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Cara: Mengunduh File di Latar Belakang

  • Artikel

Mengunduh file adalah tugas umum, dan sering kali berguna untuk menjalankan operasi yang berpotensi memakan waktu ini pada utas terpisah. BackgroundWorker Gunakan komponen untuk menyelesaikan tugas ini dengan kode yang sangat sedikit.

Contoh

Contoh kode berikut menunjukkan cara menggunakan BackgroundWorker komponen untuk memuat file XML dari URL. Ketika pengguna mengklik tombol Unduh , penanganan Click aktivitas memanggil RunWorkerAsync metode BackgroundWorker komponen untuk memulai operasi pengunduhan. Tombol dinonaktifkan selama durasi pengunduhan, lalu diaktifkan saat unduhan selesai. Menampilkan MessageBox isi file.

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
        // DoWork 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());
    }
}

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"
    ' Required designer variable.
    Private components As System.ComponentModel.IContainer = Nothing

    ' Clean up any resources being used.
    ' <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

    ' Required method for Designer support - do not modify
    ' the contents of this method with the code editor.
    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

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

Mengunduh file

File diunduh pada BackgroundWorker utas pekerja komponen, yang menjalankan penanganan DoWork aktivitas. Utas ini dimulai ketika kode Anda memanggil RunWorkerAsync metode .

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 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

Menunggu BackgroundWorker selesai

Penanganan downloadButton_Click aktivitas menunjukkan cara menunggu BackgroundWorker komponen menyelesaikan tugas asinkronnya.

Jika Anda hanya ingin aplikasi merespons peristiwa dan tidak ingin melakukan pekerjaan apa pun di utas utama saat Anda menunggu utas latar belakang selesai, cukup keluar dari handler.

Jika Anda ingin terus melakukan pekerjaan di utas utama, gunakan IsBusy properti untuk menentukan apakah BackgroundWorker utas masih berjalan. Dalam contoh, bilah kemajuan diperbarui saat unduhan sedang diproses. Pastikan untuk memanggil Application.DoEvents metode untuk menjaga antarmuka pengguna tetap responsif.

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 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

Menampilkan hasil

Metode backgroundWorker1_RunWorkerCompleted ini menangani RunWorkerCompleted peristiwa dan dipanggil ketika operasi latar belakang selesai. Metode ini pertama-tama AsyncCompletedEventArgs.Error memeriksa properti . Jika AsyncCompletedEventArgs.Error adalah null, maka metode ini menampilkan konten file. Kemudian mengaktifkan tombol unduh, yang dinonaktifkan ketika unduhan dimulai, dan mengatur ulang bilah kemajuan.

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;
}

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

Mengompilasi Kode

Contoh ini membutuhkan:

  • Referensi ke rakitan System.Drawing, System.Windows.Forms, dan System.Xml.

Pemrograman yang Kuat

Selalu periksa AsyncCompletedEventArgs.Error properti di penanganan aktivitas Anda RunWorkerCompleted sebelum mencoba mengakses RunWorkerCompletedEventArgs.Result properti atau objek lain yang mungkin terpengaruh oleh DoWork penanganan aktivitas.

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