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front end - How to catch the "Preemptive" evaluations from within the main loop?


When trying to make an optimized version of my plotRange function I faced a problem of catching the "Preemptive" evaluations generated by the FrontEnd from withing the main evaluation loop.


Consider the following:


ClearAll["Global`*"]
xr[r__] := (xRange = {r}; None);
yr[r__] := (yRange = {r}; None);
nb = CreateDocument[{Plot[Sin[x], {x, 0, 6 Pi}, Ticks -> {xr, yr}]}];
{xRange, yRange}
Pause[1];
{xRange, yRange}

NotebookClose[nb]


{xRange, yRange}


{{-0.392699, 19.2423}, {-1.04167, 1.04167}}



One can see that the first evaluation of {xRange, yRange} returns undefined symbols because at the moment of its evaluation the functions xr and yr are still not evaluated via a "Preemptive" link by the FrontEnd. But after waiting one second these functions are already evaluated and xRange, yRange are defined. Because I need these values for further evaluations I need to delay the main loop until the "Preemptive" evaluations will be done. (An alternative solution would be to force the FrontEnd to perform its evaluations in the main loop.)


One possible way to solve the problem is to initiate a Dialog which will wait for the Return generated by the FrontEnd. But "Preemptive" evaluations go past the Dialog and do not allow the return from it (the following code freezes the kernel):


Dialog[xr[r__] := (xRange = {r}; None);
 yr[r__] := (yRange = {r}; None; NotebookClose[nb]; 

   Return[{xRange, yRange}]);
 nb = CreateDocument[{Plot[Sin[x], {x, 0, 6 Pi}, 
     Ticks -> {xr, yr}]}];]

Is it possible to catch the "Preemptive" evaluations from the main loop in some way?


UPDATE


Here is a comparison of my original Rasterize-based version and the new FinishDynamic-based:


gr = Plot[Sin[x], {x, 0, 6 Pi}];
plotRange[plot : (_Graphics | _Graphics3D)] := 
 Quiet@Last@

   Last@Reap[
     Rasterize[
      Show[plot, PlotRangePadding -> None, Axes -> True, 
       Ticks -> (Sow[{##}] &), DisplayFunction -> Identity], 
      ImageResolution -> 1]]
Table[plotRange[gr], {100}]; // AbsoluteTiming

plotRange[plot : (_Graphics | _Graphics3D)] := 
 Quiet@Last@
   Last@Reap[(nb = 

       CreateDocument[{Show[plot, Axes -> True, 
          Ticks -> (Sow[{##}] &), PlotRangePadding -> None, 
          DisplayFunction -> Identity]}, ShowCellBracket -> False, 
        WindowFrame -> "Frameless", WindowElements -> {}, 
        WindowSize -> 1, Antialiasing -> False, Editable -> False, 
        Magnification -> 1, Saveable -> False, 
        WindowClickSelect -> False, WindowFloating -> False, 
        WindowFrameElements -> {}, WindowMovable -> False, 
        WindowSelected -> False, WindowTitle -> ""];
      FinishDynamic[]; NotebookClose[nb])]

Table[plotRange[gr], {100}]; // AbsoluteTiming


{2.828125, Null}


{8.406250, Null}



The original Rasterize-based version is 3 times faster.



Answer



After the dynamic evaluation (in this case, your plot) has been displayed to the screen but before you wish to access the variables, evaluate:


FinishDynamic[];


Note that this function isn't selective. It will force every Dynamic in the entire system to update before it continues.


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