LocatorPane, Dynamic and deleting locators

Question

Compare the following two code snippets :

Version 1

  pts = {{1, 1}, {0.2, 0.3}, {0.5, .3}}
  locpts = pts;
  Panel[
   LocatorPane[
    Dynamic[locpts,
     {(locpts = #; pts = #
        ) &}],
    Dynamic@
     Graphics[{Point[locpts], Dynamic[Polygon[locpts]]}, 
       PlotRange -> {{0, 1}, {0, 1}}],
    LocatorAutoCreate -> All]]

Version 2

  pts = {{1, 1}, {0.2, 0.3}, {0.5, .3}}
  locpts = pts;
  Panel[
   LocatorPane[
    Dynamic[locpts,
     {(locpts = #; pts = #
        ) &}],
        Dynamic@
     Graphics[{Point[locpts], Dynamic[Polygon[pts]]}, 
      PlotRange -> {{0, 1}, {0, 1}}],
    LocatorAutoCreate -> All]]

Both versions are the same except for the third line from below where Polygon[locpts] was changed to Polygon[pts].

In the first version a delete of a locator is immediately propagated in the graphic while this is not the case in the second version. I would expect no difference since I set both pts and locpts to #.

Question: what is the explanation of this behavior?

Heike · Accepted Answer · 2012-05-23T15:07:46.527

3

It looks like the second argument of Dynamic in LocatorPane[Dynamic[locpts, ...], ...] isn't called when a locator is removed. Consider for example

pts = {{1, 1}, {0.2, 0.3}, {0.5, .3}}
locpts = pts;
Panel[
 LocatorPane[Dynamic[locpts, (Print[#]; locpts = #; pts = #) &], 
  Dynamic@Graphics[{Point[locpts], Polygon[pts]}, 
   PlotRange -> {{0, 1}, {0, 1}}, ImageSize -> 350],
  LocatorAutoCreate -> All]]

then the Print statement will only output a line when a locator is added or moved.

edited May 23 '12 at 15:07

answered May 23 '12 at 13:54

Heike

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This snippet does not run, but I get your point. – nilo de roock May 23 '12 at 14:59
broken code: brackets do not match – Mr.Wizard May 23 '12 at 14:59
Yes, this is an accurate reflection of the question. Is this expected behavior? How can a removal of points best be trapped? – nilo de roock May 23 '12 at 15:03
1

@ndroock1 I've fixed the code now. – Heike May 23 '12 at 15:08
Please see my previous comment ( if you haven't done so already. ) – nilo de roock May 23 '12 at 15:17
@ndroock1 I have no idea whether this is known behaviour or not. To trap the removal of points, you could compare the lengths of locpts and pts just before Graphics.... If Length[locpts]<Length[pts], then a points was removed, and you should take appropriate action. – Heike May 25 '12 at 12:55
Sorry, for the late accept. – nilo de roock Jun 04 '12 at 08:21

LocatorPane, Dynamic and deleting locators

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