在基于事件的异步模式上使用任务并行库

Question

我正在写一个网络应用程序.

消息通过传输方式发送:

Network.SendMessage (new FirstMessage() );

我可以注册一个事件处理程序,当这个消息类型到达时调用,如下所示:

Network.RegisterMessageHandler<FirstMessage> (OnFirstMessageReceived);

事件被解雇了:

public void OnFirstMessageReceived(EventArgs<FirstMessageEventArgs> e)
{
}

我正在为我的网络应用程序编写自定义身份验证过程,这需要大约五条消息才能完成.

不使用任务并行库,我将被迫编写前面的事件处理程序中的每个过程的下一步,如下所示:

public void OnFirstMessageReceived(EventArgs<FirstMessageEventArgs> e)
{
     Network.SendMessage( new SecondMessage() );
}

public void OnSecondMessageReceived(EventArgs<SecondMessageEventArgs> e)
{
     Network.SendMessage( new ThirdMessage() );
}

public void OnThirdMessageReceived(EventArgs<ThirdMessageEventArgs> e)
{
     Network.SendMessage( new FourthMessage() );
}

public void OnFourthMessageReceived(EventArgs<FourthMessageEventArgs> e)
{
     // Authentication is complete
}

我不喜欢跳过源代码来编写部分内容和部分内容的想法.这很难理解和编辑.

我听说任务并行库大大简化了这个解决方案.

但是,我使用任务并行库读取的许多示例都与启动一系列活动任务有关.我所说的' 主动 '是指每个任务都可以在明确调用时启动,如下所示:

public void Drink() {}
public void Eat()   {}
public void Sleep() {}

Task.Factory.StartNew(     () => Drink() )
            .ContinueWith( () => Eat()   )
            .ContinueWith( () => Sleep() );

这与我基于事件的异步模式相反,在该模式中,仅在收到消息时才调用每个事件处理程序方法.

换句话说,我不能做这样的事情(但我想):

Task.Factory.StartNew(     () => OnFirstMessageReceived()  )
            .ContinueWith( () => OnSecondMessageReceived() )
            .ContinueWith( () => OnThirdMessageReceived()  )
            .ContinueWith( () => OnFourthMessageReceived() );

我读过这篇文章,但我不太明白.看起来我需要做的事情TaskCompletionSource.如果我想从基于事件的异步模式(如上面的代码块)创建任务,它会是什么样子？

Answer 1

你对TaskCompletionSource是对的,它是将EAP(基于事件的异步模式)转换为TPL任务的关键.

这在此处记录:https://docs.microsoft.com/en-us/dotnet/standard/parallel-programming/tpl-and-traditional-async-programming#exposing-complex-eap-operations-as-tasks

这是简化的代码:

public static class Extensions  
{
    public static Task<XDocument> GetRssDownloadTask(
        this WebClient client, Uri rssFeedUri)
    {
        // task completion source is an object, which has some state.
        // it gives out the task, which completes, when state turns "completed"
        // or else it could be canceled or throw an exception
        var tcs = new TaskCompletionSource<XDocument>(); 

        // now we subscribe to completed event. depending on event result
        // we set TaskCompletionSource state completed, canceled, or error
        client.DownloadStringCompleted += (sender, e) => 
        {
                  if(e.Cancelled) 
                  {
                      tcs.SetCanceled();
                  }
                  else if(null != e.Error)
                  {
                      tcs.SetException(e.Error);
                  }
                  else
                  { 
                      tcs.SetResult(XDocument.Parse(e.Result));
                  }
        };

        // now we start asyncronous operation
        client.DownloadStringAsync(rssFeedUri);
        // and return the underlying task immediately
        return tcs.Task;
    }
}

现在,你需要做的就是制作一系列操作,只是设置你的延续(目前不太舒服,C#5等待和异步将帮助很多)

所以,这段代码可以像这样使用:

public static void Main()
{
    var client = new WebClient();

    client.GetRssDownloadTask(
        new Uri("http://blogs.msdn.com/b/ericlippert/rss.aspx"))
        .ContinueWith( t => {
            ShowXmlInMyUI(t.Result); // show first result somewhere
            // start a new task here if you want a chain sequence
        });

    // or start it here if you want to get some rss feeds simultaneously

    // if we had await now, we would add 
    // async keyword to Main method defenition and then

    XDocument feedEric = await client.GetRssDownloadTask(
        new Uri("http://blogs.msdn.com/b/ericlippert/rss.aspx"));
    XDocument feedJon = await client.GetRssDownloadTask(
        new Uri("http://feeds.feedburner.com/JonSkeetCodingBlog?format=xml"));
    // it's chaining - one task starts executing after 
    // another, but it is still asynchronous
}