1. override
void performRebuild()

Called by rebuild() after the appropriate checks have been made.

Source 

@override
void performRebuild() {
  if (_newWidget != null) {
    // _newWidget can be null if, for instance, we were rebuilt
    // due to a reassemble.
    final Widget newWidget = _newWidget;
    _newWidget = null;
    update(newWidget);
  }
  super.performRebuild();
  assert(_newWidget == null);
}