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manipulate - Trouble understanding TrackedSymbols


While developing a relatively complex demonstration for Manipulate I encountered the problem that my output was unintentionally self-updating in an endless loop.


After some digging I discovered the problem is probably related with the following minimal code:


date = DateObject[{2014, 12, 31, 0, 0, 0}];
Dynamic[GeoGraphics[NightHemisphere[date], GeoRange -> "World"], 
 SynchronousUpdating -> False]

Because the date is fixed I honestly cannot see any reason why this is self-updating in an infinte loop.


First question. Why this happens?



To stop this behavior we can modify the code as:


date = DateObject[{2014, 12, 31, 0, 0, 0}];
Dynamic[GeoGraphics[NightHemisphere[date], GeoRange -> "World"], 
 TrackedSymbols :> {date}, SynchronousUpdating -> False]

In the real case things are more complex because I have to explicitly list and keep updated a list of many symbols. Things are more complex also because with nested Dynamic the following code doesn't prevent the self-updating:


date = DateObject[{2014, 12, 31, 0, 0, 0}];
Dynamic[{
  date, Dynamic[
   GeoGraphics[NightHemisphere[date], GeoRange -> "World"], 

   TrackedSymbols :> Automatic, SynchronousUpdating -> False]
  }, TrackedSymbols :> {date}, SynchronousUpdating -> False]

I read many time the advanced documentation of Manipulate and Dynamic, and my understanding was the inner Dynamic should inherits TrackedSymbols from the outer Dynamic.


Second question. Is that true?


Of course this is safe:


date = DateObject[{2014, 12, 31, 0, 0, 0}];
Dynamic[{
  date, Dynamic[
   GeoGraphics[NightHemisphere[date], GeoRange -> "World"], 

   TrackedSymbols :> {date}, SynchronousUpdating -> False]
  }, TrackedSymbols :> {date}, SynchronousUpdating -> False]

but again very complex to apply in the real case when there are many symbols.


Third question. There is a simpler way to prevent this behavior?




Hello,


Thank you for your email.


I have filed a bug report for the issue of NightHemisphere triggering updates with the development team.



The issue about TrackedSymbols->Automatic is not complete clear from the documentation: if you would like to let the inner Dynamic content inherits the tracked symbols of the enclosing constructs, you can use Refresh:


For example, the following code


Dynamic[{x, Refresh[{x, y}, TrackedSymbols -> Automatic]},
TrackedSymbols -> {x}]


will only update when x is changed.


Setting the tracked symbols correctly is a little tricky -- if the issue with NightHemisphere is causing great difficulty in your code, could you please send us the notebook and maybe the developer might provide some suggestion?


I hope this helps.


Thank you very much for giving us feedback and hopefully this issue would be improved in future release.


Sincerely, Wolfram Technical Support





GeoGraphics[{NightHemisphere@DateObject[{2015, 3, 21}]}] // InputForm


GeoGraphics[Graphics[{}]]





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