[core][android] Introduce mbgl::style::LayerTypeInfo

The `LayerTypeInfo` contains static meta data about certain layer type.

Each layer module should have a single immutable `LayerTypeInfo` instance
for the represented layer type. Both `LayerImpl` and `LayerFactory` from the
module always refer to the same `LayerTypeInfo` instance, so address of this
instance can be used as a layer module Id during the process life time.

include/mbgl/style/layer.hpp

@@ -18,6 +18,21 @@ namespace style {
 class LayerObserver;
 class Filter;
 
+/**
+ * @brief Holds static data for a certain layer type.
+ */
+struct LayerTypeInfo {
+    /**
+     * @brief contains the layer type as defined in the style specification;
+     */
+    const char* type;
+    /**
+     * @brief contains \c SourceRequired if the corresponding layer type requires source;
+     * contains \c SourceNotRequired otherwise.
+     */
+    const enum { SourceRequired, SourceNotRequired } source;
+};
+
 /**
  * The runtime representation of a [layer](https://www.mapbox.com/mapbox-gl-style-spec/#layers) from the Mapbox Style
  * Specification.
@@ -83,12 +98,16 @@ public:
     // identical platform-native peers.
     util::peer peer;
     Layer(Immutable<Impl>);
+
+    const LayerTypeInfo* getTypeInfo() const noexcept;
+
 protected:
     virtual Mutable<Impl> mutableBaseImpl() const = 0;
 
     LayerObserver* observer;
 };
 
+
 /**
  * @brief The LayerFactory abstract class
  * 
@@ -97,8 +116,8 @@ protected:
 class LayerFactory {
 public:
     virtual ~LayerFactory() = default;
-    /// Returns \c true if this factory can produce layers of the given type of the layers; returns \c false otherwise.
-    virtual bool supportsType(const std::string& type) const noexcept = 0;
+    /// Returns the layer type data.
+    virtual const LayerTypeInfo* getTypeInfo() const noexcept = 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;
 

include/mbgl/style/layers/background_layer.hpp

@@ -59,15 +59,11 @@ protected:
 class BackgroundLayerFactory : public LayerFactory {
 public:
     BackgroundLayerFactory();
-    // LayerFactory overrides.
     ~BackgroundLayerFactory() override;
-    bool supportsType(const std::string& type) const noexcept final;
+
+    // LayerFactory overrides.
+    const LayerTypeInfo* getTypeInfo() const noexcept final;
     std::unique_ptr<style::Layer> createLayer(const std::string& id, const conversion::Convertible& value) final;
-
-    static BackgroundLayerFactory* get() noexcept;
-
-private:
-    static BackgroundLayerFactory* instance;
 };
 
 } // namespace style

include/mbgl/style/layers/circle_layer.hpp

@@ -107,15 +107,11 @@ protected:
 class CircleLayerFactory : public LayerFactory {
 public:
     CircleLayerFactory();
-    // LayerFactory overrides.
     ~CircleLayerFactory() override;
-    bool supportsType(const std::string& type) const noexcept final;
+
+    // LayerFactory overrides.
+    const LayerTypeInfo* getTypeInfo() const noexcept final;
     std::unique_ptr<style::Layer> createLayer(const std::string& id, const conversion::Convertible& value) final;
-
-    static CircleLayerFactory* get() noexcept;
-
-private:
-    static CircleLayerFactory* instance;
 };
 
 } // namespace style

include/mbgl/style/layers/custom_layer.hpp

@@ -90,13 +90,9 @@ public:
     CustomLayerFactory();
     // LayerFactory overrides.
     ~CustomLayerFactory() override;
-    bool supportsType(const std::string& type) const noexcept final;
+
+    const LayerTypeInfo* getTypeInfo() const noexcept final;
     std::unique_ptr<style::Layer> createLayer(const std::string& id, const conversion::Convertible& value) final;
-
-    static CustomLayerFactory* get() noexcept;
-
-private:
-    static CustomLayerFactory* instance;
 };
 
 } // namespace style

include/mbgl/style/layers/fill_extrusion_layer.hpp

@@ -83,15 +83,11 @@ protected:
 class FillExtrusionLayerFactory : public LayerFactory {
 public:
     FillExtrusionLayerFactory();
-    // LayerFactory overrides.
     ~FillExtrusionLayerFactory() override;
-    bool supportsType(const std::string& type) const noexcept final;
+
+    // LayerFactory overrides.
+    const LayerTypeInfo* getTypeInfo() const noexcept final;
     std::unique_ptr<style::Layer> createLayer(const std::string& id, const conversion::Convertible& value) final;
-
-    static FillExtrusionLayerFactory* get() noexcept;
-
-private:
-    static FillExtrusionLayerFactory* instance;
 };
 
