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Avoid cross-thread access violation in an MVC scenario?
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Scenario: a controller is updating the data that's represented in a visualization. The visualization component should redraw whenever the data changes. I have created a minimal representation of the idea, below. The code works beautifully in practice, but the debugger screams about cross-thread violation at the moment of referral to the component when the "repaint" event is fired.

Are there idiomatic ways of getting the required event into the required thread?

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#light 

open System
open System.Windows.Forms
open System.Threading
open Idioms

type Controller =
    class
        val somethingChanged: (unit -> unit) * IEvent<unit>
        new () = { somethingChanged = IEvent.create() }
        member x.ChangeEvent = (snd x.somethingChanged)
        member x.Fire() = (fst x.somethingChanged)()
    end
    
type Visualization =
    class
        inherit Control
        val controller: Controller
        new(c) as this = {inherit Control(); controller = c} then
            // HOW DO WE DO THIS WITHOUT THE CROSS-THREAD VIOLATION?
            this.controller.ChangeEvent.Add(fun () -> this.Refresh())
            this.Paint.Add(fun (e) -> printf "paint called successfully\n")
    end
    
let form = new Form(Width = 400, Height = 400, Visible = true, Text = "demo")
let controller = new Controller()
let visualization = new Visualization(controller, Dock = DockStyle.Fill)
form.Controls.Add(visualization)

let t = new Thread(new ThreadStart(fun () -> while true do controller.Fire(); Thread.Sleep(500)))
t.IsBackground <- true
t.Start()

[<STAThread>]    
do Application.Run(form)
.

I've determined that the following line of code works, in the sense that it eliminates the cross-thread access violation.

let f = new MethodInvoker(fun () -> this.Refresh()) in this.controller.Repaint.Add(fun () -> this.BeginInvoke(f) |> ignore)

Unfortunately, it also destroys the responsiveness of the GUI. When calling Refresh behind the thread's back, I can get a smooth 45 frames per second of updates, with live resizing; when using BeginInvoke, the animation frequently stutters, and I usually lose the ability to resize the window or click in the close box.

Will I have to do something verbose and sophisticated?

By on 12/24/2006 10:54 AM ()

Is there anything more... I don't know... <i>poignant<i> than one of those threads where a person answers their own question? I wanted Control.Invalidate, not Control.Refresh. This enables me to get in and get out quickly in the GUI thread, and the control repaints itself at will. Any other comments are, of course, very much welcome.

By on 12/24/2006 1:02 PM ()

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