placeholderBuilder property Null safety
final
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;