();
// Uncomment the following line to
// simulate a noticeable latency.
//Thread.Sleep(5000);
// Replace this file name with a valid file name.
document.Load(@"http://www.tailspintoys.com/sample.xml");
}
Attesa della fine di un BackgroundWorker
Il gestore eventi downloadButton_Click illustra come viene atteso il completamento di un'attività asincrona da parte di un componente BackgroundWorker.
Se si desidera solo che l'applicazione risponda agli eventi e che non esegua alcuna operazione nel thread principale mentre si attende il completamento del thread di background, uscire dal gestore.
Se si desidera proseguire il lavoro nel thread principale, utilizzare la proprietà IsBusy per determinare se il thread BackgroundWorker è ancora in esecuzione. Nell'esempio, viene aggiornato un indicatore di stato mentre è in corso il download. Assicurarsi di chiamare il metodo Application.DoEvents per mantenere reattiva l'interfaccia utente.
Private Sub downloadButton_Click( _
ByVal sender As Object, _
ByVal e As EventArgs) _
Handles downloadButton.Click
' Start the download operation in the background.
Me.backgroundWorker1.RunWorkerAsync()
' Disable the button for the duration of the download.
Me.downloadButton.Enabled = False
' Once you have started the background thread you
' can exit the handler and the application will
' wait until the RunWorkerCompleted event is raised.
' If you want to do something else in the main thread,
' such as update a progress bar, you can do so in a loop
' while checking IsBusy to see if the background task is
' still running.
While Me.backgroundWorker1.IsBusy
progressBar1.Increment(1)
' Keep UI messages moving, so the form remains
' responsive during the asynchronous operation.
Application.DoEvents()
End While
End Sub
private void downloadButton_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.downloadButton.Enabled = false;
// Once you have started the background thread you
// can exit the handler and the application will
// wait until the RunWorkerCompleted event is raised.
// Or if you want to do something else in the main thread,
// such as update a progress bar, you can do so in a loop
// while checking IsBusy to see if the background task is
// still running.
while (this.backgroundWorker1.IsBusy)
{
progressBar1.Increment(1);
// Keep UI messages moving, so the form remains
// responsive during the asynchronous operation.
Application.DoEvents();
}
}
Visualizzazione del risultato
Il metodo backgroundWorker1_RunWorkerCompleted gestisce l'evento RunWorkerCompleted e viene chiamato quando l'operazione in background viene completata. Questo metodo controlla innanzitutto la proprietà AsyncCompletedEventArgs.Error. Se AsyncCompletedEventArgs.Error è null, questo metodo visualizza il contenuto del file. Abilita quindi il pulsante del download, disabilitato all'inizio del download, e reimposta l'indicatore di stato.
Private Sub backgroundWorker1_RunWorkerCompleted( _
ByVal sender As Object, _
ByVal e As RunWorkerCompletedEventArgs) _
Handles backgroundWorker1.RunWorkerCompleted
' Set progress bar to 100% in case it isn't already there.
progressBar1.Value = 100
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
' Enable the download button and reset the progress bar.
Me.downloadButton.Enabled = True
progressBar1.Value = 0
End Sub
private void backgroundWorker1_RunWorkerCompleted(
object sender,
RunWorkerCompletedEventArgs e)
{
// Set progress bar to 100% in case it's not already there.
progressBar1.Value = 100;
if (e.Error == null)
{
MessageBox.Show(document.InnerXml, "Download Complete");
}
else
{
MessageBox.Show(
"Failed to download file",
"Download failed",
MessageBoxButtons.OK,
MessageBoxIcon.Error);
}
// Enable the download button and reset the progress bar.
this.downloadButton.Enabled = true;
progressBar1.Value = 0;
}
Compilazione del codice
L'esempio presenta i seguenti requisiti:
- Riferimenti agli assembly System.Drawing, System.Windows.Forms e System.Xml.
Per informazioni sulla compilazione di questo esempio dalla riga di comando per Visual Basic o Visual C#, vedere Building from the Command Line (Visual Basic) o Compilazione dalla riga di comando con csc.exe. È anche possibile compilare questo esempio in Visual Studio incollando il codice in un nuovo progetto. Per ulteriori informazioni, vedere Procedura: compilare ed eseguire un esempio di codice Windows Form completo tramite Visual Studio e Procedura: compilare ed eseguire un esempio di codice Windows Form completo tramite Visual Studio e Procedura: compilare ed eseguire un esempio di codice Windows Form completo tramite Visual Studio e Procedura: compilare ed eseguire un esempio di codice Windows Form completo tramite Visual Studio e Procedura: compilare ed eseguire un esempio di codice Windows Form completo tramite Visual Studio.
Programmazione efficiente
Verificare sempre la presenza della proprietà AsyncCompletedEventArgs.Error nel gestore eventi RunWorkerCompleted in uso prima di tentare di accedere alla proprietà RunWorkerCompletedEventArgs.Result o ad eventuali altri oggetti che possano essere influenzati dal gestore eventi DoWork.
Vedere anche
Attività
Procedura: eseguire un'operazione in background
Procedura: implementare un form che utilizza un'operazione in background