Add const layer visitor

modules/core/inc/tactile/core/map/layer/group_layer.hpp

@@ -18,6 +18,8 @@ class TACTILE_CORE_API GroupLayer final : public ILayer {
 
   void accept(ILayerVisitor& visitor) override;
 
+  void accept(IConstLayerVisitor& visitor) const override;
+
   void set_persistent_id(Maybe<int32> id) override;
 
   void set_opacity(float opacity) override;

modules/core/inc/tactile/core/map/layer/layer.hpp

@@ -11,6 +11,7 @@
 namespace tactile {
 
 class ILayerVisitor;
+class IConstLayerVisitor;
 
 /**
  * \interface ILayer
@@ -29,6 +30,11 @@ class TACTILE_CORE_API ILayer : public IMetaContext {
    */
   virtual void accept(ILayerVisitor& visitor) = 0;
 
+  /**
+   * \copydoc accept(ILayerVisitor&)
+   */
+  virtual void accept(IConstLayerVisitor& visitor) const = 0;
+
   /**
    * \brief Sets the associated identifier used in save files.
    *
@@ -78,7 +84,8 @@ class TACTILE_CORE_API ILayer : public IMetaContext {
    * \brief Creates a clone of the layer.
    *
    * \note The layer clone may not be identical to the source layer, the only guarantee
-   *       is that the layer clone is logically equivalent.
+   *       is that the layer clone is logically equivalent. For example, associated
+   *       identifiers are not cloned.
    *
    * \return the new layer.
    */

modules/core/inc/tactile/core/map/layer/layer_visitor.hpp

@@ -43,4 +43,33 @@ class TACTILE_CORE_API ILayerVisitor {
   {}
 };
 
+/**
+ * \brief Visitor type used to inspect const layers.
+ */
+class TACTILE_CORE_API IConstLayerVisitor {
+ public:
+  TACTILE_INTERFACE_CLASS(IConstLayerVisitor);
+
+  /**
+   * \brief Visits a tile layer.
+   *
+   * \param layer the visited layer.
+   */
+  virtual void visit([[maybe_unused]] const TileLayer& layer) {}
+
+  /**
+   * \brief Visits an object layer.
+   *
+   * \param layer the visited layer.
+   */
+  virtual void visit([[maybe_unused]] const ObjectLayer& layer) {}
+
+  /**
+   * \brief Visits a group layer.
+   *
+   * \param layer the visited layer.
+   */
+  virtual void visit([[maybe_unused]] const GroupLayer& layer) {}
+};
+
 }  // namespace tactile

modules/core/inc/tactile/core/map/layer/object_layer.hpp

@@ -23,12 +23,14 @@ class TACTILE_CORE_API ObjectLayer final : public ILayer {
 
   void accept(ILayerVisitor& visitor) override;
 
+  void accept(IConstLayerVisitor& visitor) const override;
+
   /**
    * \brief Adds an object to the layer.
    *
    * \note An existing object in the layer with the specified ID will be overwritten.
    *
-   * \param id     the ID to associate with the object.
+   * \param id     the ID of the object.
    * \param object the object to add.
    */
   void add_object(ObjectID id, Shared<Object> object);

modules/core/inc/tactile/core/map/layer/tile_layer.hpp

@@ -30,6 +30,8 @@ class TACTILE_CORE_API TileLayer final : public ILayer {
 
   void accept(ILayerVisitor& visitor) override;
 
+  void accept(IConstLayerVisitor& visitor) const override;
+
   /**
    * \brief Changes the size of the layer.
    *

modules/core/src/tactile/core/map/layer/group_layer.cpp

@@ -16,6 +16,11 @@ void GroupLayer::accept(ILayerVisitor& visitor)
   visitor.visit(*this);
 }
 
+void GroupLayer::accept(IConstLayerVisitor& visitor) const
+{
+  visitor.visit(*this);
+}
+
 void GroupLayer::set_persistent_id(const Maybe<int32> id)
 {
   mDelegate.set_persistent_id(id);

modules/core/src/tactile/core/map/layer/object_layer.cpp

@@ -19,6 +19,11 @@ void ObjectLayer::accept(ILayerVisitor& visitor)
   visitor.visit(*this);
 }
 
+void ObjectLayer::accept(IConstLayerVisitor& visitor) const
+{
+  visitor.visit(*this);
+}
+
 void ObjectLayer::add_object(const ObjectID id, Shared<Object> object)
 {
   mObjects[id] = std::move(object);

modules/core/src/tactile/core/map/layer/tile_layer.cpp

@@ -28,6 +28,11 @@ void TileLayer::accept(ILayerVisitor& visitor)
   visitor.visit(*this);
 }
 
+void TileLayer::accept(IConstLayerVisitor& visitor) const
+{
+  visitor.visit(*this);
+}
+
 void TileLayer::resize(const usize row_count, const usize col_count)
 {
   if (row_count < 1) {