ExpansionPanelRadio: how to lazy build body widget when opened?

ExpansionPanelList.radio(
  children: [
    ExpansionPanelRadio(
        headerBuilder: (-,-) {},
        body: _complexWidget(),
    ),
    ExpansionPanelRadio(
        headerBuilder: (-,-) {},
        body: _complexWidget2(),
    ),
    ExpansionPanelRadio(
        headerBuilder: (-,-) {},
        body: _complexWidget3(),
    ),
    ...
  ],
);

How to build the body widget only when the panel is opened? There is no callback when the panel open/collapse. It is getting very slow when there are many such ExpansionPanelRadios.