例如：factory Simple.fromJson(Map<String, dynamic> json) => Simple(
firstField: json['firstField'] as String,
secondField: json['secondField'] as int,
thirdField: json['third_field'] as bool?,
); 其中的firstField可能是空的需要赋值默认值，或者可能需要转数字，或者可能需要转DateTime等等情况