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Re: Problems with ChoiceDialog inside a Dynamic: Bug or not (well)

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  • Subject: [mg107855] Re: Problems with ChoiceDialog inside a Dynamic: Bug or not (well)
  • From: John Fultz <jfultz at>
  • Date: Mon, 1 Mar 2010 04:42:27 -0500 (EST)

On Sun, 28 Feb 2010 04:53:10 -0500 (EST), Albert Retey wrote:
> Hi,
>> Synchronous Dynamics (which is what you get by default) are stop-the-
>> world sorts
>> of evaluations.  When they're running, any ongoing kernel computation is
>> completely halted, and the Dynamic evaluation is completely
>> uninterruptible.
>> Even another Dynamic evaluation cannot interrupt it.  Likewise, the
>> front end is
>> locked into a mode where it can only respond to a very limited number
>> of events
>> while it awaits the resolution of the Dynamic.
>> For this reason, all synchronous Dynamics are evaluated, by default,
>> with a
>> TimeConstrained wrapper using a time derived from the
>> EvaluationDynamicTimeout
>> option in the kernel.  This option can be tailored on a per-Dynamic
>> basis if you
>> like using the Style[] function to set the option.  The goal is that
>> absolutely
>> no Dynamic should be allowed to lock up the front end and kernel in
>> such away
>> that they appear unresponsive to users.
>> ChoiceDialog[], or any other function which awaits user input, causes
>> the kernel
>> to wait for results, but doesn't stop the timer in TimeConstrained[]
>> from
>> expiring.
>> Any Dynamic which you expect to take a long time should be using
>> SynchronousDynamic->False.  This causes the Dynamic to be put into the
>> same
>> queue as Shift+Enter evaluations, and it can take as long as you want.
>> So it's
>> not evaluated urgently, but there will be no need for it to evaluate
>> urgently,
>> either.
>> Incidentally, the same thing applies with Method->{"Queued"} (as
>> opposed to
>> "Preemptive") in Button and ActionMenu.
> I hear your words, but probably don't understand....
> While my experience shows that Method->"Queued" in Buttons and
> ActionMenus fixes any problems with kernel blocking dialogs, using
> SynchronosUpdating->False (I guess that's what you meant) does not help,
> at least with my version 7 on Windows XP this also hangs:
> DynamicModule[{planets, x, habitantq},
> planets = {"Mercury", "Venus", "Earth"};
> x = First[planets];
> Column[{
> PopupMenu[Dynamic[x, {x = #;
> If[StringMatchQ[x, "Earth"],
> habitantq = ChoiceDialog[Column[{"Do you live on earth?"}],
> {"Yes" -> True, "No" -> False}]]} &,
> SynchronousUpdating -> False], planets],
> Dynamic[x],
> Dynamic[habitantq]
> }]]
> and so does this:
> DynamicModule[{planets, x, habitantq},
> planets = {"Mercury", "Venus", "Earth"};
> x = First[planets];
> Column[{
> PopupMenu[Dynamic[x, {x = #;
> If[StringMatchQ[x, "Earth"],
> habitantq = ChoiceDialog[Column[{"Do you live on earth?"}],
> {"Yes" -> True, "No" -> False}]]} &,
> SynchronousUpdating -> False], planets],
> Dynamic[x, SynchronousUpdating -> False],
> Dynamic[habitantq, SynchronousUpdating -> False]
> }]]
> is there any known way to get something like this working?
> regards,
> albert

Sorry...yes, of course I meant SynchronousUpdating.

You raise an important issue which I've never taken the opportunity to 
completely clarify on this group (although I think I touched on it briefly a 
couple of months ago).  The issue is that there are really two different kinds 
of Dynamics.  They share so many properties that many people really don't need 
to understand the difference, but there are a small number of ways in which they 
are distinctly different.

We don't have official names for these things really, so I'll make up some names 
which you may or may not like, but probably shouldn't consider official Wolfram 
lingo.  I'll refer to "instancing" Dynamics and "controller" Dynamics.

