@immutable // 不可变
abstract class Widget extends DiagnosticableTree { //DiagnosticableTree提供调试信息
  const Widget({ this.key });

  final Key? key;//canUpdate中判断前后的key是否相同，则使用新的Widget配置更新Element对象，否则创建新的Element对象

  @protected
  @factory
  Element createElement();//每个Widget都有对应的Element。Widget树根Element树对应

  
  @override
  String toStringShort() {
    final String type = objectRuntimeType(this, 'Widget');
    return key == null ? type : '$type-$key';
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.defaultDiagnosticsTreeStyle = DiagnosticsTreeStyle.dense;
  }

  @override
  @nonVirtual
  bool operator ==(Object other) => super == other;

  @override
  @nonVirtual
  int get hashCode => super.hashCode;


  // 是否可以用newWidget修改前一帧oldWidget生成的Element，而不是创建新的Element
  static bool canUpdate(Widget oldWidget, Widget newWidget) {
    return oldWidget.runtimeType == newWidget.runtimeType
        && oldWidget.key == newWidget.key;
  }

}