基于事件的异步模式—

　　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。

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。

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;

}

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。

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。

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);

}

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;

}

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 = ;

}

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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基于事件的异步模式——BackgroundWorker

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