Map-wide variables

[stevage]

Motivation

Expressions can respond to three things (afaik): properties of the data, zoom, and temporary feature state of the data. But sometimes it would be helpful to respond to something map-wide, like a "dark mode" or whether a satellite basemap is present, or whether the user has requested large fonts.

Design Alternatives

Currently, the way to handle this is to call map.setPaintProperty on each specific layer when that map-wide property changes. But that's not ideal, because it often means defining layer properties twice: once as initially shown, and then later when reset:

map.addLayer({ ... "line-color": "dark-green") })
...
on("basemapchange", () => map.setPaintProperty("my-lines", "line-color", "light-green")

Design

Something like:

"line-color": ["case", ["variable", "dark-mode"], "dark-green", "green"]

map.setVariable("dark-mode", true)

[peterqliu]

We used to have style classes, but removed them because they were hard to design for in Studio and the use case was perceived to be obscure. I've found them useful.

[asheemmamoowala]

Duplicate of #7946