Add "placeholder-loading-*" properties

[lucaswoj]

Play with the demo!

This PR allows developers to show a pattern or color in lieu of a still-loading raster tile.

• placeholder-loading-color
• placeholder-loading-pattern
• placeholder-loading-opacity

This feature is useful when loading raster tiles that use transparency to represent "no data." In the image below 👇 is there really no storm over SF? Or has the tile not loaded? Or has the tile errored?

In some industries, there is a regulatory requirement that a pattern be shown to disambiuate between "no data", "data loading", and "data errored"

cc @lbud @jfirebaugh @mourner

fixes #3280

Launch Checklist

• create a new style layer type
• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs
• post benchmark scores
• manually test the debug page

[jfirebaugh]

This would be useful for vector sources as well. Can we make it more general, taking into account the ideas in mapbox/mapbox-gl-style-spec#219?

[lucaswoj]

I see the value in making this it's own layer type. That would give developers the ability to use this feature with vector tiles and control the z-ordering of the loading pattern.

Glomming this onto the raster layer type was just a prototyping expediency 😄

[lucaswoj]

The latest commit adds a new layer type called source. I like how self-explanitory the property names are: source-loading-pattern, source-loading-opacity, etc.

Open to other naming ideas that convey a layer which displays source metadata, anticipating the need for errored-patterns in the near future. Also open to creating separate source-loading and source-errored layer types.

[lucaswoj]

Updated to put the functionality on a new layer type called placeholder

• placeholder-loading-color
• placeholder-loading-pattern
• placeholder-loading-opacity

Thoughts @jfirebaugh?

[lucaswoj]

ping @jfirebaugh @lbud

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I'm thinking about redoing the style spec such that it consists of

• placeholder-color
• placeholder-pattern
• placeholder-opacity
• placeholder-type: loading or errored

This will simplify the API and give users more control over layering order

[lucaswoj]

Gonna take this back to the drawing board somewhat 😄