How to use the gaphor.core.transactional function in gaphor

    @transactional
    def _on_auto_dependency_change(self, button):
        self.item.auto_dependency = button.get_active()
        self.update()

    @transactional
    def tree_view_create_diagram(self):
        element = self._namespace.get_selected_element()
        while not isinstance(element, UML.Package):
            element = element.namespace
        diagram = self.element_factory.create(UML.Diagram)
        diagram.package = element

        if element:
            diagram.name = f"{element.name} diagram"
        else:
            diagram.name = "New diagram"

        self.select_element(diagram)
        self.event_manager.handle(DiagramOpened(diagram))
        self.tree_view_rename_selected()

    @transactional
    def _on_dependency_type_change(self, combo):
        combo = self.combo
        cls = combo.get_model().get_value(combo.get_active())
        self.item.dependency_type = cls
        if self.item.subject:
            self.element_factory.swap_element(self.item.subject, cls)
            self.item.request_update()

    @transactional
    def _on_aggregation_change(self, combo):
        self.subject.aggregation = ("none", "shared", "composite")[combo.get_active()]

    @transactional
    def connect(self, sink):
        """
        Create connection at handle level and at model level.
        """
        handle = self.handle
        item = self.item
        cinfo = item.canvas.get_connection(handle)

        try:
            callback = DisconnectHandle(self.item, self.handle)
            if cinfo and cinfo.connected is sink.item:
                # reconnect only constraint - leave model intact
                log.debug('performing reconnect constraint')
                constraint = sink.port.constraint(item.canvas, item, handle, sink.item)
                item.canvas.reconnect_item(item, handle, constraint=constraint)
            elif cinfo:

    @transactional
    def delete_selected_items(self):
        assert self.view
        items = self.view.selected_items
        for i in list(items):
            if isinstance(i, UML.Presentation):
                i.unlink()
            else:
                if i.canvas:
                    i.canvas.remove(i)

    @transactional
    def update_text(text):
        item.subject.name = text
        popover.popdown()
        return True

    @transactional
    def _on_ordering_change(self, combo):
        value = self.ORDERING_VALUES[combo.get_active()]
        self.subject.ordering = value

    @transactional
    def tree_view_delete(self):
        element = self._namespace.get_selected_element()
        if isinstance(element, UML.Package):
            element.unlink()
        elif isinstance(element, UML.Diagram):
            m = Gtk.MessageDialog(
                None,
                Gtk.DialogFlags.MODAL,
                Gtk.MessageType.QUESTION,
                Gtk.ButtonsType.YES_NO,
                "Do you really want to delete diagram %s?\n\n"
                "This will possibly delete diagram items\n"
                "that are not shown in other diagrams." % (element.name or ""),
            )
            if m.run() == Gtk.ResponseType.YES:
                for i in reversed(element.canvas.get_all_items()):

    @transactional
    def tree_view_create_package(self):
        element = self._namespace.get_selected_element()
        package = self.element_factory.create(UML.Package)
        package.package = element

        if element:
            package.name = f"{element.name} package"
        else:
            package.name = "New model"

        self.select_element(package)
        self.tree_view_rename_selected()