didUpdateWidget method
Override this method to respond when its StatefulWidget is re-created.
The framework always calls build after calling didUpdateWidget, which
means any calls to setState in didUpdateWidget are redundant.
Implementation
@override
@mustCallSuper
void didUpdateWidget(covariant T oldWidget) {
  /// No 'setState()' functions are allowed
//    _setStateAllowed = false;
  for (final con in controllerList) {
    con.didUpdateWidget(oldWidget);
  }
  super.didUpdateWidget(oldWidget);
  /// Re-enable setState() function
  _setStateAllowed = true;
  /// No 'setState()' functions are necessary
  _setStateRequested = false;
  // Record the triggered event
  assert(() {
    if (_printEvents) {
      debugPrint('============ Event: didUpdateWidget() in $this');
    }
    return true;
  }());
}