[core] #6071 - extract GeoJSONOptions conversion from GeoJSONSource conversion

* [core] geojson_options - retain original error message

* [core] tests - initial style conversion stub methods

* [core] geojsonoptions conversion - initial unit tests

* [core] tests - fix forward reference issue

* [core] geojsonoptions conversion - unit tests

* [core] geojsonoptions conversion - renamed Holder to Value

1 files changed, 111 insertions, 0 deletions

diff --git a/test/src/mbgl/test/conversion_stubs.hpp b/test/src/mbgl/test/conversion_stubs.hpp
new file mode 100644
index 0000000000..60f110ea0d
--- /dev/null
+++ b/test/src/mbgl/test/conversion_stubs.hpp
@@ -0,0 +1,111 @@
+#pragma once
+
+#include <mbgl/style/conversion.hpp>
+#include <mbgl/util/feature.hpp>
+#include <mbgl/util/optional.hpp>
+#include <mbgl/util/variant.hpp>
+
+#include <string>
+#include <map>
+
+namespace mbgl {
+namespace style {
+namespace conversion {
+
+class Value;
+using ValueMap = std::map<std::string, Value>;
+using ValueVector = std::vector<Value>;
+class Value : public mbgl::variant<std::string, float, bool, ValueMap, ValueVector> {
+     using variant::variant;
+};
+
+inline bool isUndefined(const Value&) {
+    //Variant is always intialized
+    return false;
+}
+
+inline bool isArray(const Value& value) {
+    return value.is<ValueVector>();
+}
+
+inline std::size_t arrayLength(const Value& value) {
+    return value.get<ValueVector>().size();
+}
+
+inline Value arrayMember(const Value& value, std::size_t i) {
+    return value.get<ValueVector>()[i];
+}
+
+inline bool isObject(const Value& value) {
+    return value.is<ValueMap>();
+}
+
+inline optional<Value> objectMember(const Value& value, const char* key) {
+    auto map = value.get<ValueMap>();
+    auto iter = map.find(key);
+
+    if (iter != map.end()) {
+        return iter->second;
+    } else {
+        return {};
+    }
+}
+
+using EachMemberFn = std::function<optional<Error>(const std::string&, const Value&)>;
+
+optional<Error> eachMember(const Value& value, EachMemberFn&& fn) {
+    auto map = value.get<ValueMap>();
+    auto iter = map.begin();
+
+    while (iter != map.end()) {
+        optional<Error> result = fn(iter->first, iter->second);
+        if (result) {
+            return result;
+        }
+
+        ++iter;
+    }
+
+    return {};
+}
+
+inline optional<bool> toBool(const Value& value) {
+    if (value.is<bool>()) {
+        return value.get<bool>();
+    } else {
+        return {};
+    }
+}
+
+inline optional<float> toNumber(const Value& value) {
+    if (value.is<float>()) {
+        return value.get<float>();
+    } else {
+        return {};
+    }
+    return {};
+}
+
+inline optional<std::string> toString(const Value& value) {
+    if (value.is<std::string>()) {
+        return value.get<std::string>();
+    } else {
+        return {};
+    }
+}
+
+inline optional<mbgl::Value> toValue(const Value& value) {
+    if (value.is<bool>()) {
+        return { value.get<bool>() };
+    } else if (value.is<std::string>()) {
+        return { value.get<std::string>() };
+    } else if (value.is<float>()) {
+        return { value.get<float>() };
+    } else {
+        return {};
+    }
+}
+
+} // namespace conversion
+} // namespace style
+} // namespace mbgl