Add support for expressions

This ticket is here to start discussions around expressions. See https://blog.mapbox.com/announcing-expressions-in-gl-js-a72b55d0a6af for a good introduction to the subject.

Questions we need to answer

• How are users currently using expressions, is the JSON editor sufficient?
• If not how would you like to see expressions implemented?

Currently unsupported features, due to lack of expression support

• line-gradient
• *-sort-key

[orangemug]

Hey @pathmapper, so I think I’ve landed on a solution that feels more ‘correct’. The editor now has one ‘map style’ which it uses for all the editor UI (except for the map surface), this ‘map style’ can valid or invalid. An invalid ‘map style’ shows error messages in the UI as before.

The change is that the map surface, in the case of an invalid style tries to render what it can. So with the attached broken_osm_liberty.json the map will render everything but the invalid property showing an error both in the layers/layer panel and the error bar at the bottom of the map. Now the user can fix map style issues one by one and the map surface will update as these are fixed.

The undo/redo functionality has been fixed with these changes because we’re now using a single authoritative ‘map style’ as we were previously. This also means undo/redo works on an invalid style, so if you’re fixing a style with multiple issues, you can undo during the process of fixing the style.

Also, fixed the disabled button styling issue raised previously

Demo https://2152-84182601-gh.circle-artifacts.com/0/artifacts/build/index.html#0.25/0/0

[nspringer]

@orangemug this is a good UX solution. While it requires knowledge of the MB expressions rather than an expression builder it is a great first step and something that is badly needed in Maputnik. I wish I knew more React and could help, but I fully support this solution.