C#中的异步调用及异步设计模式(三)——基于事件的异步模式

四、基于事件的异步模式（设计层面）
基于事件的C#异步编程模式是比IAsyncResult模式更高级的一种异步编程模式，也被用在更多的场合。该异步模式具有以下优点：
· “在后台”执行耗时任务（例如下载和数据库操作），但不会中断您的应用程序。

· 同时执行多个操作，每个操作完成时都会接到通知（在通知中可以区分是完成了哪个操作）。

· 等待资源变得可用，但不会停止（“挂起”）您的应用程序。

· 使用熟悉的事件和委托模型与挂起的异步操作通信。

对于相对简单的应用程序可以直接用 .Net 2.0 新增的 BackgroundWorker 组件来很方便的实现，对于更复杂的异步应用程序则需要自己实现一个符合基于事件的C#异步编程模式的类。在实现基于事件的异步模式的设计前，需要了解基于事件的异步模式的实现原理是什么。基于事件的异步模式需要以下三个类型的帮助。
AsyncOperation：提供了对异步操作的生存期进行跟踪的功能，包括操作进度通知和操作完成通知，并确保在正确的线程或上下文中调用客户端的事件处理程序。
public void Post(SendOrPostCallback d,Object arg);
public void PostOperationCompleted(SendOrPostCallback d,Object arg);
通过在异步辅助代码中调用Post方法把进度和中间结果报告给用户，如果是取消异步任务或提示异步任务已完成，则通过调用PostOperationCompleted方法结束异步操作的跟踪生命期。在PostOperationCompleted方法调用后，AsyncOperation对象变得不再可用，再次访问将引发异常。在此有个问题：在该异步模式中，通过AsyncOperation的Post函数来通知进度的时候，是如何使SendOrPostCallback委托在UI线程上执行的？针对该问题下文有具体分析。

AsyncOperationManager：为AsyncOperation对象的创建提供了便捷方式，通过CreateOperation方法可以创建多个AsyncOperation实例，实现对多个异步操作进行跟踪。

WindowsFormsSynchronizationContext：该类继承自SynchronizationContext类型，提供 Windows 窗体应用程序模型的同步上下文。该类型是基于事件异步模式通信的核心。之所以说该类型是基于事件异步模式的通信核心，是因为该类型解决了“保证SendOrPostCallback委托在UI线程上执行”的问题。它是如何解决的？请看AsyncOperation类型的Post方法的实现：

/// <summary>
/// AsyncOperation类型的Post方法的实现
/// </summary>
public void Post(SendOrPostCallback d, object arg)
{
this.VerifyNotCompleted();
this.VerifyDelegateNotNull(d);
this.syncContext.Post(d, arg);
}

在AsyncOperation类型的Post方法中，直接调用了SynchronizationContext类型的Post方法，再看该Post方法的实现：

/// <summary>
/// WindowsFormsSynchronizationContext类型的Post方法的实现
/// </summary>
public override void Post(SendOrPostCallback d, object state)
{
if (this.controlToSendTo != null)
{
this.controlToSendTo.BeginInvoke(d, new object[] { state }); //此处保证了SendOrPostCallBack委托在UI线程上执行

}
}

有以上三个类型（AsyncOpertion，AsyncOperationManager和SynchronizationContext）作为基础，实现基于事件的异步模式的进度通知和完成通知就轻松多了。下面用一个基于事件的异步模型的例子来结束本文章。

using System;
using System.Collections.Generic;
using System.Text;
using System.ComponentModel;
using System.Collections.Specialized;
using System.Threading;

namespace test
{
/// <summary>
/// 任务1的进度通知代理
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
public delegate void Work1ProgressChangedEventHandler(object sender, Work1ProgressChangedEventArgs e);
/// <summary>
/// 任务1的进度通知参数
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
public delegate void Work1CompletedEventHandler(object sender, Work1CompletedEventArgs e);

public class BasedEventAsyncWorker
{
private delegate void WorkerEventHandler(int maxNumber, AsyncOperation asyncOp);
private HybridDictionary userStateToLifetime = new HybridDictionary();

public BasedEventAsyncWorker()
{ }

#region DoWork1的基于事件的异步调用
public void DoWork1Async(object userState, int maxNumber)
{
AsyncOperation asyncOp = AsyncOperationManager.CreateOperation(userState);

