add on attach for emit signal

src/application/application.hpp

@@ -28,7 +28,12 @@ namespace VE
       void push_layer(Args*... args) 
       { 
         (m_layers_stack.push_back(args), ...); 
-        attach_layers();
+      }
+
+      template<typename... Args>
+      void on_attach(Args*... args) 
+      { 
+        (args->on_attach(), ...);
       }
 
     private: