Temporary 'instance()' property of MetaClasses for singletons (but I

do not like it), fix for Graphite-in-blender demo: we need to be able
to detect that we already have a SceneGraph (and reuse it).

Author: BrunoLevy

plugins/OGF/gompy/interpreter/python_interpreter.cpp

@@ -199,6 +199,7 @@ namespace OGF {
 	    PyDict_SetItemString(sys_modules, "gompy.gom", py_gom);
 	    PyDict_SetItemString(sys_modules, "gompy.types", py_meta_types);
 	    PyDict_SetItemString(sys_modules, "gompy.types.OGF", py_OGF);
+
 	    return;
 	}
 

src/lib/OGF/gom/reflection/meta_class.cpp

@@ -107,6 +107,7 @@ namespace OGF {
         if(!abstract) {
             set_factory(new FactoryMetaClass(this));
         }
+	instance_ = nullptr;
     }
 
     MetaClass::MetaClass(

src/lib/OGF/gom/reflection/meta_class.h

@@ -299,6 +299,15 @@ namespace OGF {
          */
         virtual bool is_subclass_of(const MetaClass* other) const;
 
+    gom_properties:
+	/**
+	 * \brief For singletons, returns instance
+	 * \return the unique instance or nullptr if not already created
+	 */
+	Object* get_instance() const {
+	    return instance_;
+	}
+
     public:
 
         /**
@@ -455,6 +464,13 @@ namespace OGF {
          */
         std::string get_doc() const override;
 
+	/**
+	 * \brief For singletons, sets instance
+	 * \param[in] object the unique instance or nullptr
+	 */
+	void set_instance(Object* object) {
+	    instance_ = object;
+	}
 
     protected:
 
@@ -525,6 +541,7 @@ namespace OGF {
         bool abstract_;
         Factory_var factory_;
         friend class ::OGF::MetaConstructor;
+	Object* instance_; // For singletons
     };
 
     /**

src/lib/OGF/scene_graph/types/scene_graph.cpp

@@ -69,9 +69,13 @@ namespace OGF {
 	application_(nullptr) {
         Grob::scene_graph_ = this;
         SceneGraphLibrary::instance()->set_scene_graph(this, transfer_ownership);
+	Any value;
+	value.set_value(this);
+	meta_class()->set_instance(this);
     }
 
     SceneGraph::~SceneGraph() {
+	meta_class()->set_instance(nullptr);
     }
 
     void SceneGraph::set_scene_graph_shader_manager(