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I'm trying to wrap some dynamic code into DynamicModule to localize all the variables, yet when I do that, the variables stop updating.

That does not happen if I use Module instead. However, the variables get highlighted in red and when I hover over them with the mouse pointer the following message pops up: "A variable was used where it is probably not going to be evaluated before going out of scope".

The code in question is from Pickett's answer to my earlier question (AJAX-style HTTP calls triggered by a variable change):

url = "https://maps.googleapis.com/maps/api/geocode/json";
callback[_, "data", data_] := choices = extractsAddresses@First@FromCharacterCode[data]

Needs["GeneralUtilities`"]
extractsAddresses[data_] := ToAssociations[ImportString[data, "JSON"]][["results", All, "formatted_address"]]

fetchChoices[addr_] := URLFetchAsynchronous[
  url,
  callback,
  "Parameters" -> {"address" -> addr}
  ]

Column[{
  InputField[
   Dynamic[s, (fetchChoices[#]; s = #) &],
   String,
   ContinuousAction -> True
   ],
  Dynamic@choices
  }]

And here is the version wrapped in DynamicModule:

Needs["GeneralUtilities`"]
DynamicModule[{url, callback, choices = "", fetchChoices, s},
 url = "https://maps.googleapis.com/maps/api/geocode/json";
 callback[_, "data", data_] := 
  choices = extractsAddresses@First@FromCharacterCode[data];
 extractsAddresses[data_] := 
  ToAssociations[ImportString[data, "JSON"]][["results", All, 
    "formatted_address"]];
 fetchChoices[addr_] := 
  URLFetchAsynchronous[url, callback, 
   "Parameters" -> {"address" -> addr}];
 Column[{InputField[Dynamic[s, (fetchChoices[#]; s = #) &], String, 
    ContinuousAction -> True], Dynamic@choices}]
 ]

What am I doing wrong?

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  • 1
    $\begingroup$ I suspect that it has something to do with the asynchronous nature of the callback -- somehow Dynamic@choices is not tracking that kind of change. As a quick and dirty work-around, try Dynamic[choices, UpdateInterval->0] instead. $\endgroup$ – WReach Jun 19 '15 at 20:21
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I think you are actually not doing anything wrong but have found one of those cases where the automatic dependency tracking doesn't work (I think this is a bug and is also discussed in other questions, e.g. here, here or here). You can make the code work (version 10.1 on Windows) by explicitly telling the last Dynamic to track choices:

 Dynamic[choices,TrackedSymbols:>{choices}]

There are some things in your code that I probably would solve different (e.g. url is a constant and it is also unusual to localize function names like callback and fetchChoices as DynamicModule variables, also your function definitions would probably be better put into the Initialization option of DynamicModule). Here is a version which should work using the usual conventions:

Needs["GeneralUtilities`"];
With[{url="https://maps.googleapis.com/maps/api/geocode/json"},
    DynamicModule[{choices="",string},
        Column[{
            InputField[
                Dynamic[string,(fetchChoices[#];string=#)&],
                String,ContinuousAction->True
            ],
            Dynamic[choices,TrackedSymbols:>{choices}]
        }],

        Initialization:>(
            Needs["GeneralUtilities`"];
            callback[_,"data",data_]:=(
                choices=extractAddresses@First@FromCharacterCode[data]
            );
            extractAddresses[data_]:=ToAssociations[
                ImportString[data,"JSON"]
            ][["results",All,"formatted_address"]];
            fetchChoices[addr_]:=URLFetchAsynchronous[
                url,callback,"Parameters"->{"address"->addr}
            ];
        )
    ]
]

when playing with this you will find that sometimes the shown choices seem to not match the current string. I think that is because it is not guaranteed that the asynchronous callbacks will return in the order in which they are issued. You might need to handle that for a real application, e.g. only update choices when the received value is from a later call then the currently shown...

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  • $\begingroup$ Albert, thanks for your comments. Just to clarify, if I were to put function definitions into the Initialization option, wouldn't that create global variables? Or should I wrap the whole DynamicModule with Block or With and move static variables and function definitions there? $\endgroup$ – verse Jun 19 '15 at 20:53
  • $\begingroup$ No, Block and With won't help or work for these. You can make the symbols used as function names local to DynamicModule, the code in the Initialization option is also having access to the local variables. Doing that seems to work alright, but I'm not sure if I can forsee all the consequences that would have, though. An alternative would be to put them in an own namespace (Context), e.g. one which is unique to the cell in which that DynamicModule lives... $\endgroup$ – Albert Retey Jun 19 '15 at 21:39

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