Export in Manipulate, infinite loop

Question

I want to make export of a plot inside Manipulate, but only once. However, this code runs infinitely and does export infinitely:

Manipulate[Module[{pl},
  pl = Plot[Sin[a x + b], {x, 0, 6}];
  Export["pl.pdf", pl];
  pl]
 , {a, 1, 4}, {b, 0, 10}]

If I remove Export[] from Manipulate, problem disappears, but I need it inside Manipulate for other reason;)

Can we control number of evaluations of certain code inside Manipulate?

Answer 1

What about this (sans Module):

Manipulate[pl = Plot[Sin[a x + b], {x, 0, 6}], {a, 1, 4}, {b, 0, 10}, 
Button["Export", Export["pl.pdf", pl]]]

Answer 2

It is a Dynamic problem. Export seems to be tagged dynamic somewhere. This is workaround. When I saw this, I remembered similar problem few days ago, and I used the same trick given there and it worked.

I set SynchronousUpdating -> False and ContinuousAction -> False else it will be very slow since you'll be trying to make a pdf file for each small tick change. Now a PDF file will be created when you release the mouse only from the slider.

SetDirectory[NotebookDirectory[]]; (*so we know where the PDF file is *)

Manipulate[

 Module[{x},
  pl = Plot[Sin[a x + b], {x, 0, 6}];
  myExport["myPlot.pdf", pl];
  pl
  ],

 {a, 1, 4},
 {b, 0, 10},

 SynchronousUpdating -> False,
 ContinuousAction -> False,

 Initialization :> 
  (
   myExport[type_String, p_] := myExport[type, p] = Export[type, p]
   )
 ]

reference: Manipulate keeps updating due to a function

Update

The problem as I saw it, is why Manipulate continue to update when using an Export when though Export is clearly not updating any of the control variables. On V 9.01, these are animations showing the problem. First animation is the original code posted above, and the second one, after adding SynchronousUpdating -> False.

The real question I assumed why: Why does this happen.

and