[core, android] Factor JSON string conversions

This adds a `convertJSON` template, to be used like:

   Error error
   optional<Foo> foo = convertJSON<Foo>(string, error);

Internally, it parses the string with RapidJSON and then calls `convert<Foo>(parsed, error)`.

While here, rationalize GeoJSON converters and fix error handling for Tileset conversion in OfflineDownload.

2 files changed, 12 insertions, 4 deletions

diff --git a/include/mbgl/style/conversion/geojson.hpp b/include/mbgl/style/conversion/geojson.hpp
index 6e6e97dbc0..0b594f066c 100644
--- a/include/mbgl/style/conversion/geojson.hpp
+++ b/include/mbgl/style/conversion/geojson.hpp
@@ -1,14 +1,22 @@
 #pragma once
 
 #include <mbgl/style/conversion.hpp>
-#include <mbgl/style/sources/geojson_source.hpp>
+#include <mbgl/util/geojson.hpp>
 
 namespace mbgl {
 namespace style {
 namespace conversion {
 
-template <class V>
-optional<GeoJSON> convertGeoJSON(const V& value, Error& error);
+template <>
+struct Converter<GeoJSON> {
+public:
+    optional<GeoJSON> operator()(const std::string&, Error&) const;
+
+    // This is explicitly specialized in the .cpp file for JSValue. It may also be explicitly
+    // specialized for SDK-specific types (e.g. mbgl::android::Value).
+    template <class V>
+    optional<GeoJSON> operator()(const V&, Error&) const;
+};
 
 } // namespace conversion
 } // namespace style
diff --git a/include/mbgl/style/conversion/source.hpp b/include/mbgl/style/conversion/source.hpp
index 2371183b1c..dc7cdc0d42 100644
--- a/include/mbgl/style/conversion/source.hpp
+++ b/include/mbgl/style/conversion/source.hpp
@@ -122,7 +122,7 @@ private:
         auto result = std::make_unique<GeoJSONSource>(id, *options);
 
         if (isObject(*dataValue)) {
-            optional<GeoJSON> geoJSON = convertGeoJSON(*dataValue, error);
+            optional<GeoJSON> geoJSON = convert<GeoJSON>(*dataValue, error);
             if (!geoJSON) {
                 return {};
             }