I'm writing a networked application.
Messages are sent over the transport as such:
Network.SendMessage (new FirstMessage() );
I can register an event handler to be called when this message type arrives, like so:
Network.RegisterMessageHandler<FirstMessage> (OnFirstMessageReceived);
And the event gets fired:
public void OnFirstMessageReceived(EventArgs<FirstMessageEventArgs> e)
{
}
I'm writing a custom authentication procedure for my networked application, which requires around five messages to complete.
Without using the Task Parallel Library, I would be forced to code the next step of each procedure in the preceding event handler, like so:
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
}
I don't like the idea of jumping around the source code to code a portion of this and a portion of that. It's hard to understand and edit.
I hear the Task Parallel Library substantially simplifies this solution.
However, many of the examples I read using the Task Parallel Library were related to starting a chain of active tasks. What I mean by 'active', is that each task could start when called explicitly, like so:
public void Drink() {}
public void Eat() {}
public void Sleep() {}
Task.Factory.StartNew( () => Drink() )
.ContinueWith( () => Eat() )
.ContinueWith( () => Sleep() );
This is opposite from my event-based async pattern, in which each event handler method is called only when the message is received.
In other words, I can't do something like this (but I want to):
Task.Factory.StartNew( () => OnFirstMessageReceived() )
.ContinueWith( () => OnSecondMessageReceived() )
.ContinueWith( () => OnThirdMessageReceived() )
.ContinueWith( () => OnFourthMessageReceived() );
I've read this article, but I don't quite understand it. It seems like what I need has to do with TaskCompletionSource. If I wanted to make a task from my event-based async pattern like the code block above, what would it look like?
You're right about TaskCompletionSource, it's the key to transforming EAP (event-based asynchronous pattern) to TPL's Task.
This is documented here: https://docs.microsoft.com/en-us/dotnet/standard/parallel-programming/tpl-and-traditional-async-programming#exposing-complex-eap-operations-as-tasks
Here is the simplified code:
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;
}
}
Now, all you need to do, to make a chain of those operations, is just to set your continuations (which is not very comfortable at the moment, and the C# 5 await and async will help alot with it)
So, this code could be used like this:
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
}
Jeremy Likness has a blog entry title Coroutines for Asynchronous Sequential Workflows using Reactive Extensions (Rx) that might interest you. Here is the question he tries to answer:
The concept is straightforward: there are often times we want an asynchronous set of operations to perform sequentially. Perhaps you must load a list from a service, then load the selected item, then trigger an animation. This can be done either by chaining the completed events or nesting lambda expressions, but is there a cleaner way?
