[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, 4 insertions, 9 deletions

diff --git a/platform/qt/src/qmapboxgl.cpp b/platform/qt/src/qmapboxgl.cpp
index edb8afa28a..ce7e237afb 100644
--- a/platform/qt/src/qmapboxgl.cpp
+++ b/platform/qt/src/qmapboxgl.cpp
@@ -1303,7 +1303,7 @@ void QMapboxGL::updateSource(const QString &id, const QVariantMap &params)
 
     if (params.contains("data")) {
         Error error;
-        auto result = convertGeoJSON(params["data"], error);
+        auto result = convert<mbgl::GeoJSON>(params["data"], error);
         if (result) {
             sourceGeoJSON->setGeoJSON(*result);
         }
diff --git a/platform/qt/src/qt_geojson.hpp b/platform/qt/src/qt_geojson.hpp
index 7d12660aec..a6958b7edc 100644
--- a/platform/qt/src/qt_geojson.hpp
+++ b/platform/qt/src/qt_geojson.hpp
@@ -178,25 +178,20 @@ namespace style {
 namespace conversion {
 
 template <>
-optional<GeoJSON> convertGeoJSON(const QMapbox::Feature& feature, Error&) {
-    return GeoJSON { asMapboxGLFeature(feature) };
-}
-
-template <>
-optional<GeoJSON> convertGeoJSON(const QVariant& value, Error& error) {
+optional<GeoJSON> Converter<GeoJSON>::operator()(const QVariant& value, Error& error) const {
 #if QT_VERSION >= 0x050000
     if (value.typeName() == QStringLiteral("QMapbox::Feature")) {
 #else
     if (value.typeName() == QString("QMapbox::Feature")) {
 #endif
-        return convertGeoJSON(value.value<QMapbox::Feature>(), error);
+        return GeoJSON { asMapboxGLFeature(value.value<QMapbox::Feature>()) };
     } else if (value.type() != QVariant::ByteArray) {
         error = { "JSON data must be in QByteArray" };
         return {};
     }
 
     QByteArray data = value.toByteArray();
-    return convertGeoJSON(std::string(data.constData(), data.size()), error);
+    return convert<GeoJSON>(std::string(data.constData(), data.size()), error);
 }
 
 } // namespace conversion