Is it possible to make your own output format that uses the "Data not in notebook. Store now" mechanism?

Question

Several functions in WL have a custom elided typesetting form that does not automatically embed large amounts of data into the notebook but presents you with the option to do so if you wish. SparseArray is a good example of this:

SparseArray[RandomInteger[1, 10^8]]

You can make objects like these with BoxForm`ArrangeSummaryBox as explained here, but sometimes the rigid form of ArrangeSummaryBox is insufficient for what you want to do. Is there a way to "hijack" this mechanism?

Answer 1

I spent some time reverse-engineering the ArrangeSummaryBox code and came up with the function AddCustomTypesetting. It's easy to use and automatically incorporates the delayed embedding button if the expression is larger than a certain limit (given by $NotebookInlineStorageLimit). Consider this example from the Neat Examples section:

myObj[assoc_?AssociationQ][key_String] := assoc[key];
myObj[assoc_?AssociationQ][fun_] := fun@assoc;

ResourceFunction["AddCustomTypesetting"][
 myObj[_?AssociationQ], Function[{obj},
  RawBoxes @ Cell[
     BoxData @ ToBoxes @ Row[{
        "myObj", "[", "\[LeftAssociation]",
        Panel @ Grid[{
           {"Length:", obj[Length]},
           {"Keys:", Multicolumn[obj[Keys], 4]},
           {"ByteCount:", obj[ByteCount]}
           }, Alignment -> Left],
        "\[RightAssociation]", "]"
        }],
    ShowStringCharacters -> False
    ]
  ]
 ]

myObj[<|"a" -> RandomReal[1, 10], "b" -> 2|>]
myObj[<|"a" -> RandomReal[1, 10^6], "b" -> 2|>]

I tend to tinker a bit with functions like these. The latest version can always be found here on GH. The current version (that I will push to ResourceFunction["AddCustomTypesetting"] soon) will also allow you to remove the typesetting rule again with:

AddCustomTypesetting[myObj[_?AssociationQ], None]