How to update a dialog Form dynamiclly

Hi,

    In my code I try to update a String field dynamiclly and it does not work, in the string filed is only displayed at the end Step 30, the dialog is blocked during the call back function, what can I do here?

procedure(myCreateForm()
let((str button)

str = hiCreateStringField(
?name 'testStr
?prompt "Test String :"
?editable nil
)

button = hiCreateButton(
?name 'testButton
?callback "myCallBack(hiGetCurrentForm())"
?buttonText "Start"
?invisible nil
?enabled t)

hiCreateAppForm(
?name 'myFrame
?formTitle "Test"
?fields
list(str button)
?buttonLayout 'Close
?dontBlock t
)
))

procedure(myCallBack(theForm)
let((num)
num = 0
while( num < 30
num = num+1
theForm->testStr->value = sprintf(a "Step %d" num)
printf("Step %d\n" num)
;do something here
ipcSleep(1)
)
))

dialog = myCreateForm()

hiDisplayForm(dialog)