placeholderBuilder property
Placeholder widget left in place as the Hero's child once the flight takes
off.
By default the placeholder widget is an empty SizedBox keeping the Hero
child's original size, unless this Hero is a source Hero of a Navigator
push transition, in which case child will be a descendant of the placeholder
and will be kept Offstage during the Hero's flight.
Implementation
final HeroPlaceholderBuilder? placeholderBuilder;