问题描述

我希望能够根据对象的另一个属性推断对象的属性类型，而不必像Newtype那样声明T。而是声明Newtype并从属性推断T。见下图：

export interface Action {
  type: K;
  payload: K extends "UPDATE_FIVE_DAY"
    ? {
        fiveDayForecast?: FiveDayForecast;
        fiveDayExpiresAt?: Moment;
        fiveDayLocationFor?: Location;
      }
    : K extends "UPDATE_LOADING"
    ? { loading: boolean }
    : K extends "UPDATE_LOCATION"
    ? { location: Location }
    : K extends "UPDATE_SETTINGS"
    ? { settings: Settings }
    : undefined;
}

我遇到的问题是变量K不存在，我可以这样设置它：

export interface Action<K extends 'UPDATE_FIVE_DAY' | 'UPDATE_LOADING' | ...etc

我要在通用减速器中使用此操作：

export default (state: State = initialState, action: Action): State => {
  switch (action.type) {
    case "UPDATE_LOADING":
      return {
        ...state,
        loading: action.payload,
      };
  }
});

推荐答案

我按照@shlang的建议，使用区别对待的联合使其正常工作。

减速机：

export default (state: State = initialState, action: Actions): State => {
  switch (action.type) {
    case "UPDATE_LOADING":
      return {
        ...state,
        loading: action.payload.loading,
      };
   }
});

和操作的类型：

export type Action = {
  type: AllActionTypes;
  payload: Partial<State> | undefined;
};

export type Actions =
  | UpdateFiveDayAction
  | UpdateLoadingAction;


export interface UpdateLoadingAction extends Action {
  type: "UPDATE_LOADING";
  payload: { loading: State["loading"] };
}


export interface UpdateFiveDayAction extends Action {
  type: "UPDATE_FIVE_DAY";
  payload: {
    fiveDayForecast: State["fiveDayForecast"];
    fiveDayExpiresAt: State["fiveDayExpiresAt"];
    fiveDayLocationFor: State["fiveDayLocationFor"];
  };
}

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