Synchronizing code in Dynamics with other procedures

Question

I have huge code and some problem. I don't know how to make example. I'm afraid I can't make good SMALL example to demonstrate you the problem. So I'll try to explain. Maybe somebody know about such problem.

I have some DynamicModule. Large grid of plots, controls and tables of expressions some of them Dynamic.

Dynamic[myplots=Grid[{{Plot[...], Plot[...]}}], TrackedSymbols:>{expr}]

I have some function that execute some calculations what change Dynamic plots and Tables.

What's the problem:

expr=values;
Export["file.png", myplots];

When I'm trying to export calculated data and dynamically updating data from grid I get empty grid.

Changing expr launch calculating Dynamic grids. And Export saves file until calculations complete. So I get empty file.

If I add:

expr=values;
Pause[5];
Export["file.png", myplots];

All calculations complete and I have right result in "file.png".

But I never know how much time I need. It depends on input data size. Sometimes it 1 sec, sometimes it 30sec. So always have to wait 30sec to guarantee right result it's a bad idea.

I look at some commands as WaitAll:

expr=values;
WaitAll[myplots];
WaitAsynchronousTask[myplots];
Export["file.png", myplots];

Still nothing. I just don't understand.

What did I do wrong and how can I fix it?

Answer 1

I think you should change your setup a little bit.

Including procedures inside Dynamics is useful somethimes, for better events handling, but nothing fancy is going on here. Thus it is better schedule calculations in a more stable way, by moving the Grid generation outside Dynamic.

Your exporting procedure clearly expects myplots to have an up to date value with respect to expr.

expr=values;
Export["file.png", myplots];

So keeping myplots=Grid[...] in a Dynamic is not a stable solution.

Why it is better to keep "procedures" outside and use Dynamics only for displaying things?

• You can force FinishDynamic[] before the Export but if your Dynamic[myplots...] is not currently visible on a screen it won't be updated and assigned anyway.

• ref: "FinishDynamic[] does not return until all Dynamic objects are updated and displayed. " - "all" sounds like an overkill for our purpose, doesn't it?

• Dynamic[ myplots=Grid[...] ] requests this Grid calculation over the PreemptiveLink, it will timeout after DynamicEvaluationTimeout (6 seconds by default) and it will freeze other FrontEnd activities till it is finished.

  Which isn't critical since you can icrease the timeout limit or use SynchronousUpdating -> False (runs on the MainLink), but why complicate things.

If something is not clear I'm encouraging you to lookup those symbols in Help and read Advanced Dynamic Functionality tutorial (and related)

What I'd change is the following:

• create a plot updating procedure (for flexibility and readability)

  updatePlot[]:= (myplots=Grid[...]);
  

  You can put it in DynamicModule's Initialization.

• insert it in the exporting procedure

  expr=values;
  updatePlot[];
  Export["file.png", myplots];
  

• change your Dynamic content:

  Dynamic[expr;myplots, TrackedSymbols:>{expr}]
  

  Notice expr;, even though it is mentioned in TrackedSymbols it has to be explicitly stated in Dynamic's body to be properly tracked.

• additionaly you can introduce progress state variable to show a ProgressIndicator instead of myPlots before they are recalculated. Analogously to Palette button with progress bar, which is nice from user perspective.