[plt-scheme] Parameters and Eventspaces Question

From: Matthew Flatt (mflatt at cs.utah.edu)
Date: Tue Oct 28 20:20:35 EDT 2008

At Tue, 28 Oct 2008 12:41:08 -0600, "Doug Williams" wrote:
> I assume the problem is with my mental model of parameters and eventspaces.
> I had assumed that each eventspace would have, in addition to its event
> queues, etc,  its own execution thread, and copies of any parameters. 

That's correct.

> Since
> it allows me to parameterize or set the current-frame in a new eventspace, I
> assume the eventspace 'knows about' my current-frame parameter.  What I
> don't understand is why a call to update-text-field by a frame that I think
> is running under 'new' eventspace doesn't get a value for the current-frame
> parameter. 


 (parameterize ([current-eventspace E])

the `parameterize' sets the `current-eventspace' parameter to `E' while
evaluating `B', it still evaluates `B' in the current thread --- not
the handler thread of `E'. So if you mutate a parameter within `B',
then it still mutates the parameter for the current thread, not for the
thread of `E'.

Use `queue-callback' to jump to another eventspace's handler thread.
The `queue-callback' function uses the `current-eventspace' parameter
(of the current thread) to determine where to queue the callback, and
the callback is invoked in that eventspace's handler thread.

 (define (call-in-other-eventspace e thunk)
   (let ([ch (make-channel)])
     (parameterize ([current-eventspace e])
       (queue-callback (lambda ()
                         (channel-put ch (thunk)))))
     (channel-get ch)))

  (lambda ()
    (parameterize ((current-frame (instantiate my-frame%
                                               ("Frame 2"))))
      (send (current-frame) show #t)

  (lambda ()
    (let ((frame-3 (instantiate my-frame%
                                ("Frame 3"))))
      (current-frame frame-3)
      (send (current-frame) show #t)

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