 } // namespace style

include/mbgl/style/layers/fill_layer.hpp

@@ -83,15 +83,11 @@ protected:
 class FillLayerFactory : public LayerFactory {
 public:
     FillLayerFactory();
-    // LayerFactory overrides.
     ~FillLayerFactory() override;
-    bool supportsType(const std::string& type) const noexcept final;
+
+    // LayerFactory overrides.
+    const LayerTypeInfo* getTypeInfo() const noexcept final;
     std::unique_ptr<style::Layer> createLayer(const std::string& id, const conversion::Convertible& value) final;
-
-    static FillLayerFactory* get() noexcept;
-
-private:
-    static FillLayerFactory* instance;
 };
 
 } // namespace style

include/mbgl/style/layers/heatmap_layer.hpp

@@ -72,15 +72,11 @@ protected:
 class HeatmapLayerFactory : public LayerFactory {
 public:
     HeatmapLayerFactory();
-    // LayerFactory overrides.
     ~HeatmapLayerFactory() override;
-    bool supportsType(const std::string& type) const noexcept final;
+
+    // LayerFactory overrides.
+    const LayerTypeInfo* getTypeInfo() const noexcept final;
     std::unique_ptr<style::Layer> createLayer(const std::string& id, const conversion::Convertible& value) final;
-
-    static HeatmapLayerFactory* get() noexcept;
-
-private:
-    static HeatmapLayerFactory* instance;
 };
 
 } // namespace style

include/mbgl/style/layers/hillshade_layer.hpp

@@ -77,15 +77,11 @@ protected:
 class HillshadeLayerFactory : public LayerFactory {
 public:
     HillshadeLayerFactory();
-    // LayerFactory overrides.
     ~HillshadeLayerFactory() override;
-    bool supportsType(const std::string& type) const noexcept final;
+
+    // LayerFactory overrides.
+    const LayerTypeInfo* getTypeInfo() const noexcept final;
     std::unique_ptr<style::Layer> createLayer(const std::string& id, const conversion::Convertible& value) final;
-
-    static HillshadeLayerFactory* get() noexcept;
-
-private:
-    static HillshadeLayerFactory* instance;
 };
 
 } // namespace style

include/mbgl/style/layers/layer.hpp.ejs

@@ -75,15 +75,11 @@ protected:
 class <%- camelize(type) %>LayerFactory : public LayerFactory {
 public:
     <%- camelize(type) %>LayerFactory();
-    // LayerFactory overrides.
     ~<%- camelize(type) %>LayerFactory() override;
-    bool supportsType(const std::string& type) const noexcept final;
+
+    // LayerFactory overrides.
+    const LayerTypeInfo* getTypeInfo() const noexcept final;
     std::unique_ptr<style::Layer> createLayer(const std::string& id, const conversion::Convertible& value) final;
-
-    static <%- camelize(type) %>LayerFactory* get() noexcept;
-
-private:
-    static <%- camelize(type) %>LayerFactory* instance;
 };
 
 } // namespace style

include/mbgl/style/layers/line_layer.hpp

@@ -128,15 +128,11 @@ protected:
 class LineLayerFactory : public LayerFactory {
 public:
     LineLayerFactory();
-    // LayerFactory overrides.
     ~LineLayerFactory() override;
-    bool supportsType(const std::string& type) const noexcept final;
+
+    // LayerFactory overrides.
+    const LayerTypeInfo* getTypeInfo() const noexcept final;
     std::unique_ptr<style::Layer> createLayer(const std::string& id, const conversion::Convertible& value) final;
-
-    static LineLayerFactory* get() noexcept;
-
-private:
-    static LineLayerFactory* instance;
 };
 
 } // namespace style

include/mbgl/style/layers/raster_layer.hpp

@@ -89,15 +89,11 @@ protected:
 class RasterLayerFactory : public LayerFactory {
 public:
     RasterLayerFactory();
-    // LayerFactory overrides.
     ~RasterLayerFactory() override;
-    bool supportsType(const std::string& type) const noexcept final;
+
+    // LayerFactory overrides.
+    const LayerTypeInfo* getTypeInfo() const noexcept final;
     std::unique_ptr<style::Layer> createLayer(const std::string& id, const conversion::Convertible& value) final;
-
-    static RasterLayerFactory* get() noexcept;
-
-private:
-    static RasterLayerFactory* instance;
 };
 
 } // namespace style

include/mbgl/style/layers/symbol_layer.hpp

@@ -277,15 +277,11 @@ protected:
 class SymbolLayerFactory : public LayerFactory {
 public:
     SymbolLayerFactory();
-    // LayerFactory overrides.
     ~SymbolLayerFactory() override;
-    bool supportsType(const std::string& type) const noexcept final;
+
+    // LayerFactory overrides.
+    const LayerTypeInfo* getTypeInfo() const noexcept final;
     std::unique_ptr<style::Layer> createLayer(const std::string& id, const conversion::Convertible& value) final;
-
-    static SymbolLayerFactory* get() noexcept;
-
-private:
-    static SymbolLayerFactory* instance;
 };
 
 } // namespace style