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/layer.hpp.ejs b/include/mbgl/style/layers/layer.hpp.ejs
index a5d4be14e6..b19b501713 100644
--- a/include/mbgl/style/layers/layer.hpp.ejs
+++ b/include/mbgl/style/layers/layer.hpp.ejs
@@ -72,5 +72,13 @@ protected:
     Mutable<Layer::Impl> mutableBaseImpl() const final;
 };
 
+class <%- camelize(type) %>LayerFactory : public LayerFactory {
+protected:
+    // LayerFactory overrides.
+    ~<%- camelize(type) %>LayerFactory() 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