[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.

1 files changed, 28 insertions, 0 deletions

diff --git a/src/mbgl/style/conversion/json.hpp b/src/mbgl/style/conversion/json.hpp
new file mode 100644
index 0000000000..0817ac09df
--- /dev/null
+++ b/src/mbgl/style/conversion/json.hpp
@@ -0,0 +1,28 @@
+#include <mbgl/style/conversion.hpp>
+#include <mbgl/style/rapidjson_conversion.hpp>
+
+#include <string>
+#include <sstream>
+
+namespace mbgl {
+namespace style {
+namespace conversion {
+
+template <class T, class...Args>
+optional<T> convertJSON(const std::string& json, Error& error, Args&&...args) {
+    JSDocument document;
+    document.Parse<0>(json.c_str());
+
+    if (document.HasParseError()) {
+        std::stringstream message;
+        message << document.GetErrorOffset() << " - " << rapidjson::GetParseError_En(document.GetParseError());
+        error = { message.str() };
+        return {};
+    }
+
+    return convert<T, JSValue>(document, error, std::forward<Args>(args)...);
+}
+
+} // namespace conversion
+} // namespace style
+} // namespace mbgl