How can I make a stylesheet that counts subcells?

Question

The example cells in the documentation each have a count of the cells inside their section:

 Cell[TextData[{"Basic Examples", "  ", Cell["(4)", "ExampleCount"]}],
   "ExampleSection", "ExampleSection"]

But this is static content, how exactly would this work dynamically? I'd like to make my own cell style where the cell dingbat counts the number of cells inside the cell group it contains (and updates itself dynamically of course).

I've looked at the stylesheet for outline-styled notebooks and then tried using the Counter* options, but these are for dynamic tallying, not content counting and there's not much documentation on these esoteric front-end things like

CounterBoxOptions->{CounterFunction:>CapitalRomanNumeral}]

Any help would be appreciated.

Answer 1

I think when it's done each time you save the notebook it should be nice enough :)

SetOptions[
 EvaluationNotebook[],
 NotebookEventActions -> {
   {"MenuCommand", "Save"} :> (Scan[
      Module[{nr},
        SelectionMove[#, All, CellGroup, AutoScroll -> False];
        nr = Length @ Select[
           SelectedCells[], 
           Experimental`CellStyleNames[#] === "Input" & (*1*)
        ];
        SetOptions[#, CellDingbat -> "(" <> ToString[nr] <> ")"];
      ] &
      ,
      Cells[CellStyle -> "Section"] (*2*)
   ]),
   PassEventsDown -> True
   }
 ]

Ad 1. Cell style to count

Ad 2. Cell style whose parent group end "resets the counter"

You can use it in stylesheets too.

---

Update from Question's Author:

As Kuba's comment fixes the raggedness:

SetOptions[EvaluationNotebook[], 
     NotebookEventActions -> {{"MenuCommand", 
         "Save"} :> (Scan[
          Module[{nr}, 
            SelectionMove[#, All, CellGroup, AutoScroll -> False];

            nr = Length@
              Select[SelectedCells[], 
               Experimental`CellStyleNames[#] === 
                 "ItemNumbered" & (*1*)];

            SetOptions[#, 
             CellDingbat -> 
              Cell[BoxData[
                PaneBox[
                 StyleBox[ToString[nr] <> "   ", 
                  RGBColor[0.5, 0.5, 0.67, 0.81], 
                  FontFamily -> "Continuum Light", 15], 
                 Alignment -> Right, ImageSize -> 40]], 
               Background -> White]];] &, 
          Cells[CellStyle -> "Subsection"] (*2*)]), 
       PassEventsDown -> True}]

Answer 2

Here is a stylesheet solution that gives "Section" cells a dingbat that counts the number of "Input" cells in the section:

SetOptions[
    EvaluationNotebook[],
    StyleDefinitions -> Notebook[
        {
        Cell[StyleData[StyleDefinitions -> "Default.nb"]], 
        Cell[StyleData["Input"],
            CounterIncrements -> "CellCount"
        ],
        Cell[StyleData["Notebook"],
            TaggingRules -> "TotalCells" -> Dynamic[Length[Cells[CellStyle -> "Input"]]]
        ],
        Cell[StyleData["Section"], 
            CellDingbat -> Cell @ BoxData @ DynamicBox[
                RowBox[{
                    "(",
                    ToBoxes[
                        AbsoluteCurrentValue[ParentCell[EvaluationCell[]], {TaggingRules, "NextSection"}] - 
                        AbsoluteCurrentValue[ParentCell[EvaluationCell[]], {TaggingRules, "ThisSection"}]
                    ],
                    ")"
                }]
            ], 
            TaggingRules -> {
                "NextSection" -> Dynamic @ Replace[
                    NextCell[CellStyle -> "Section"], 
                    {
                    None :> CurrentValue[EvaluationNotebook[], {TaggingRules, "TotalCells"}], 
                    o_ :> CurrentValue[o, {"CounterValue", "CellCount"}]
                    }
                ],
                "ThisSection" -> Dynamic[CurrentValue[EvaluationCell[], {"CounterValue", "CellCount"}]]}], 
        }, 
        StyleDefinitions -> "PrivateStylesheetFormatting.nb"
    ]
]

Note that all the dynamic content might cause the notebook to be laggy.