Gui: add function to observe before changing a property

src/Gui/Application.h

@@ -109,6 +109,8 @@ public:
     boost::signals2::signal<void (const Gui::ViewProvider&)> signalNewObject;
     /// signal on deleted Object
     boost::signals2::signal<void (const Gui::ViewProvider&)> signalDeletedObject;
+    /// signal on changed Object
+    boost::signals2::signal<void (const Gui::ViewProvider&, const App::Property&)> signalBeforeChangeObject;
     /// signal on changed object property
     boost::signals2::signal<void (const Gui::ViewProvider&, const App::Property&)> signalChangedObject;
     /// signal on renamed Object

src/Gui/DocumentObserverPython.cpp

@@ -83,6 +83,7 @@ DocumentObserverPython::DocumentObserverPython(const Py::Object& obj) : inst(obj
     FC_PY_ELEMENT_ARG1(ActivateDocument, ActiveDocument)
     FC_PY_ELEMENT_ARG1(CreatedObject, NewObject)
     FC_PY_ELEMENT_ARG1(DeletedObject, DeletedObject)
+    FC_PY_ELEMENT_ARG2(BeforeChangeObject, BeforeChangeObject)
     FC_PY_ELEMENT_ARG2(ChangedObject, ChangedObject)
     FC_PY_ELEMENT_ARG1(InEdit, InEdit)
     FC_PY_ELEMENT_ARG1(ResetEdit, ResetEdit)
@@ -190,6 +191,27 @@ void DocumentObserverPython::slotDeletedObject(const Gui::ViewProvider& Obj)
     }
 }
 
+void DocumentObserverPython::slotBeforeChangeObject(const Gui::ViewProvider& Obj,
+                                                    const App::Property& Prop)
+{
+    Base::PyGILStateLocker lock;
+    try {
+        Py::Tuple args(2);
+        args.setItem(0, Py::Object(const_cast<Gui::ViewProvider&>(Obj).getPyObject(), true));
+        // If a property is touched but not part of a document object then its name is null.
+        // In this case the slot function must not be called.
+        const char* prop_name = Obj.getPropertyName(&Prop);
+        if (prop_name) {
+            args.setItem(1, Py::String(prop_name));
+            Base::pyCall(pyBeforeChangeObject.ptr(),args.ptr());
+        }
+    }
+    catch (Py::Exception&) {
+        Base::PyException e; // extract the Python error text
+        e.ReportException();
+    }
+}
+
 void DocumentObserverPython::slotChangedObject(const Gui::ViewProvider& Obj,
                                                const App::Property& Prop)
 {

src/Gui/DocumentObserverPython.h

@@ -65,6 +65,8 @@ private:
     /** Checks if the given object is about to be removed. */
     void slotDeletedObject(const Gui::ViewProvider& Obj);
     /** The property of an observed object has changed */
+    void slotBeforeChangeObject(const Gui::ViewProvider& Obj, const App::Property& Prop);
+    /** The property of an observed object has changed */
     void slotChangedObject(const Gui::ViewProvider& Obj, const App::Property& Prop);
     /** The object was set into edit mode */
     void slotInEdit(const Gui::ViewProviderDocumentObject& Obj);
@@ -90,6 +92,7 @@ private:
     Connection pyActivateDocument;
     Connection pyCreatedObject;
     Connection pyDeletedObject;
+    Connection pyBeforeChangeObject;
     Connection pyChangedObject;
     Connection pyInEdit;
     Connection pyResetEdit;

src/Gui/ViewProvider.cpp

@@ -514,6 +514,13 @@ int ViewProvider::getDefaultMode() const {
     return viewOverrideMode>=0?viewOverrideMode:_iActualMode;
 }
 
+void ViewProvider::onBeforeChange(const App::Property* prop)
+{
+    Application::Instance->signalBeforeChangeObject(*this, *prop);
+
+    App::TransactionalObject::onBeforeChange(prop);
+}
+
 void ViewProvider::onChanged(const App::Property* prop)
 {
     Application::Instance->signalChangedObject(*this, *prop);

src/Gui/ViewProvider.h

@@ -522,6 +522,8 @@ protected:
     SoPickedPoint* getPointOnRay(const SbVec3f& pos, const SbVec3f& dir,
                                  const View3DInventorViewer* viewer) const;
     /// Reimplemented from subclass
+    void onBeforeChange(const App::Property* prop);
+    /// Reimplemented from subclass
     void onChanged(const App::Property* prop);
 
 

src/Gui/ViewProviderDocumentObject.cpp

@@ -169,6 +169,8 @@ void ViewProviderDocumentObject::onBeforeChange(const App::Property* prop)
             onBeforeChangeProperty(doc, prop);
         }
     }
+
+    ViewProvider::onBeforeChange(prop);
 }
 
 void ViewProviderDocumentObject::onChanged(const App::Property* prop)