[core, ios, android, qt] Observe style layer mutations rather than requiring SDKs to use Map::update

This paves the way for updates to filter and layout properties to trigger a source reload, without each SDK having to participate in the implementation.

1 files changed, 15 insertions, 0 deletions

diff --git a/include/mbgl/style/property_value.hpp b/include/mbgl/style/property_value.hpp
index d8f83a0fb3..d7e160fdca 100644
--- a/include/mbgl/style/property_value.hpp
+++ b/include/mbgl/style/property_value.hpp
@@ -8,12 +8,17 @@ namespace style {
 
 class Undefined {};
 
+inline bool operator==(const Undefined&, const Undefined&) { return true; }
+inline bool operator!=(const Undefined&, const Undefined&) { return false; }
+
 template <class T>
 class PropertyValue {
 private:
     using Value = variant<Undefined, T, Function<T>>;
     Value value;
 
+    template <class S> friend bool operator==(const PropertyValue<S>&, const PropertyValue<S>&);
+
 public:
     PropertyValue()                     : value()         {}
     PropertyValue(         T  constant) : value(constant) {}
@@ -34,5 +39,15 @@ public:
     }
 };
 
+template <class T>
+bool operator==(const PropertyValue<T>& lhs, const PropertyValue<T>& rhs) {
+    return lhs.value == rhs.value;
+}
+
+template <class T>
+bool operator!=(const PropertyValue<T>& lhs, const PropertyValue<T>& rhs) {
+    return !(lhs == rhs);
+}
+
 } // namespace style
 } // namespace mbgl