pub trait WidgetUnitData {
    // Required method
    fn id(&self) -> &WidgetId;
    // Provided method
    fn get_children(&self) -> Vec<&WidgetUnit> { ... }
}pub trait WidgetUnitData {
    // Required method
    fn id(&self) -> &WidgetId;
    // Provided method
    fn get_children(&self) -> Vec<&WidgetUnit> { ... }
}