flutter - @freezed copyWith 缺少密封类

Question

在以下代码段中，state.copyWith 函数不可用。

@freezed
class MyState with _$MyState {
  @JsonSerializable(fieldRename: FieldRename.snake, explicitToJson: true)
  const factory MyState({@Default(0) int counter,}) = _MyState;
  const factory MyState.initial({@Default(0) int counter}) = Initial;
  const factory MyState.loading() = Loading;
  const factory MyState.one() = One;
  const factory MyState.two() = Two;

  factory MyState.fromJson(Map<String, dynamic> json) =>
      _$MyStateFromJson(json);
}

class MyStateNotifier extends StateNotifier<MyState> {
  MyStateNotifier() : super(MyState.initial());

  Future<void> one() async {
    state = MyState.loading();
    await Future.delayed(Duration(seconds: 5));
    state.copyWith(counter: 1);
  }
}

但是，当我删除密封类时，copyWith 函数可用。

@freezed
class MyState with _$MyState {
  @JsonSerializable(fieldRename: FieldRename.snake, explicitToJson: true)
  const factory MyState({@Default(0) int counter,}) = _MyState;
  // const factory MyState.initial({@Default(0) int counter}) = Initial;
  // const factory MyState.loading() = Loading;
  // const factory MyState.one() = One;
  // const factory MyState.two() = Two;

  factory MyState.fromJson(Map<String, dynamic> json) =>
      _$MyStateFromJson(json);
}

class MyStateNotifier extends StateNotifier<MyState> {
  MyStateNotifier() : super(MyState());

  Future<void> one() async {
    await Future.delayed(Duration(seconds: 5));
    state.copyWith(counter: 1);
  }
}

我需要更改什么才能使第一个代码段中的 copyWith 可用？

Answer 1

只有所有构造函数共有的属性才会生成 copyWith 方法，如 README docs 中所述.

We can use copyWith with properties defined on all constructors...

假设您有一个 Loading 实例，您希望它有什么 copyWith 方法？它没有属性，因此它不能有任何 copyWith 方法，因此所有类型的 union 也不能。

但是，您可以使用模式匹配在正确类型的实例上调用 copyWith。

在你的例子中，这样的事情会起作用:

MyState myState = ...;

myState.maybeMap(
    initial: (v: Initial) => v.copyWith(counter: v.counter + 1),
    orElse: () => ...,
);

或者使用when:

MyState myState = ...;

myState.when(
    (int counter) => MyState.initial(counter + 1),
    loading: () => loading,
    one: () => one,
    two: () => two,
);