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Synchronously wait for F# event
f#

Hi,

I'm trying to use F# events as a messaging system to coordinate a GUI with a controller on a different thread. (This is a toy for learning F#: a tic-tac-toe game on WPF.) My problem: the Async.RunSynchronously call on the controller thread never terminates.

Code is below. Any idea how I can troubleshoot this?

-Max

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open System

open TTT

open TTT.WPF

open TTTGui

open System.Collections.ObjectModel

type FSController() = class

    member x.SquareIsReady = new Event<_>()

    interface Controller with

        member x.ClickSquare(i) =

            Console.WriteLine("Square {0} clicked", i)

            x.SquareIsReady.Trigger(i)

            Console.WriteLine("Square {0} triggered", i)            

        member x.GetView() : SquareView Collection =

            null

end

type FSPlayer() = class

    inherit HumanPlayer()

    let controller = new FSController()

    let gui = WPFGui.Make(controller)    

    override x.PlaceX(pos) =

        gui.ViewModel.[pos - 1].XVisible <- System.Windows.Visibility.Visible;

        gui.ViewModel.[pos - 1].NotifyAll()

    override x.PlaceO(pos) =

        gui.ViewModel.[pos - 1].OVisible <- System.Windows.Visibility.Visible;

        gui.ViewModel.[pos - 1].NotifyAll()

    override x.GetMove() =

        Console.WriteLine("Getting sq")

        let sq = Async.RunSynchronously(Async.AwaitEvent(controller.SquareIsReady.Publish))

        Console.WriteLine("Got {0}", sq)

        sq

end

    

let rf = new Referee(new FSPlayer(), new FSPlayer());

rf.RunGameToCompletion();

Console.WriteLine("The winner is " + rf.Board.DetermineWinner().ToString());

// output

Getting sq

Square 1 clicked

Square 1 triggered

Square 2 clicked

Square 2 triggered

Square 4 clicked

Square 4 triggered

[etc.]

.

Oh, wait:

member x.SquareIsReady = new Event()

every time you read that property, it creates a new, distinct event. I think you want

let e = new Event()

member x.SquareIsReady = e

By on 6/29/2010 5:34 PM ()

Avoid Async.RunSynchronously in favor of Async.StartImmediate in many cases.

That said, I don't think that general advice applies here. It seems (from what you're asking) like you really do want to block the UI while waiting for an event to be fired on the background controller thread (yes?). But RunSynchronously is called on the UI thread, right (that's what the code suggests to me, though your phrasing of the question suggests otherwise)? Please give a little clarification, before I try to offer advice. :)

In any case, stack traces of all threads at the point of deadlock is a useful way to troubleshoot.

By on 6/29/2010 5:19 PM ()

Avoid Async.RunSynchronously in favor of Async.StartImmediate in many cases.

That said, I don't think that general advice applies here.  It seems (from what you're asking) like you really do want to block the UI while waiting for an event to be fired on the background controller thread (yes?).  But RunSynchronously is called on the UI thread, right (that's what the code suggests to me, though your phrasing of the question suggests otherwise)?  Please give a little clarification, before I try to offer advice.  :)

Sorry, the detail I didn't post is that the UI is created on a separate STAThread when you create the FSPlayers. So RunSyncrhonously is called on the controller thread, i.e. I'm blocking the controller thread while waiting for the UI, just like Console.ReadLine().

Your next post is spot-on about the problem: I hadn't considered that a new event was being created every time. Your fix worked, too, and I now know how to declare events correctly in F#. Thanks!

-Max

By on 6/30/2010 7:40 AM ()

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