如何：在后台下载文件

  • 项目

下载文件是常规任务，通常情况下，最好以单独的线程来运行这项可能很耗时的操作。 使用 BackgroundWorker 组件可以用非常少的代码完成此任务。

示例

下面的代码示例演示如何使用 BackgroundWorker 组件从 URL 加载 XML 文件。 用户单击**“下载”**按钮时，Click 事件处理程序将调用 BackgroundWorker 组件的 RunWorkerAsync 方法来启动下载操作。 在下载过程中，将禁用该按钮，然后在下载完成后再启用该按钮。 MessageBox 将显示文件的内容。

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());
    }
}

下载文件

文件下载在 BackgroundWorker 组件的辅助线程上进行，该线程运行 DoWork 事件处理程序。 当代码调用 RunWorkerAsync 方法时，将启动此线程。

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");
}

等待 BackgroundWorker 完成

downloadButton_Click 事件处理程序演示如何等待 BackgroundWorker 组件完成它的异步任务。

如果您希望在等待后台线程完成时应用程序仅仅响应事件，而不在主线程中执行任何工作，则退出相应的处理程序即可。

如果您希望在主线程中继续执行工作，请使用 IsBusy 属性来确定 BackgroundWorker 线程是否仍然在运行。 在示例中，在下载的过程中将会更新进度栏。 请务必调用 Application.DoEvents 方法来使用户界面保持响应状态。

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();
    }
}

显示结果

backgroundWorker1_RunWorkerCompleted 方法将处理 RunWorkerCompleted 事件，并在后台操作完成后被调用。 此方法首先检查 AsyncCompletedEventArgs.Error 属性。 如果 AsyncCompletedEventArgs.Error 为 null，则此方法将显示文件内容， 然后启用下载按钮（此按钮在下载开始后已禁用），并重置进度栏。

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;
}

编译代码

此示例需要：

  • 对 System.Drawing、System.Windows.Forms 和 System.Xml 程序集的引用。

有关从 Visual Basic 或 Visual C# 的命令行生成此示例的信息，请参见从命令行生成 (Visual Basic) 或在命令行上使用 csc.exe 生成。 也可以通过将代码粘贴到新项目，在 Visual Studio 中生成此示例。 有关更多信息，请参见 如何：使用 Visual Studio 编译和运行完整的 Windows 窗体代码示例 和 如何：使用 Visual Studio 编译和运行完整的 Windows 窗体代码示例 和 如何：使用 Visual Studio 编译和运行完整的 Windows 窗体代码示例 和 如何：使用 Visual Studio 编译和运行完整的 Windows 窗体代码示例 和 如何：使用 Visual Studio 编译和运行完整的 Windows 窗体代码示例.

可靠编程

在尝试访问 RunWorkerCompletedEventArgs.Result 属性或可能已受 DoWork 事件处理程序影响的任何其他对象之前，一定要在 RunWorkerCompleted 事件处理程序中检查 AsyncCompletedEventArgs.Error 属性。

请参见

任务

如何：在后台运行操作

如何：实现使用后台操作的窗体

参考

BackgroundWorker