A predicate to check whether a selection is empty or not

[kanitw]

Our selection can include empty: "all" | "none". However, sometimes, we want to apply each of the modes to a different part of the same visualization.

For example, in https://vega.github.io/vega-lite/examples/interactive_bar_select_highlight.html:

• We want the fillOpacity to activate the selection predicate both when there are selected data for the select selection and when there is none.
• However, we want the strokeWidth to appear only when there are selected data for the select selection, but not when there is none. Currently, I had to add a hack where I have to combine an expression with the selection predicate:

           test": {
            "and": [
              {"selection": "select"},
              "length(data(\"select_store\"))"
            ]
          }

Note that the expression "length(data(\"select_store\"))" is a hack as I know the underlying data store name in Vega.

It would be more natural to provide a syntax for this as a part of a selection predicate (and a condition definition).

The tricky part is that the new syntax that we can add here may be overriding the "empty" defined.

Some alternatives (given a selection named select):

a) Add {empty: "sel_name"} to the selection logical chain

{
  "condition": {"selection": {"or": ["select", {"empty": "select"}], "value": ...}
}

{
  "condition": {"selection": {"and": ["select", {"not": {"empty": "select"}}], "value": ...}
}

b) Add {empty: "sel_name"} to the predicate chain

{
  "condition": {"test": {"or": [{"selection": "select"}, {"empty": "select"}], "value": ...}
}

{
  "condition": {"test": {"and": [{"selection": "select"}, {"not": {"empty": "select"}}], "value": ...}
}

c) Add boolean empty to both the selection predicate and condition predicate

{
  "condition": {
    // Don't trigger predicate if empty (equivalent to "and not empty") 
    "selection": "select", "empty": false,  
    "value": ...
  } 
}

{
  "condition": {
    //Trigger predicate if empty (equivalent to "or empty")
    "selection": "select", "empty": true,  
    "value": ...
  } 
}

For this c) design we can also add "empty" to the selection predicate too: {"selection": "select", "empty": false}

---

After writing these alternatives, I think c) is probably the most user-friendly.

@arvind @domoritz

[domoritz]

C definitely makes sense to me. Why do we even need an empty property on the selection itself? I feel how we handle empty is something that should be in the condition or the filter, not the definition of the selection.

[kanitw]

I think if we have to pick one having only on the predicate make more sense.

However i can see that if we apply the predicate multiple times like in multi-view plots having it in the selection is good for setting the default empty for a certain selection.

[kanitw]

Plus, we already have these on selection — better avoid breaking changes.

[kanitw]

Reading this again, I still like option c).

Plus, we already have these on selection — better avoid breaking changes.

Given we may introduce this in VL 5.0, which will already introduce a number of changes. It might make sense to just move this "empty" bit to predicate.