[core] Convert style properties to a tuple-based approach

This converts the style property classes (CirclePaintProperties and so on) to the same tuple-based approach as gl::Attribute and gl::Uniform. The approach is outlined in https://github.com/mapbox/cpp/blob/master/C%2B%2B%20Structural%20Metaprogramming.md.

The main advantage of this approach is it allows writing algorithms that work on sets of style properties, without resorting to code generation or manually repetitive code. This lets us iterate on approaches to data-driven properties more easily.

Another advantage is that the cascading, unevaluated, and evaluated states of a set of properties exist as independent structures, instead of individual properties holding their own state. This is a more functional approach that makes data flow clearer and reduces state.

1 files changed, 14 insertions, 9 deletions

diff --git a/src/mbgl/style/layers/background_layer_properties.hpp b/src/mbgl/style/layers/background_layer_properties.hpp
index 78a35a4f0c..792bf3de94 100644
--- a/src/mbgl/style/layers/background_layer_properties.hpp
+++ b/src/mbgl/style/layers/background_layer_properties.hpp
@@ -9,18 +9,23 @@
 namespace mbgl {
 namespace style {
 
-class CascadeParameters;
-class CalculationParameters;
+struct BackgroundColor : PaintProperty<Color> {
+    static Color defaultValue() { return Color::black(); }
+};
 
-class BackgroundPaintProperties {
-public:
-    void cascade(const CascadeParameters&);
-    bool recalculate(const CalculationParameters&);
+struct BackgroundPattern : CrossFadedPaintProperty<std::string> {
+    static std::string defaultValue() { return ""; }
+};
 
-    PaintProperty<Color> backgroundColor { Color::black() };
-    PaintProperty<std::string, CrossFadedPropertyEvaluator> backgroundPattern { "" };
-    PaintProperty<float> backgroundOpacity { 1 };
+struct BackgroundOpacity : PaintProperty<float> {
+    static float defaultValue() { return 1; }
 };
 
+class BackgroundPaintProperties : public PaintProperties<
+    BackgroundColor,
+    BackgroundPattern,
+    BackgroundOpacity
+> {};
+
 } // namespace style
 } // namespace mbgl