【C#学习笔记】使用C#中BackgroundWorker

使用前提

在WPF程序中，有一些比较耗时的后台操作时，比如向远程服务器请求数据，或者通过TCP/IP为某台设备提供升级固件服务等等。为了防止这类操作freeze用户界面，造成用户体验下降，即程序假死的状况出现。一种常见的，更user friendly的方式是，提供一个进度条窗口，提示用户该操作的完成进度。并提供取消操作的选项。

C#中的 BackgroundWorker Class 则是执行该任务的最佳选择。

The BackgroundWorker class allows you to run an operation on a separate, dedicated thread. Time-consuming operations like downloads and database transactions can cause your user interface (UI) to seem as though it has stopped responding while they are running. When you want a responsive UI and you are faced with long delays associated with such operations, theBackgroundWorker class provides a convenient solution.
-MSDN

关于 BackgroundWorker 类

主要属性

• CancellationPending - 只读属性，default值为false，执行CancelAsync方法后，值为true。表明应用程序请求了取消后台操作。
• IsBusy - 如果后台异步操作开始执行，值为true，否则为false
• WorkerReportProgress - 如果BackgroundWorker支持后台操作进程更新，设置值为true，default值为false

主要事件

• DoWork
• ProgressChanged
• RunWorkerCompleted
  不要再DoWork事件处理程序中对UI线程中的对象进行操作，操作应该放在ProgressChanged和RunWorkerCompleted的事件处理程序中。

主要方法

• RunWorkerAsync() - 执行后台操作，激发DoWork事件
• ReportProgress()- 激发ProgressChanged事件
• CancelAsync() - 提交终止后台操作的请求，并将CancellationPending属性值设为true。在程序其他地方要定时检查CancellationPending属性的值，作出相应操作，比如

if (worker.CancellationPending)
{
    e.Cancel = true;
}

示例程序

XAML

<Window x:Class="BackgroundWorkerExample.MainWindow"  
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"  
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"  
        Title="MainWindow" Height="150" Width="300">
    <StackPanel>
        <ProgressBar Name="progressBar" Height="20" Width="250" Margin="10"></ProgressBar>
        <TextBox Name="textBox" Width="50" Height="20" HorizontalAlignment="Center"></TextBox>
        <Button Name="btnProcess" Width="100" Click="btnProcess_Click" Margin="5">Start</Button>
        <Button Name="btnCancel" Width="100" Click="btnCancel_Click" Margin="5">Cancel</Button>
    </StackPanel>
</Window>

C#

namespace BackgroundWorkerExample
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        BackgroundWorker bgworker = new BackgroundWorker();
        public MainWindow()
        {
            InitializeComponent();

            bgworker.WorkerReportsProgress = true;
            bgworker.WorkerSupportsCancellation = true;
            bgworker.DoWork += bgworker_DoWork;
            bgworker.ProgressChanged += bgworker_ProgressChanged;
            bgworker.RunWorkerCompleted += bgworker_RunWorkerCompleted;
        }


        void bgworker_DoWork(object sender, DoWorkEventArgs e)
        {
            BackgroundWorker worker = sender as BackgroundWorker;
            for (int i = 1; i <= 100; i++)
            {
                if (worker.CancellationPending)
                {
                    e.Cancel = true;
                }
                else
                {
                    worker.ReportProgress(i);
                    Thread.Sleep(100);
                }
            }
        }
       
        void bgworker_ProgressChanged(object sender, ProgressChangedEventArgs e)
        {
            progressBar.Value = e.ProgressPercentage;
            textBox.Text = e.ProgressPercentage.ToString();
        }

        void bgworker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            progressBar.Value = 0;
            if (e.Cancelled)
            {
                MessageBox.Show("Background task has been canceled", "info");
            }
            else
            {
                MessageBox.Show("Background task finished", "info");
            }
        }

        private void btnProcess_Click(object sender, RoutedEventArgs e)
        {
            if (!bgworker.IsBusy)
            {
                bgworker.RunWorkerAsync();
            }
        }

        private void btnCancel_Click(object sender, RoutedEventArgs e)
        {
            bgworker.CancelAsync();
        }
    }
}

演示

BackgroundWorkerExample.gif