What is the TPL equivalent of rx's Observable.FromEventPattern? - task-parallel-library

Question

In rx you can write :
var oe = Observable.FromEventPattern<SqlNotificationEventArgs>(sqlDep, "OnChange");
and then subscribe to the observable to convert the OnChange event on the sqlDep object into an observable.
Similarily, how can you create a Task from a C# event using the Task Parallel Library ?
EDIT: clarification
The solution pointed by Drew and then written explicitely by user375487 works for a single event. As soon as the task finished ... well it is finished.
The observable event is able to trigger again at any time. It is can be seen as an observable stream. A kind of ISourceBlock in the TPL Dataflow. But in the doc http://msdn.microsoft.com/en-us/library/hh228603(v=vs.110).aspx there is no example of ISourceBlock.
I eventually found a forum post explaining how to do that: http://social.msdn.microsoft.com/Forums/en/tpldataflow/thread/a10c4cb6-868e-41c5-b8cf-d122b514db0e
public static ISourceBlock CreateSourceBlock(
Action,Action,Action,ISourceBlock> executor)
{
var bb = new BufferBlock();
executor(t => bb.Post(t), () => bb.Complete(), e => bb.Fault(e), bb);
return bb;
}
//Remark the async delegate which defers the subscription to the hot source.
var sourceBlock = CreateSourceBlock<SomeArgs>(async (post, complete, fault, bb) =>
{
var eventHandlerToSource = (s,args) => post(args);
publisher.OnEvent += eventHandlerToSource;
bb.Complete.ContinueWith(_ => publisher.OnEvent -= eventHandlerToSource);
});
I've not tryed the above code. There may be a mismatch between the async delegate and the definition of CreateSourceBlock.

Answer 1

There is no direct equivalent for the Event Asynchronous Pattern (EAP) baked into the TPL. What you need to do is using a TaskCompletionSource<T> that you signal yourself in the event handler. Check out this section on MSDN for an example of what that would look like which uses WebClient::DownloadStringAsync to demonstrate the pattern.

Answer 2

You can use TaskCompletionSource.
public static class TaskFromEvent
{
public static Task<TArgs> Create<TArgs>(object obj, string eventName)
where TArgs : EventArgs
{
var completionSource = new TaskCompletionSource<TArgs>();
EventHandler<TArgs> handler = null;
handler = new EventHandler<TArgs>((sender, args) =>
{
completionSource.SetResult(args);
obj.GetType().GetEvent(eventName).RemoveEventHandler(obj, handler);
});
obj.GetType().GetEvent(eventName).AddEventHandler(obj, handler);
return completionSource.Task;
}
}
Example usage:
public class Publisher
{
public event EventHandler<EventArgs> Event;
public void FireEvent()
{
if (this.Event != null)
Event(this, new EventArgs());
}
}
class Program
{
static void Main(string[] args)
{
Publisher publisher = new Publisher();
var task = TaskFromEvent.Create<EventArgs>(publisher, "Event").ContinueWith(e => Console.WriteLine("The event has fired."));
publisher.FireEvent();
Console.ReadKey();
}
}
EDIT Based on your clarification, here is an example of how to achieve your goal with TPL DataFlow.
public class EventSource
{
public static ISourceBlock<TArgs> Create<TArgs>(object obj, string eventName)
where TArgs : EventArgs
{
BufferBlock<TArgs> buffer = new BufferBlock<TArgs>();
EventHandler<TArgs> handler = null;
handler = new EventHandler<TArgs>((sender, args) =>
{
buffer.Post(args);
});
buffer.Completion.ContinueWith(c =>
{
Console.WriteLine("Unsubscribed from event");
obj.GetType().GetEvent(eventName).RemoveEventHandler(obj, handler);
});
obj.GetType().GetEvent(eventName).AddEventHandler(obj, handler);
return buffer;
}
}
public class Publisher
{
public event EventHandler<EventArgs> Event;
public void FireEvent()
{
if (this.Event != null)
Event(this, new EventArgs());
}
}
class Program
{
static void Main(string[] args)
{
var publisher = new Publisher();
var source = EventSource.Create<EventArgs>(publisher, "Event");
source.LinkTo(new ActionBlock<EventArgs>(e => Console.WriteLine("New event!")));
Console.WriteLine("Type 'q' to exit");
char key = (char)0;
while (true)
{
key = Console.ReadKey().KeyChar;
Console.WriteLine();
if (key == 'q') break;
publisher.FireEvent();
}
source.Complete();
Console.ReadKey();
}
}