//userStateToLifetime有可能会同时被多线程访问，在此需要lock进行同步处理
lock (userStateToLifetime.SyncRoot)
{
if (userStateToLifetime.Contains(userState))
{
throw new ArgumentException(
"userState parameter must be unique",
"userState");
}

userStateToLifetime[userState] = asyncOp;
}

//异步开始任务1
WorkerEventHandler workerDelegate = new WorkerEventHandler(DoWork1);
workerDelegate.BeginInvoke(maxNumber, asyncOp, null, null);
}

private void DoWork1(int maxNumber, AsyncOperation asyncOp)
{
Exception e = null;

//判断该userState的任务仍在处理中
if (!TaskCanceled(asyncOp.UserSuppliedState))
{
try
{
int n = 0;
int percentage = 0;
while (n < maxNumber && !TaskCanceled(asyncOp.UserSuppliedState))
{
Thread.Sleep(100); //模拟耗时操作
percentage = (int)((float)n / (float)maxNumber * 100);
Work1ProgressChangedEventArgs progressChanageArgs =
new Work1ProgressChangedEventArgs(maxNumber, percentage, asyncOp.UserSuppliedState);
//任务1的进度通知
asyncOp.Post(new SendOrPostCallback(Work1ReportProgressCB), progressChanageArgs);
n++;
}
}
catch (Exception ex)
{
e = ex;
}
}

this.Work1Complete(e, TaskCanceled(asyncOp.UserSuppliedState), asyncOp);
}

private void Work1Complete(Exception exception, bool canceled, AsyncOperation asyncOp)
{
if (!canceled)
{
lock (userStateToLifetime.SyncRoot)
{
userStateToLifetime.Remove(asyncOp.UserSuppliedState);
}
}

Work1CompletedEventArgs e = new Work1CompletedEventArgs(exception, canceled, asyncOp.UserSuppliedState);

//通知指定的任务已经完成
asyncOp.PostOperationCompleted(new SendOrPostCallback(Work1CompleteCB), e);

//调用 PostOperationCompleted 方法来结束异步操作的生存期。
//为某个特定任务调用此方法后，再调用其相应的 AsyncOperation 对象会引发异常。
}

private void Work1ReportProgressCB(object state)
{
Work1ProgressChangedEventArgs e = state as Work1ProgressChangedEventArgs;

OnWork1ProgressChanged(e);
}

private void Work1CompleteCB(object state)
{
Work1CompletedEventArgs e = state as Work1CompletedEventArgs;

OnWork1Completed(e);
}

#region Work1的进度通知和任务完成的事件
public event Work1ProgressChangedEventHandler Work1ProgressChanged;
protected virtual void OnWork1ProgressChanged(Work1ProgressChangedEventArgs e)
{
Work1ProgressChangedEventHandler temp = this.Work1ProgressChanged;
if (temp != null)
{
temp(this, e);
}
}

public event Work1CompletedEventHandler Work1Completed;
protected virtual void OnWork1Completed(Work1CompletedEventArgs e)
{
Work1CompletedEventHandler temp = this.Work1Completed;
if (temp != null)
{
temp(this, e);
}
}
#endregion
#endregion

/// <summary>
/// 取消指定userState的任务执行
/// </summary>
/// <param name="userState"></param>
public void CancelAsync(object userState)
{
AsyncOperation asyncOp = userStateToLifetime[userState] as AsyncOperation;
if (asyncOp != null)
{
lock (userStateToLifetime.SyncRoot)
{
userStateToLifetime.Remove(userState);
}
}
}

/// <summary>
/// 判断指定userState的任务是否已经被结束。返回值：true 已经结束； false 还没有结束
/// </summary>
/// <param name="userState"></param>
/// <returns></returns>
private bool TaskCanceled(object userState)
{
return (userStateToLifetime[userState] == null);
}

}

public class Work1ProgressChangedEventArgs :ProgressChangedEventArgs
{
private int totalWork = 1;

public Work1ProgressChangedEventArgs(int totalWork, int progressPercentage, object userState)
: base(progressPercentage, userState)
{
this.totalWork = totalWork;
}

/// <summary>
/// Work1的总工作量
/// </summary>
public int TotalWork
{
get
{
return totalWork;
}
}
}

public class Work1CompletedEventArgs : AsyncCompletedEventArgs
{
public Work1CompletedEventArgs(Exception e, bool canceled, object state)
: base(e, canceled, state)
{
}

}

}