Port data-driven styling (property functions) from GL JS

[ansis]

Port data-driven styling from GL JS (see mapbox/mapbox-gl-js#1626). DDS is currently exposed as property functions in the style specification.

Master list of what's needed. Not necessarily in implementation order.

• Refactor existing LayoutProperty and PaintProperty classes such that unevaluated and evaluated properties live in separate structures (in progress, @jfirebaugh).
• gl::Drawable/gl::Context need support for multiple vertex buffers.
• Rename Function ⇢ ZoomFunction.
• Add PropertyFunction and ZoomPropertyFunction. Add evaluators for them.
• Add PropertyFunction and ZoomPropertyFunction as variant types for PropertyValue.
  • Might need to split PropertyValue into one variant for properties that are known to be feature-constant (undefined, constant, or zoom function) and one for properties that are data-driven (property function or zoom property function).
• Paint properties can no longer be fully evaluated by Style. Instead, pass cascaded paint properties into the bucket. Bucket figures out which are feature constant and which are data-driven, producing a ProgramConfiguration (or PaintConfiguration?). In the ProgramConfiguration:
  • constant properties have a ZoomFunction which will be evaluated by the main thread and the result bound to a uniform
  • data-driven properties have a vertex buffer
• Decision: for feature-constant properties, should we use glDisableVertexAttribArray/glVertexAttrib* or dynamically-generated shader uniforms?
• Decision: single "interleaved" vertex buffer for all data-driven paint properties of a layer (requires implementing a custom vector type with runtime-sized elements), or one vertex buffer per data-driven property (can keep using std::vector).
• Changing a paint property needs to do a worker round trip (if changing to or from data-driven property or updating a data-driven property), or update the PaintConfiguration for each bucket in place (if not).
• Implement support in SDK bindings.

[nitrag]

Awesome!

So this will this allow MGLShapeSource parsing (mapbox built-in) to dynamically style a MGLPolyline based on feature.properties.line-color from the geojson source?

https://www.mapbox.com/mapbox-gl-js/style-spec/#paint-line-color

[boundsj]

Yes @nitrag. Platform specific documentation for iOS and macOS is still being worked on in #7924 but you can check out the examples in the tests and the test app today.