Add a GeometryCollection convert method for GeoInterface (#207)

* Add a GeometryCollection convert

* add basic gc test

* Apply suggestion from code review

Co-authored-by: Rafael Schouten <[email protected]>

---------

Co-authored-by: Rafael Schouten <[email protected]>

Author: asinghvi17

src/geo_interface.jl

@@ -176,6 +176,28 @@ function GI.convert(
         Polygon[GI.convert(Polygon, PolygonTrait(), g; context) for g in GI.getgeom(geom)]
     return MultiPolygon(polygons)
 end
+GI.convert(
+    ::Type{GeometryCollection},
+    ::GeometryCollectionTrait,
+    geom::GeometryCollection;
+    context = nothing,
+) = geom
+function GI.convert(
+    ::Type{GeometryCollection},
+    ::GeometryCollectionTrait,
+    geom;
+    context = get_global_context(),
+)
+    geometries = map(GI.getgeom(geom)) do g
+        t = GI.trait(g)
+        lg = geointerface_geomtype(t)
+        # We call the full invocation for LibGEOS directly, 
+        # so the context can be passed through, since
+        # `GI.convert(Mod, x)` does not allow kwargs.
+        GI.convert(lg, t, g; context) 
+    end
+    return GeometryCollection(geometries)
+end
 
 function GI.convert(
     t::Type{<:AbstractGeometry},

test/test_geo_interface.jl

@@ -129,6 +129,9 @@ const LG = LibGEOS
     # Plots.plot(lr)
     # Makie.plot(lr)
 
+    gi_gc = GeoInterface.GeometryCollection([ls, mls, pt])
+    @test GeoInterface.convert(LibGEOS, gi_gc) isa LibGEOS.GeometryCollection
+
     @inferred GeoInterface.ncoord(lr)
     @inferred GeoInterface.ngeom(lr)
     @inferred GeoInterface.getgeom(lr)