Working with Dynamic: Synchronization with EventActions ( and other preemptive evaluations)

Question

I would like to make a simple "game": Display a dialog with a random letter, if the same letter is pressed on the keyboard, the display changes into a string "Correct", and after a while a new random letter is generated.

However, in the below code, I am unable to get the "Correct" displayed on the screen. The Dynamic does not update timely.

rand := RandomChoice[FromCharacterCode /@ Join[Range[65, 90]]];
display = rand;

CreateDialog[
 Dynamic[Style[display, 600], TrackedSymbols :> {display}], 
 NotebookEventActions -> {"KeyDown" :> 
    Module[{k = Capitalize@CurrentValue@"EventKey", x = display},
     If[x === k,
      display = "Correct";
      Pause[1];
      display = rand]]}]

How to make it work?

Answer 1

What is going on

By default the Dynamic, EventHandlers and various controllers like Button and Slider use a PreemptiveLink to communicate with the kernel.

Synchronous versus Asynchronous Dynamic Evaluations about the PreemptiveLink:

[...] There is no queue; instead, the front end sends one evaluation at a time and waits for the result before continuing with its other work, [...] the front end is completely locked up, and no typing or other actions are possible. [...] (* so Dynamics' updates aswell *)

In your case display = "Correct"; Pause[1]; display = rand is that "one evaluation" during which the front end is locked up. Therefore there is no reaction for display's change.

Solutions

• The shortest one would be to use a MainLink (Asynchronous/Queued).

  Unfortunately for Notebook/Cell/FrontEndEventActions we do not have Method->"Queued" like in Button or ActionMenu.

  You can check that this indeed works:

   Dynamic[x]
       (*check with and without that method*)
   Button["test", x = 1; Pause[1]; x = 2, Method -> "Queued" ]
  

• FinishDynamic[]

  We can use FinishDynamic[] to tell the front end that it should (blindly) update all out of date Dynamics.

  This works well:

  display = "Correct"; FinishDynamic[]; Pause[1];  display = rand
  

  The Kernel waits till the front end confirms FinishDynamic[] end, and then proceeds with Pause. Quite often it is convenient, however, one should keep in mind that this function is going to prompt all dynamics checks.

  related: How to catch the “Preemptive” evaluations from within the main loop?

• ScheduledTasks

  Scheduling a procedure is done immediately so it doesn't block the link anymore.

  Pause[1]; expr; can be translated to RunScheduledTasks[expr, {1}]

  Nesting and synchronizing multiple tasks may be tricky but here it works as intended.

  This automatically gives us:

  display = "Correct"; RunScheduledTask[display = rand, {1}]
  

Further readings

Evaluation order of Button[]'s action

Working with ScheduledTasks inside DynamicModules

Button with progress bar