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 = ;
}
}
}代码下载：点我