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bugs - Why is PaneSelector caching nested Dynamics and how to switch it off?


Bug introduced in V10.4 or earlier and persisting through V11.3



CASE:3875733, confirmed




It seems that Dynamic content is somehow cached by PaneSelector/FrontEnd.


Even weirder, it does respond to evaluation though it uses old values, incorrectly.


Here is a minimal example:


  (*our panel, Dynamic + := may seem strange but remember this is only an example*)

paneContent[] := Dynamic[Print[1]; DateString[]]

  (*our app implementation with PaneSelector*)
DynamicModule[{pane},
 Panel @ Column[{
    Button["test", pane = "pane"],
    Button["init", pane = "init"],
    PaneSelector[{
      "default" -> "default",
      "pane" -> Dynamic[pane; Print["pane"]; paneContent[]]

      }
     , Dynamic[ pane ], ImageSize -> {200, 50}

     ]

    }]
 ]

   (*same functionality implemented with Dynamic only*)


DynamicModule[{pane = "init"},
 Panel@Column[{
    Button["test", pane = paneContent[]],
    Button["init", pane = "init"],
    Pane[Dynamic[Print["pane"]; pane], {200, 50}]

    }]
 ]

Now toggle between test and init and observe what happens.



The second panel updates date while the first show only the initial one. Even though the Print[] statement is evaluated! I don't understand this at all, it seems that FrontEnd caches paneContent[] somehow.


Can I prevent that?



Answer



Edit


Using knowledge from Dynamic triggered but renders the same content we can enhance our Dynamic to always re-render:


...
"pane" -> Dynamic @ DynamicModule[{$flag = RandomReal[]}
, pane; Print["pane"]; paneContent[]
]
...


This way Dynamic will always see different $flag and re-render.


Old answer


The bug (not documented optimization which is a pain for those who are not aware of it) is confirmed so before it is fixed we need to use something which will mimic PaneSelector instead. So simply Dynamic + Switch.


paneContent[] := Dynamic[ DateString[]]

DynamicModule[
    {pane = "init"}
  , Panel @ Column[{
        Button["test", pane = "pane"]

      , Button["init", pane = "init"]
      , Dynamic[
            Switch[pane, "init", "init", "pane", paneContent[]]
          , TrackedSymbols :> {pane}
        ] 
    }]
]

This slightly differs from the example in a question as paneContent isn't evaluated each time you click "text" button but only when pane's value changes. This way it is closer to what PaneSelector does.


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