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.

include/mbgl/style/layer.hpp

@@ -30,9 +30,9 @@ class Filter;
  * * Cloning and copying
  *
  * All other functionality lives in the derived classes. To instantiate a layer, create an instance of the desired
- * type, passing the ID:
+ * type, calling `LayerManager`:
  *
- *     auto circleLayer = std::make_unique<CircleLayer>("my-circle-layer");
+ *     auto circleLayer = LayerManager::get()->createLayer("circle", ...);
  */
 class Layer {
 public:
@@ -89,5 +89,42 @@ protected:
     LayerObserver* observer = nullptr;
 };
 
+/**
+ * @brief The LayerFactory abstract class
+ * 
+ * This class is responsible for creation of the layer objects that belong to a concrete layer type.
+ */
+class LayerFactory {
+public:
+    virtual ~LayerFactory() = default;
+    /// Returns the type of the layers, created by this factory.
+    virtual const char* type() const = 0;
+    /// Returns a new Layer instance on success call; returns `nulltptr` otherwise. 
+    virtual std::unique_ptr<Layer> createLayer(const std::string& id, const conversion::Convertible& value) = 0;
+protected:
+    optional<std::string> getSource(const conversion::Convertible& value) const;
+    bool initSourceLayerAndFilter(Layer*, const conversion::Convertible& value) const;
+};
+
+/**
+ * @brief A singleton class forwarding calls to the corresponding  \c LayerFactory instance.
+ */
+class LayerManager {
+public:
+    /**
+     * @brief A singleton getter.
+     * 
+     * @return LayerManager* 
+     */
+    static LayerManager* get();
+
+    virtual ~LayerManager() = default;
+    /// Returns a new Layer instance on success call; returns `nulltptr` otherwise.
+    virtual std::unique_ptr<Layer> createLayer(const std::string& type, const std::string& id, const conversion::Convertible& value, conversion::Error& error) = 0;
+
+protected:
+    LayerManager() = default;
+};
+
 } // namespace style
 } // namespace mbgl

include/mbgl/style/layers/background_layer.hpp

@@ -56,5 +56,13 @@ protected:
     Mutable<Layer::Impl> mutableBaseImpl() const final;
 };
 
+class BackgroundLayerFactory : public LayerFactory {
+protected:
+    // LayerFactory overrides.
+    ~BackgroundLayerFactory() 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

include/mbgl/style/layers/circle_layer.hpp

@@ -104,5 +104,13 @@ protected:
     Mutable<Layer::Impl> mutableBaseImpl() const final;
 };
 
+class CircleLayerFactory : public LayerFactory {
+protected:
+    // LayerFactory overrides.
+    ~CircleLayerFactory() 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

include/mbgl/style/layers/fill_extrusion_layer.hpp

@@ -80,5 +80,13 @@ protected:
     Mutable<Layer::Impl> mutableBaseImpl() const final;
 };
 
+class FillExtrusionLayerFactory : public LayerFactory {
+protected:
+    // LayerFactory overrides.
+    ~FillExtrusionLayerFactory() 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

include/mbgl/style/layers/fill_layer.hpp

@@ -80,5 +80,13 @@ protected:
     Mutable<Layer::Impl> mutableBaseImpl() const final;
 };
 
+class FillLayerFactory : public LayerFactory {
+protected:
+    // LayerFactory overrides.
+    ~FillLayerFactory() 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

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

include/mbgl/style/layers/hillshade_layer.hpp

@@ -74,5 +74,13 @@ protected:
     Mutable<Layer::Impl> mutableBaseImpl() const final;
 };
 
+class HillshadeLayerFactory : public LayerFactory {
+protected:
+    // LayerFactory overrides.
+    ~HillshadeLayerFactory() 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

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

include/mbgl/style/layers/line_layer.hpp

@@ -125,5 +125,13 @@ protected:
     Mutable<Layer::Impl> mutableBaseImpl() const final;
 };
 
+class LineLayerFactory : public LayerFactory {
+protected:
+    // LayerFactory overrides.
+    ~LineLayerFactory() 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

include/mbgl/style/layers/raster_layer.hpp

@@ -86,5 +86,13 @@ protected:
     Mutable<Layer::Impl> mutableBaseImpl() const final;
 };
 
+class RasterLayerFactory : public LayerFactory {
+protected:
+    // LayerFactory overrides.
+    ~RasterLayerFactory() 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

include/mbgl/style/layers/symbol_layer.hpp

@@ -274,5 +274,13 @@ protected:
     Mutable<Layer::Impl> mutableBaseImpl() const final;
 };
 
+class SymbolLayerFactory : public LayerFactory {
+protected:
+    // LayerFactory overrides.
+    ~SymbolLayerFactory() 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