Freezed类自定义copyWith运行报错，求问题原因及实现方法

我使用Freezed包生成数据类，通过@Freezed(copyWith: false)注解禁用了自动生成的copyWith方法，尝试自行实现自定义copyWith方法，代码如下：

@Freezed(copyWith: false)
class AuthenticationState with _$AuthenticationState {
  const factory AuthenticationState({
    String? userId,
    ErrorObject? errorObject,
  }) = _AuthenticationState;

  AuthenticationState copyWith({
    String? userId,
    ErrorObject? errorObject,
  }) {
    return AuthenticationState(
      userId: userId ?? this.userId,
      errorObject: errorObject, // 不传入则重置
    );
  }
}

代码生成成功且无静态分析错误，但运行应用时出现报错：

authentication_bloc.freezed.dart:32:7: Error: The non-abstract class
'_$_AuthenticationState' is missing implementations for these members:

• AuthenticationState.copyWith Try to either
• provide an implementation,
• inherit an implementation from a superclass or mixin,
• mark the class as abstract, or
• provide a 'noSuchMethod' implementation.
  class _$_AuthenticationState implements _AuthenticationState {
  ^^^^^^^^^^^^^^^^^^^^^^ lib/authentication/bloc/authentication_state.dart:28:23: Context:
  'AuthenticationState.copyWith' is defined here.

Freezed的代码生成机制决定了：你编写的AuthenticationState会被处理为抽象类，生成的具体实现类_$_AuthenticationState是实现_AuthenticationState接口，而非直接继承AuthenticationState类。你在AuthenticationState中定义的copyWith实例方法，不会被生成的子类自动继承或实现，因此运行时会提示子类缺少该方法的实现逻辑。

最优方案是使用扩展方法实现自定义copyWith，这是Freezed中添加自定义实例方法的标准方式，不会与生成代码产生冲突：

@Freezed(copyWith: false)
class AuthenticationState with _$AuthenticationState {
  const factory AuthenticationState({
    String? userId,
    ErrorObject? errorObject,
  }) = _AuthenticationState;
}

// 通过扩展为AuthenticationState添加自定义copyWith方法
extension AuthenticationStateCopyWith on AuthenticationState {
  AuthenticationState copyWith({
    String? userId,
    ErrorObject? errorObject,
  }) {
    return AuthenticationState(
      userId: userId ?? this.userId,
      errorObject: errorObject, // 不传入参数则重置为null
    );
  }
}

这种方式下，所有AuthenticationState的实例都能正常调用copyWith方法，且不会触发子类未实现方法的报错。

内容的提问来源于stack exchange，提问作者genericUser