基于事件的异步模式——BackgroundWorker

　　实现异步处理的方法很多，经常用的有基于委托的方式，今天记录的是基于事件的异步模式。利用BackgroundWorker组件可以很轻松的实现异步处理，并且该组件还支持事件的取消、进度报告等功能。本文以计算两个数X、Y的和为例。

1. 程序界面如下图，其中三个文本框分别为两个加数和处理结果，两个按钮为计算和取消，按钮下方为进度条。
   
2. 引入BackgroundWorker组件，为DoWork、ProgressChanged、RunWorkerCompleted三个事件指定方法。
   将WorkerReportsProgress属性设为true，以支持报告进度。
   将WorkerSupportsCancellation属性设置为true，以支持取消。

   public MainForm()
   {
       InitializeComponent();
       this.backgroundWorker1.DoWork += this.BackgroundWorker1DoWork;
       this.backgroundWorker1.ProgressChanged += this.BackgroundWorker1ProgressChanged;
       this.backgroundWorker1.RunWorkerCompleted += this.BackgroundWorker1RunWorkerCompleted;
       this.backgroundWorker1.WorkerReportsProgress = true;
       this.backgroundWorker1.WorkerSupportsCancellation = true;
       this.buttonCancel.Enabled = false;
   }

3. 当点击计算按钮时，调用BackgroundWorker的RunWorkerAsync方法实现异步处理，该方法支持一个object类型的参数。这里用元组Tuple<int,int>传递X、Y的值。调用RunWorkerAsync方法，将触发DoWork事件。

   this.backgroundWorker1.RunWorkerAsync(new Tuple<int,int>(Convert.ToInt32(this.textBoxX.Text), Convert.ToInt32(this.textBoxY.Text)));

4. 当点击取消按钮时，调用BackgroundWorker的CancelAsync方法，这将改变BackgroundWorker的CancellationPending属性的值为false。当执行OnWork方法时可以根据CancellationPending属性，判断用户是否要取消事件。

   this.backgroundWorker1.CancelAsync();   

5. OnWork方法中可以
   根据CancellationPending属性，判断是否取消。若取消，应在方法结束之前将DoWorkEventArgs的Cancel属性设置为ture。这将用于RunWorkerCompleted事件中判断事件取消还是正常完成。
   通过ReportProgress方法报告进度， 该方法支持一个int类型的参数，该方法将触发ProgressChanged事件。

   void BackgroundWorker1DoWork(object sender, System.ComponentModel.DoWorkEventArgs e)
   {
       for (int i = 0; i < 10; i++) {
           System.Threading.Thread.Sleep(500);
           if (backgroundWorker1.CancellationPending) {
               e.Cancel = true
               return;
           } else {
               backgroundWorker1.ReportProgress(i * 10);
           }
       }
       var t = e.Argument as Tuple<int,int>;
       e.Result = t.Item1 + t.Item2;
   }

6. ProgressChanged事件，会将控制权交给UI线程。在其实现方法中根据ProgressChangedEventArgs的ProgressPercentage属性获取进度值。

   void BackgroundWorker1ProgressChanged(object sender, System.ComponentModel.ProgressChangedEventArgs e)
   {
       this.progressBar1.Value = e.ProgressPercentage;
   }

7. OnWork退出后，将调用RunWorkerCompleted事件。
   根据RunWorkerCompletedEventArgs的Cancelled属性判断是否正常完成。
   根据RunWorkerCompletedEventArgs的Result属性获取处理结果。

   void BackgroundWorker1RunWorkerCompleted(object sender, System.ComponentModel.RunWorkerCompletedEventArgs e)
   {
       this.textBoxResult.Text = e.Cancelled ? "Canceled" : e.Result.ToString();
       this.buttonCancel.Enabled = false;
       this.buttonCalculate.Enabled = true;
       this.progressBar1.Value = 100;
   }

8. 完整代码

    1 /*
    2  * 由SharpDevelop创建。
    3  * 用户： David Huang
    4  * 日期: 2015/9/8
    5  * 时间: 14:54
    6  */
    7 using System;
    8 using System.Windows.Forms;
    9
   10 namespace BackgroundWorkerTest
   11 {
   12     /// <summary>
   13     /// Description of MainForm.
   14     /// </summary>
   15     public partial class MainForm : Form
   16     {
   17         public MainForm()
   18         {
   19             InitializeComponent();
   20             this.backgroundWorker1.DoWork += this.BackgroundWorker1DoWork;
   21             this.backgroundWorker1.ProgressChanged += this.BackgroundWorker1ProgressChanged;
   22             this.backgroundWorker1.RunWorkerCompleted += this.BackgroundWorker1RunWorkerCompleted;
   23             this.backgroundWorker1.WorkerReportsProgress = true;
   24             this.backgroundWorker1.WorkerSupportsCancellation = true;
   25             this.buttonCancel.Enabled = false;
   26         }
   27         void ButtonCalculateClick(object sender, EventArgs e)
   28         {
   29             this.buttonCalculate.Enabled = false;
   30             this.textBoxResult.Text = string.Empty;
   31             this.buttonCancel.Enabled = true;
   32             this.backgroundWorker1.RunWorkerAsync(new Tuple<int,int>(Convert.ToInt32(this.textBoxX.Text), Convert.ToInt32(this.textBoxY.Text)));
   33         }
   34         void ButtonCancelClick(object sender, EventArgs e)
   35         {
   36             this.backgroundWorker1.CancelAsync();
   37         }
   38         void BackgroundWorker1DoWork(object sender, System.ComponentModel.DoWorkEventArgs e)
   39         {
   40             for (int i = 0; i < 10; i++) {
   41                 System.Threading.Thread.Sleep(500);
   42                 if (backgroundWorker1.CancellationPending) {
   43                     e.Cancel = true;
   44                     return;
   45                 } else {
   46                     backgroundWorker1.ReportProgress(i * 10);
   47                 }
   48             }
   49             var t = e.Argument as Tuple<int,int>;
   50             e.Result = t.Item1 + t.Item2;
   51         }
   52         void BackgroundWorker1ProgressChanged(object sender, System.ComponentModel.ProgressChangedEventArgs e)
   53         {
   54             this.progressBar1.Value = e.ProgressPercentage;
   55         }
   56         void BackgroundWorker1RunWorkerCompleted(object sender, System.ComponentModel.RunWorkerCompletedEventArgs e)
   57         {
   58             this.textBoxResult.Text = e.Cancelled ? "Canceled" : e.Result.ToString();
   59             this.buttonCancel.Enabled = false;
   60             this.buttonCalculate.Enabled = true;
   61             this.progressBar1.Value = 100;
   62         }
   63     }
   64 }

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