Function background_fn 

pub fn background_fn(
    child: impl IntoUiNode,
    wgt_fn: impl IntoVar<WidgetFn<()>>,
) -> UiNode

P Custom background generated using a WidgetFn<()>.

This is the equivalent of setting background to the presenter node, but if the property is set in a style that is used by multiple widgets at the same time the wgt_fn will be called for each widget to create duplicates of the background nodes instead of moving the node to the last widget.