Support for MGLLineStyleLayer.linegradient to consider MGLfeature attributes

[jumbopilot]

Steps to reproduce

Currently, Mapbox seems to support only style layer properties determined by properties on the feature itself or, alternatively, from a computed variable such as $zoomLevel or $lineProgress. geojson-vt is responsible for varying $lineProgress along the path of the line feature. Beyond that one variable, there isn’t a general mechanism for integrating external data that varies along the path of the line.

Here's a simple use case.

1. Create a MGLPolylineFeature layer

let polyline = MGLPolylineFeature(coordinates: &coordinates, count: UInt(coordinates.count))
self.polylineTrackSource = MGLShapeSource(identifier: "my-identifier", features: [polyline], options: [MGLShapeSourceOption.lineDistanceMetrics : true])
style.addSource(self.polylineTrackSource)

2. Create a MGLFeature with attributes for above layer

let feature = MGLPointFeature()
feature.coordinate = location.coordinate       
feature.attributes["elevation"] = location.altitude

3. Style MGLLineStyleLayer.linegradient

layer.lineGradient = NSExpression(format: "mgl_interpolate:withCurveType:parameters:stops:(elevation, 'linear', nil, %@)", [0: UIColor.blue, 10000: UIColor.red])

Expected behavior

Styling of MGLLineStyleLayer.linegradient based on feature attributes is supported.
The MGLLineStyleLayer.linegradient property does support applying interpolation or step functions to feature attributes.

Actual behavior

Styling of MGLLineStyleLayer.linegradient based on feature attributes (e.g. elevation) is not supported.
The MGLLineStyleLayer.linegradient property does not support applying interpolation or step functions to feature attributes.

Configuration

Mapbox SDK versions:
v4.6.0
iOS/macOS versions:
v12.x
Device/simulator models:
iPhone 7++
Xcode version:
V10, Swift 4

[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.

[jumbopilot]

Can this be addressed in conjunction with Issue232?

Do I need to create a new (linked) issue or can this one be reopened?