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

Question

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?

paneContent[]:=Dynamic[pane;Print[Stack[_]];DateString[]] with "pane" -> paneContent[] seems working. So it looks like the Dynamic in paneContent[] needs to be triggered explicitly. But I don't understand what is the relation between the global pane and the localized pane. — Silvia, Mar 22 '17 at 20:12
@Silvia paneContent is independent from DynamicModule so can't use it. The more that bare Dynamic works, I just won't use PaneSelector. For me it is just crazy that this prints Print["pane"]; paneContent[] but it doesn't evaluate paneContent[]. I suppose it is a side effect of something which should have made our lifes easier, again. Thanks for your attention :)\ — Kuba, Mar 22 '17 at 20:23

Kuba · Accepted Answer · 2018-06-07T10:10:28.387

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.

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

1 Answers1

Edit

Old answer

Linked