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Pressing the button below will freeze my front end. An unpainted white dialog box comes up, then it completely freezes, and I need to forcibly kill the front end. CPU usage stays at zero during this.

Button["Ask", If[ChoiceDialog["OK?"], Print["OK!"]]]

Why does this happen?

What is the proper way to ask for confirmation before an action?

Answers:

My first question has been answered below and in the comment by @JohnFultz: use Button[..., Method -> "Queued"]. But this will make a button which doesn't work while evaluations are ongoing.

I need a button which works even during evaluations. I came up with the following solution:

Button["Ask", 
 CreateDialog[
  Column[{"OK?", 
    ChoiceButtons[{"OK"}, {Print["OK!"]; DialogReturn[]}]}]]]

CreateDialog returns immediately without waiting for an input, so the front end is not blocked. The functionality is moved completely into the dialog's button.

You can test that the button works and reacts immediately even while Pause[10] is evaluating.

Quoting the two most enlightening comments:

The "Preemptive" method interrupts queued (e.g., Shift+Enter) evaluations to do its work and cannot itself be interrupted. I.e., you can't preempt a preemptive calculation. ChoiceDialog is putting up an interface which independently needs the attention of the Mathematica kernel, but can't get it because Button has the kernel's sole attention - John Fultz

 

Button[..., Method->"Preemptive"] (the default Method value) and InputString[] are effectively exactly the opposite of each other. Button[..., Method->"Preemptive"] will pause the user interface (causing it to stop responding to events) until the computation is complete. InputString[] will pause the computation until the user has input some value. When you combine these two the user interface is waiting for the computation to complete while at the same time the computation is waiting for user interface input. - ragfield

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  • $\begingroup$ My vague guess is this has to do with evaluating things on the main or preemptive link, but I'm not sure ... $\endgroup$
    – Szabolcs
    Commented Jan 16, 2012 at 19:15
  • $\begingroup$ @NasserM.Abbasi Thanks, ragfield's description there is very instructive. $\endgroup$
    – Szabolcs
    Commented Jan 17, 2012 at 1:03
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    $\begingroup$ It's not a bug. Please see this question. $\endgroup$
    – Szabolcs
    Commented May 8, 2012 at 19:39
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    $\begingroup$ Please also see here $\endgroup$
    – Szabolcs
    Commented May 8, 2012 at 19:40
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    $\begingroup$ I flagged those two questions to be moved here. I propose we merge all three (I volunteer to do the editing). $\endgroup$
    – Szabolcs
    Commented May 8, 2012 at 19:43

3 Answers 3

Reset to default
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Try the following two alternatives, pressing the button while the Do loop is waiting.

Button["Click Here", Print[10!], Method -> "Preemptive"]
Do[Print[x]; Pause[5], {2}]
(*
 -> x
 -> 3628800
 -> x
*)

and 

Button["Click Here", Print[10!], Method -> "Queued"]
Do[Print[x]; Pause[5], {2}]
(*
 -> x
 -> x
 -> 3628800
*)

Some Explanation:

You can see that in the first case, the Do loop is interrupted for processing the Button procedure, but in the second case it is just queued and the Button processing is postponed until the Do loop ends.

In the case of the "Preemptive" method, Mma wants to assure that you are not stealing the CPU forever and impeding the Do loop finalization, so it gives you a certain time slice to process the Button procedure. If you yield control to human input (as in InputString[]), that time slice expires and thus the results you are getting.

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$\begingroup$

Try Method-> "Queued". This works for me, although I cannot say just why.

Button["Ask", If[ChoiceDialog["OK?"], Print["OK!"]], Method -> "Queued"]

Here's the explanation from the docs:

enter image description here

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  • 7
    $\begingroup$ The "Preemptive" method interrupts queued (e.g., Shift+Enter) evaluations to do its work and cannot itself be interrupted. I.e., you can't preempt a preemptive calculation. ChoiceDialog is putting up an interface which independently needs the attention of the Mathematica kernel, but can't get it because Button has the kernel's sole attention. $\endgroup$
    – John Fultz
    Commented Jan 16, 2012 at 22:10
  • $\begingroup$ @John Fultz Thanks. That now makes sense. $\endgroup$
    – DavidC
    Commented Jan 16, 2012 at 22:23
  • $\begingroup$ I'm also wondering if there's a way to "unfreeze" the front end if this happens ... $\endgroup$
    – Szabolcs
    Commented Jan 17, 2012 at 0:39
  • $\begingroup$ @Szabolcs Every time the front end froze and I called up Force Quit (on my Mac), a dialogue box from mma popped up asking me if I wanted to abort the evaluation. Then aborting worked. This may have been a mere coincidence but it happened 5 or 6 times. $\endgroup$
    – DavidC
    Commented Jan 17, 2012 at 0:44
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For this, you need to use Method->"Queued"

Button["hi", InputString[], Method -> "Queued"]
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  • $\begingroup$ Thanks! Interesting. I can't say I know enough of the inner-workings of Mathematica to understand why the hang-up occurs, although I just went into Documentation Center and saw that it crops up under "Possible Issues". $\endgroup$
    – user854688
    Commented Jul 20, 2011 at 20:18
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    $\begingroup$ Button[..., Method->"Preemptive"] (the default Method value) and InputString[] are effectively exactly the opposite of each other. Button[..., Method->"Preemptive"] will pause the user interface (causing it to stop responding to events) until the computation is complete. InputString[] will pause the computation until the user has input some value. When you combine these two the user interface is waiting for the computation to complete while at the same time the computation is waiting for user interface input. $\endgroup$
    – ragfield
    Commented Jul 21, 2011 at 1:35

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