An instancing Dynamic is one which creates an area on the screen containing some 
contents.  MakeBoxes, when run on such a Dynamic, turns it into a DynamicBox.  
If such a Dynamic were unable to resolve, you'd see a gray box on the screen 
representing the area it would have taken (with a few exceptions...a Dynamic 
inside of a graphic might not show the gray box).  You can think of an 
instancing Dynamic as an output-only zone.  You can't somehow click on an 
instancing Dynamic and directly affect its properties...not without some 
mechanism of causing additional evaluations.  Therefore, the second argument of 
Dynamic is meaningless to an instancing Dynamic.

A controller Dynamic is one which does not represent an area on the screen, but 
merely sets the value of another thing which displays on the screen.  For 
example, the Dynamic argument of Slider, Checkbox and Locator.  Any coordinates 
of a graphics primitive specified with Dynamic are using a controller Dynamic.  
Dynamics on the right-hand side of front end options are also controller 
Dynamics.  MakeBoxes of such a Dynamic returns Dynamic, not DynamicBox.  It 
doesn't return DynamicBox because what's important is the *value* of the Dynamic 
result, not its appearance.  The appearance of a Slider is always as a slider; 
the Dynamic argument merely controls the value the slider appears to be set to.  
In nearly all cases (excepting the rhs of FE options), a controller Dynamic 
represents a two-way interface.  It controls the display of its value, but also 
accepts the setting of new values through some interactive means, which makes 
the second argument of Dynamic a very meaningful concept here.

As I said, most of the functionality of Dynamic is shared between both types.  
But here's a list of things (well, the things I could think of...might not be 
complete) which you can only meaningfully use within one of the types...

* Second argument of Dynamic - controller Dynamic
* ShrinkingDelay option of Dynamic - instancing Dynamic
* SynchronousUpdating option of Dynamic - instancing Dynamic

The first two limitations are conceptual and really quite necessary.  If an 
instancing Dynamic allowed you to change it, it would be an InputField (with 
Appearance->None).  There are many times when that's really not what you want, 
and if it is, you should simply use InputField instead.  ShrinkingDelay just 
isn't meaningful for an object which has no concept of size, such as a 
controller Dynamic.

SynchronousUpdating is a bit trickier.  There are extra difficulties with 
implementing an asynchronous version of controller Dynamic (particularly, having 
a control which is in an "unknown" state for a possibly long period of time).  
And that extra difficulty is exactly why it's not implemented in controller 
Dynamics for now.  But one can imagine that this as a meaningful concept, and so 
this might change for future versions.

But for now, SynchronousUpdating only works for instancing Dynamics.  Which 
means that controller Dynamics are generally locked into what I referred to 
before as "stop-the-world" evaluations.

Note that it's generally not necessary to implement things this way, though.  
You can make an controller Dynamic which sets immediately, and then triggers an 
instancing Dynamic that operates asynchronously.  Or, perhaps a controller 
Dynamic isn't necessary at all...but instead some sort of Button or ActionMenu 

Here's an interesting concept I worked up for someone recently which you might 
find ActionMenu which works like a PopupMenu by virtue of 
containing an instancing Dynamic for the appearance.  The simplest case...

DynamicModule[{x = 1}, 
 ActionMenu[Dynamic[x], {1 :> (x = 1), 2 :> (x = 2), 3 :> (x = 3)}]]

which is equivalent to

DynamicModule[{x = 1}, PopupMenu[Dynamic[x], {1, 2, 3}]]

except that it guarantees the evaluation of the "x =" statements every time you 
choose the menu, rather than only when 'x' would change.  Your example could 
very easily be reformulated using such a mechanism.  Then setting 
Method->"Queued" would allow you to put things such as ChoiceDialog[] in the 


John Fultz
jfultz at
User Interface Group
Wolfram Research, Inc.

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