Structured attribution #20

As explained in mapbox/mapbox-gl-native#2723 (comment), native map controls need attribution information in a format other than HTML source code. We should allow the attribution property to be set to something more structured that can be converted to the appropriate data structures on the client side. For example, it could be an array of objects, each with a required title property (interpreted as literal text) and an optional url property (for linking):

{
  "attribution": [
    {
      "title": "© OpenStreetMap contributors",
      "url": "http://www.openstreetmap.org/about/"
    },
    {
      "title": "Creative Commons Attribution-ShareAlike 2.0",
      "url": "http://www.openstreetmap.org/copyright/"
    }
  ]
}

Other considerations:

• Should the title property be localizable? If so, perhaps it should be set to an object mapping ISO 639 language codes to literal strings.
• Mobile clients are often space-constrained. In the example above, it may be advantageous to abbreviate the two items as “© OSM” and “CC BY-SA”. These abbreviations could go in a short_title property beside the title property.
• Should other formatting be allowed? The current specification doesn’t disallow any HTML tags, so it’s theoretically possible to boldface text or embed images. While such content may be feasible in Web clients, native clients may have difficulty displaying it.
• Perhaps there should be a separate feedback property beside the attribution property. Many TileJSON documents hosted by Mapbox include an “Improve This Map” link alongside copyright statements. Native clients may want to open an alternative map feedback tool (Map feedback view controller mapbox-gl-native#6001); as things stand, they’d have to look for and omit a specific URL. In space-constrained environments, the client may choose to omit feedback items from the attribution UI to ensure that all the legally required copyright statements are visible.

/cc @kkaefer @jfirebaugh