I have a function which takes relatively long to compute.

longtask[str_String] := (Pause[0.5]; ToUpperCase[str])

I need to show the result of this function "live" as applied to an input dynamically typed into a text box. I need typing to be fluid and fast. I expect the output to follow with some lag.

Here's an example:

DynamicModule[{str = ""},
    InputField[Dynamic[str], String, FieldSize -> {30, 2}, ContinuousAction -> True],
     SynchronousUpdating -> False]

The problem: Sometimes the character that I just typed gets immediately deleted. Why? How can I prevent this?

To test this, just quickly type any sentence into the text box. For example, if I type "Once upon a time there was a little girl.", I get something like this:

enter image description here

If you watch carefully while typing, you will notice that all characters appear, but some of them get immediately removed.

Is there a way to prevent this without affecting the responsiveness of the input field?

To put it differently: I am dealing with slow evaluations in Dynamic. I want ContinuousUpdating -> True. I understand that the output will respond slowly to the input, but I do not want this to affect the performance of the input. Is this technically possible?

Part of the reason why I want ContinuousUpdating -> True is that an InputField won't update otherwise until it loses focus.

  • $\begingroup$ @Kuba I didn't realize that. I can actually do that. $\endgroup$
    – Szabolcs
    Commented Nov 18, 2016 at 12:25

1 Answer 1


My guess it is a problem with FE-Kernel synchornization of variables state (str copy is here and there).

Notice that str which is not a DynamicModule's variable works well. Quite often it is not a major problem. Last value won't be preserved across sessions but at least it can be properly initialized in Initialization.

One needs to pick a less conflict prone name e.g. MyPackage`Gui`specificname but the code itself doesn't have to be changed:

With[{str := MyPackage`Gui`whatever}, 
  Deploy@Column[{InputField[Dynamic[str], String, 
      FieldSize -> {30, 2}, ContinuousAction -> True], 
     Dynamic[longtask[str], SynchronousUpdating -> False]}], 
  Initialization :> (str = "")]
  • 1
    $\begingroup$ Thanks! i.sstatic.net/A602t.gif $\endgroup$
    – Szabolcs
    Commented Nov 20, 2016 at 11:36
  • $\begingroup$ @Szabolcs Nice! $\endgroup$
    – Kuba
    Commented Nov 20, 2016 at 11:48

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