when the start of an “object” element is reached, a XMLWidgetObject instance is created and pushed onto a stack of the objects created: such object in turn calls the appropriate ``XML builder'' function (got from the “common.widgets_from_xml” dictionary) that creates the widget: this function is similar to the factory function used to build the widget during an interactive session, see the code for details and differences

when the end of an “object” element is reached, the object at the top of the stack is removed, and its widget (see the source of XmlWidgetObject) is laid out

when the end of a Property element is reached, the appropriate setter function of the owner of the Property is called. This is the default behaviour, suitable for simple properties. For more complex properties, whose XML representation consists of more sub-elements, each widget can define a particular handler: see for example FontHandler in edit_windows.WindowBase