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Re: Re: How to find which variable caused the trigger in Manipulate[]

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  • Subject: [mg104159] Re: [mg104136] Re: How to find which variable caused the trigger in Manipulate[]
  • From: "Nasser M. Abbasi" <nma at>
  • Date: Wed, 21 Oct 2009 06:31:22 -0400 (EDT)
  • References: <2009102020541.263124@jfultz2winlap>
  • Reply-to: "Nasser M. Abbasi" <nma at>

From: "John Fultz"

"Now this code has the distinct advantage that it works, and it's not so
complicated, but there's one fatal flaw in it.  It relies on the same
undocumented assumption I discussed earlier...that Dynamics are validated
left-to-right.  It would fall down if you flipped the order of the Row[], or in
a hypothetical future implementation of Dynamic that violates this 
So that's not my final solution.  But I wanted you to see this so you could
understand better how I built my final solution.  My final solution takes 
same approach, but tracks the Dynamics as they're initialized, forcing the
Text[] Dynamic to evaluate (possibly again) after the others.  The solution
doesn't seem very elegant, but it does work.  I'm afraid that the 
value of this example will require some study of the code, but you seem 
to do that.  I reversed the Row[] in this example to prove that it does, in
fact, work as advertised.


      {True, True, False}, dynamicsInitialized[[3]] = True;r = initialState,
      {True, True, True}, initialState = r];
      Text[r], TrackedSymbols -> {y, x, dynamicsInitialized}]],

   Dynamic[Refresh[dynamicsInitialized[[1]] = True; r = x; 
"",TrackedSymbols -> {x}]],

   Dynamic[Refresh[dynamicsInitialized[[2]] = True; r = y; 
"",TrackedSymbols -> {y}]]}

 {x, 0, 1, 0.01},
 {y, 0, 1, 0.01},
 {r, "", "", ControlType -> None},
 {{initialState, 0},   ControlType ->None},
 {{dynamicsInitialized, {False, False, False}}, ControlType -> None},

 Initialization :>  (dynamicsInitialized = {False, False, False})


Brilliant John as always.

Studying your code, I just learned something I never knew about Manipulate, 
which is probably why I could not come up with a solution.

I was wondering, since in the code above, dynamicsInitialized variable is 
initialized to {False, False, False}, then, assuming does not "touch" the 
"y" slider at all the whole time, then how could the Switch statement, which 
checks only for  {True, True, False} and  {True, True, True}, ever gets 

Since only when the "y" slider is moved, would you set the 
dynamicsInitialized[[2]] to true.

Yet, when I look at the DynamicModule[] code generated by the hitting the 
"Paste snapshot" button in the corner of the Manipulate display, it says 
that dynamicsInitialized has the value  {True, True, True} each time !  But 
how could it be so, I have not even touched the "y" slider! even if I do not 
touch the "x" slider, this variable will have this value.

But when I click the "initial settings" button, then look at the "Paste 
snaphot", then I see this variable having the value {False, False, False}.

I came up with a theory on how this could be:

Just by the act of displaying the Manipulate result on the screen, or 
scrolling over it, all the control variables will be "activated", or 
"triggered" initially, even when one has not physically moved the slides/the 
controls yet.

This means there will be some initial evaluations of the Manipulate 
expression. 3 initial evaluations that occur behind the scene before I ever 
get to touch the sliders.

The first evaluation detects that "x" is triggered (or maybe "y", since the 
order is not defined), so this will cause dynamicsInitialized to become 
{True, False, False}.

The second evaluation will detect that "y" control variable is triggered, so 
now dynamicsInitialized becomes {True, True, False}.

And now in the third evaluation, since dynamicsInitialized is being tracked 
also, and it has changed from the above evaluations, then now and only now, 
would one of the Switch statement conditions be used, the one which checks 
for {True, True, False}.and this will finally cause dynamicsInitialized 
becomes {True, True, True}. And now the rest will follow as the user moves 
the "x" or the "y" slides.

Thanks again for your help John.


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