[core] Extract Function::evaluation to a separate class (#4811)

This allows the Function<Faded<T>> partial specialization to be eliminated, giving all property functions a consistent storage type.

1 files changed, 6 insertions, 3 deletions

diff --git a/src/mbgl/style/paint_property.hpp b/src/mbgl/style/paint_property.hpp
index e742b2c109..124b0a7c0a 100644
--- a/src/mbgl/style/paint_property.hpp
+++ b/src/mbgl/style/paint_property.hpp
@@ -4,6 +4,7 @@
 #include <mbgl/style/class_dictionary.hpp>
 #include <mbgl/style/property_parsing.hpp>
 #include <mbgl/style/function.hpp>
+#include <mbgl/style/function_evaluator.hpp>
 #include <mbgl/style/property_transition.hpp>
 #include <mbgl/style/style_cascade_parameters.hpp>
 #include <mbgl/style/style_calculation_parameters.hpp>
@@ -16,10 +17,11 @@
 
 namespace mbgl {
 
-template <typename T, typename Result = T>
+template <class T, template <class S> class Evaluator = NormalFunctionEvaluator>
 class PaintProperty {
 public:
-    using Fn = Function<Result>;
+    using Fn = Function<T>;
+    using Result = typename Evaluator<T>::ResultType;
 
     explicit PaintProperty(T fallbackValue)
         : value(fallbackValue) {
@@ -114,7 +116,8 @@ public:
         }
 
         Result calculate(const StyleCalculationParameters& parameters) {
-            Result final = value.evaluate(parameters);
+            Evaluator<T> evaluator;
+            Result final = evaluator(value, parameters);
             if (!prior) {
                 // No prior value.
                 return final;