None (same as `nil`, but named `None`)

main.dart

///
/// [Author] Alex (https://github.com/AlexV525)
/// [Date] 2022/04/28 11:12
///
import 'package:flutter/widgets.dart';

const None none = None();

class None extends Widget {
  const None({Key? key}) : super(key: key);

  @override
  Element createElement() => _NoneElement(this);
}

class _NoneElement extends Element {
  _NoneElement(Widget widget) : super(widget);

  @override
  bool get debugDoingBuild => false;

  @override
  void mount(Element? parent, dynamic newSlot) {
    assert(
      parent is! MultiChildRenderObjectElement,
      'You are using None under a MultiChildRenderObjectElement. '
      'This suggests a possibility that the None '
      'is not needed or is being used improperly. '
      "Make sure it can't be replaced with an inline conditional or "
      'omission of the target widget from a list.',
    );
    super.mount(parent, newSlot);
  }

  @override
  void performRebuild() {}
}