Enumerators for layers and fields within a source

[1ec5]

It should be possible for the developer to programmatically discover the layers in a vector source and the properties (fields) within those layers.

In the style specification, the source-layer documentation implies that it isn’t required for a single-layer vector source, that the sole layer will be autodetected. However, #6780 points out that layers aren’t currently autodetected, so perhaps this feature will pave the way for that enhancement. (See also mapbox/mapbox-gl-style-spec#547, which is about clarifying the style specification documentation.) Having a way to enumerate the layers and properties would help us avoid hard-coding Mapbox Streets properties for #7031 and similar features. A list of valid properties could also help us validate property names used in property functions.

It looks like Mapbox-hosted vector TileJSON manifests specify the necessary data in an undocumented vector_layers property and fields properties within each layer. (Layer and property descriptions are also present, but they probably aren’t worth parsing and keeping in memory for the likely use cases.) mbgl::style::TileSourceImpl::parseTileJSON() doesn’t currently parse these properties. If it isn’t a good idea to rely on this TileJSON information, I wonder if there’s a way to get access to mbgl::VectorTileLayer::keys.

At the iOS and macOS SDK level, I think we’d want a attributeNamesByLayer property that maps source layer identifiers to sets of property names. attributeNamesByLayer would either live on MGLVectorSource or on a new MGLVectorTileSet that inherits from MGLTileSet.

/cc @jfirebaugh @incanus @boundsj @ericrwolfe

[stale]

This issue has been automatically detected as stale because it has not had recent activity and will be archived. Thank you for your contributions.