prepare view provider for undo/redo

src/Gui/ViewProviderDocumentObject.h

@@ -117,6 +117,8 @@ protected:
       If a value different to 0 is returned it is guaranteed to be a 3d view.
      */
     Gui::MDIView* getInventorView() const;
+    /// get called before the value is changed
+    virtual void onBeforeChange(const App::Property* prop);
     /// Gets called by the container whenever a property has been changed
     virtual void onChanged(const App::Property* prop);
     /** Searches in all view providers that are attached to an object that
@@ -128,6 +130,13 @@ protected:
      */
     SoNode* findFrontRootOfType(const SoType& type) const;
 
+    /** @name Transaction handling
+     */
+    //@{
+    virtual bool isAttachedToDocument() const;
+    virtual const char* detachFromDocument();
+    //@}
+
 protected:
     App::DocumentObject *pcObject;