Introduce the style::Layer factory classes

This patch introduces the initial implementation of
- A `LayerFactory` abstract class that creates `style::Layer` instances of
  a certain layer type (line, hillshade, round, ..)
- A singleton `LayerManager` class, which is responsible for initializing
  the `LayerFactory` instances and forwarding the `create()` calls to the
  corresponding factory.

1 files changed, 8 insertions, 0 deletions

diff --git a/include/mbgl/style/layers/heatmap_layer.hpp b/include/mbgl/style/layers/heatmap_layer.hpp
index 5069eef165..3f53877523 100644
--- a/include/mbgl/style/layers/heatmap_layer.hpp
+++ b/include/mbgl/style/layers/heatmap_layer.hpp
@@ -69,5 +69,13 @@ protected:
     Mutable<Layer::Impl> mutableBaseImpl() const final;
 };
 
+class HeatmapLayerFactory : public LayerFactory {
+protected:
+    // LayerFactory overrides.
+    ~HeatmapLayerFactory() override;
+    const char* type() const final;
+    std::unique_ptr<style::Layer> createLayer(const std::string& id, const conversion::Convertible& value) override;
+};
+
 } // namespace style
 } // namespace mbgl