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。

　   通过反编译可以看到，这个组件内部也是通过异步委托实现的，报告进度、取消事件等运用了事件技术实现，而事件的本质其实就是委托。

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)));

   通过反编译看到以下实现代码：

           public void RunWorkerAsync(object argument)
           {
               if (this.isRunning)
               {
                   throw new InvalidOperationException(SR.GetString("BackgroundWorker_WorkerAlreadyRunning"));
               }
               this.isRunning = true;
               this.cancellationPending = false;
               this.asyncOperation = AsyncOperationManager.CreateOperation(null);
               this.threadStart.BeginInvoke(argument, null, null);
           }

   可以看到其内部也是通过异步委托实现的，其中threadStart的定义：private delegate void WorkerThreadStartDelegate(object argument);就是一个委托类型。

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ture。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();   

   再看看其内部：

           public void CancelAsync()
           {
               if (!this.WorkerSupportsCancellation)
               {
                   throw new InvalidOperationException(SR.GetString("BackgroundWorker_WorkerDoesntSupportCancellation"));
               }
               this.cancellationPending = true;
           }

   如上面所说，改变只读属性CancellationPending的值为ture。

5. 这里在DoWork方法中
   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， 调用该方法时传递了一个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 = ; i < ; i++) {
           System.Threading.Thread.Sleep();//模拟耗时操作
           if (backgroundWorker1.CancellationPending) {
               e.Cancel = true
               return;
           } else {
               backgroundWorker1.ReportProgress(i * );
           }
       }
       var t = e.Argument as Tuple<int,int>;
       e.Result = t.Item1 + t.Item2;
   }

   再看报告进度是怎么基于事件实现的：

   1.我们调用的ReportProgress方法
   public void ReportProgress(int percentProgress)
   {
       this.ReportProgress(percentProgress, null);
   }
   2.ReportProgress另一个重载，可以带一个自定义参数，方便传递其他内容，例如一般安装程序中的“正在复制文件”，“正在注册相关组件”提示信息等。
   public void ReportProgress(int percentProgress, object userState)
   {
       if (!this.WorkerReportsProgress)
       {
           throw new InvalidOperationException(SR.GetString("BackgroundWorker_WorkerDoesntReportProgress"));
       }
       ProgressChangedEventArgs progressChangedEventArgs = new ProgressChangedEventArgs(percentProgress, userState);
       if (this.asyncOperation != null)
       {
           this.asyncOperation.Post(this.progressReporter, progressChangedEventArgs);
           return;
       }
       this.progressReporter(progressChangedEventArgs);
   }
   3.触发事件
   private void ProgressReporter(object arg)
   {
       this.OnProgressChanged((ProgressChangedEventArgs)arg);
   }

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 = ;
   }

8. 完整代码

    /*
     * 由SharpDevelop创建。
     * 用户： David Huang
     * 日期: 2015/9/8
     * 时间: 14:54
     */
    using System;
    using System.Windows.Forms;
   
    namespace BackgroundWorkerTest
    {
        /// <summary>
        /// Description of MainForm.
        /// </summary>
        public partial class MainForm : Form
        {
            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;
            }
            void ButtonCalculateClick(object sender, EventArgs e)
            {
                this.buttonCalculate.Enabled = false;
                this.textBoxResult.Text = string.Empty;
                this.buttonCancel.Enabled = true;
                this.backgroundWorker1.RunWorkerAsync(new Tuple<int,int>(Convert.ToInt32(this.textBoxX.Text), Convert.ToInt32(this.textBoxY.Text)));
            }
            void ButtonCancelClick(object sender, EventArgs e)
            {
                this.backgroundWorker1.CancelAsync();
            }
            void BackgroundWorker1DoWork(object sender, System.ComponentModel.DoWorkEventArgs e)
            {
                for (int i = ; i < ; i++) {
                    System.Threading.Thread.Sleep();
                    if (backgroundWorker1.CancellationPending) {
                        e.Cancel = true;
                        return;
                    } else {
                        backgroundWorker1.ReportProgress(i * );
                    }
                }
                var t = e.Argument as Tuple<int,int>;
                e.Result = t.Item1 + t.Item2;
            }
            void BackgroundWorker1ProgressChanged(object sender, System.ComponentModel.ProgressChangedEventArgs e)
            {
                this.progressBar1.Value = e.ProgressPercentage;
            }
            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 = ;
            }
        }
    }

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