8.6.1 Updating windows in real time

If your code needs to cause visible updates whilst continuing to do further computation, then you should run your computation in a separate thread which is not directly associated with the CAPI window.

Consider the following example where real work is represented by calls to sleep :

1. Evaluate this code:

(defun change-text (win text)

  (setf (title-pane-text win)

        text))

(defun my-callback (win)

  (change-text win "Go")

  (loop

   for i from 0 to 20 do 

   (change-text win (format nil "~D" i))

   (sleep 0.1)))

(defun test ()

  (let* ((p1 (make-instance 'title-pane

                            :text "init"))

         (p2 (make-instance

              'button :text "Go"

              :callback-type :none

              :callback #'(lambda ()

                            (my-callback p1)))))

    (contain

     (make-instance 'row-layout :description (list p1 nil p2))

     :width 200 :height 200)))

2. Run (test) and note that the updates do not appear until my-callback returns. This is because it uses only one thread.
3. Now try this modified callback which uses a worker thread to perform the calculations:

(defun my-work-function ()

  (let ((mbox (mp:ensure-process-mailbox)))

    ;; This should really have an error handler.

    (loop (let ((event (mp:process-read-event mbox

                        "Waiting for events")))

            (cond ((consp event)

                   (apply (car event) (cdr event)))

                  ((functionp event)

                   (funcall event)))))))

(setf *worker*

      (mp:process-run-function "Worker process" ()

                               'my-work-function))

(defun change-text (win text)

  (apply-in-pane-process win

                         #'(setf title-pane-text)

                         text win))

(defun my-callback (win)

  (mp:process-send

   *worker*

   #'(lambda ()

       (change-text win "Go")

       (loop

        for i from 0 to 20 do 

        (change-text win (format nil "~D" i))

        (sleep 0.1)))))

4. Run (test) again: you should see the updates appear immediately.

A real application might also display an Abort button during the computation, with a callback that aborts the worker process.

CAPI User Guide (Windows version) - 30 Aug 2011