Enable data-driven values for text-font

[jfirebaugh]

Enables support for data-driven text-font values. Fixes #5568.

The port to native will need to take into account mapbox/mapbox-gl-native#9939 -- my proposed resolution is described in mapbox/mapbox-gl-native#9939 (comment).

[anandthakker]

The port to native will need to take into account mapbox/mapbox-gl-native#9939 -- my proposed resolution is described in mapbox/mapbox-gl-native#9939 (comment).

@jfirebaugh I was thinking we would impose this restriction at the level of spec validation (and so in GL JS as well), not just in -native.

[jfirebaugh]

Oh, yeah, that's probably a good idea.

[anandthakker]

Nice.

We could at some point elaborate possibleValues() for at and var, but 👍 for keeping it conservative for now.

[anandthakker]

Nit: let's include "text-font" in the id for these layers, since that's what they're testing.

[jfirebaugh]

Do you think this is necessary given that text-font is in the fixture file name?

[anandthakker]

Oh! Whoops -- I'd assumed without looking that this was in the functions fixture. 👍 as is

[jfirebaugh]

Hmm, validation also needs to prohibit identity functions.

[anandthakker]

Oof, I always forget about those. Another option would be to require data-driven values to be expressions (not functions)

[jfirebaugh]

I thought about that, but I think it might be confusing for a long-existing property to have that restriction. We did do it for heatmap-color, but that was (essentially) a new property, and it has a special-case input.

[anandthakker]

I think it might be confusing for a long-existing property to have that restriction.

Hm, yeah good point -- I suppose if someone was already using a zoom-only stop function, they'd expect to be able to upgrade it to a zoom-and-property stop function after this.

[nickidlugash]

@jfirebaugh looks great – thanks for pushing on this. Can't wait to test out some expression values for text-font! /cc @aparlato

[jplante]

@jfirebaugh
Just verifying:

1. This being data-driven does not appear to be updated on the API documentation Style Specification. text-font is listed as "data-driven styling" and "Not yet supported" across platforms.

2. Even if it does work, does it support identity functions?

[anandthakker]

@jplante Yes, the API docs need to be updated -- thanks for noting that!

does it support identity functions

No, identity functions aren't supported for text-font. (And, to generalize this, text-font expressions must contain all possible output values as literals within the expression)