Add MGLShape from GeoJSON example

[1ec5]

Added an example of deserializing an MGLShape from a GeoJSON file.

/ref #7622 (comment)
/cc @nitrag

[mention-bot]

@1ec5, thanks for your PR! By analyzing this pull request, we identified @incanus to be potential reviewers.

[ericrwolfe]

Huh, didn't even know this was available 😃.

Is there an advantage of using MGLShape's convenience initializer over -[MGLShapeSource initWithIdentifier:URL:options:] or vise versa (other than perhaps that one can be used for MGLOverlays while the other is runtime-styling specific)?

At some point, we should define a preference and link between the two in the docs.

[1ec5]

Is there an advantage of using MGLShape's convenience initializer over -[MGLShapeSource initWithIdentifier:URL:options:] or vise versa (other than perhaps that one can be used for MGLOverlays while the other is runtime-styling specific)?

MGLShapeSource.URL is primarily useful for create a shape source with the contents of a GeoJSON file stored remotely. To be sure, you can specify a local file URL, but if you populate a shape source with a URL instead of a shape, the shape source is a black box: the only way you can access the shape data is via feature querying, which returns tiled data.

+[MGLShape shapeWithData:encoding:error:] is more versatile. You can use it to deserialize GeoJSON data, avoiding the mess of typecasts that results from NSJSONSerialization, even without using MGLMapView at all. Additionally, when you create an MGLShapeSource using a shape, such as the result of +[MGLShape shapeWithData:encoding:error:], you retain the ability to access the shape losslessly. Access to the shape can be important if, for example, you intend to read a LineString from GeoJSON at launch but continually append to the polyline at runtime.