reducers.ts

type Reducer（该文件中最重要的）

export type Reducer<S = any, A extends Action = AnyAction> = (
  state: S | undefined,
  action: A
) => S

reducer(也称为reducer函数)是一个接受累加和一个值并返回新累加的函数。它们用于将值的集合缩减为单个值.

注意⚠️：Reducers并不是Redux独有的，它们是函数式编程中的基本概念，比如JS，他内置的reducing API是Array.prototype.reduce()

在Redux, 累积值是state object(状态对象), 而累加的值就是actions(动作)。Reducers 根据之前的state(状态)和 an action(一个动作)来计算出新状态，他们必须是纯函数(对于给定的输入返回完全相同的输出的函数),他们也应该没有副作用.
注意：⚠️⚠️⚠️**Reducers是Redux中最重要的概念

ReducersMapObject

值为对应不同reducer函数的对象

export type ReducersMapObject<S = any, A extends Action = AnyAction> = {
  [K in keyof S]: Reducer<S[K], A>
}

StateFromReducersMapObject

从ReducersMapObject推断一个组合的state形状

export type StateFromReducersMapObject<M> = M extends ReducersMapObject

ReducerFromReducersMapObject

从ReducersMapObject推断reducer的联合类型

export type ReducerFromReducersMapObject<M> = M extends {
  [P in keyof M]: infer R
}
  ? R extends Reducer<any, any>
    ? R
    : never
  : never

ActionFromReducer

从reducer函数推断action类型

export type ActionFromReducer<R> = R extends Reducer<any, infer A> ? A : never

ActionFromReducersMapObject

从ReducersMapObject推断action(动作)联合类型

export type ActionFromReducersMapObject<M> = M extends ReducersMapObject
  ? ActionFromReducer<ReducerFromReducersMapObject<M>>
  : never