基于事件的异步模式——BackgroundWorker

转自strangeman原文 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。

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;
}

 

 

 

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 = 0; i < 10; i++) {

System.Threading.Thread.Sleep(500);//模拟耗时操作

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;
}

 

 

 

 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 = 100;

}

 

 

 

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 = 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;
        }
        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 = 100;
        }
    }
}

 

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