解释一下 Redux 中间件
explain Redux Middleware
一句话：中间件是夹在「派发 action」和 「reducer 收到 action」之间的一层， 能拦截、改写、延迟、 甚至吞掉一个 action。
签名是三层柯里化—— 这个形状本身常被问到：store => next => action => {}。 多个中间件靠 next串成一条链，和 Express 的中间件是同一个模式。
为什么需要它：因为reducer 必须是纯的， 所以异步和副作用无处安放。 中间件就是专门给副作用留的位置。
常见的几个：
redux-thunk—— 让你能 dispatch 一个函数而不只是对象， 在里面做异步。最简单，RTK 默认装。redux-saga—— 用 generator 描述复杂异步流程 （可取消、可重试、能编排多个请求）。 能力强但学习成本高。redux-logger—— 打印每个 action 前后的 state。
会追问：「thunk 和 saga 怎么选？」—— 大部分项目 thunk 够了；只有在需要「取消、去抖、 复杂的流程编排」时 saga 才值那份复杂度。
「能自己写一个吗？」—— 能，而且面试常让手写一个 logger。
In one line: middleware is a layer between “an action is dispatched” and “the reducer receives it”. It can intercept, rewrite, delay or even swallow an action.
The signature is curried three levels deep — the shape itself gets asked about: store => next => action => {}. Several middlewares chain together through next, and it is the same pattern as Express middleware.
Why you need it: because the reducer has to be pure, async work and side effects have nowhere to live. Middleware is the place reserved for side effects.
The common ones:
redux-thunk— lets you dispatch a function instead of only an object, and do your async work inside it. Simplest option, and RTK installs it by default.redux-saga— describes complex async flows with generators (cancellable, retryable, able to orchestrate several requests). Powerful, but a steep learning curve.redux-logger— prints the state before and after each action.
Follow-up: “thunk or saga?” — thunk is enough for most projects; saga only earns its complexity when you need cancellation, debouncing or real flow orchestration.
“Could you write one?” — yes, and interviewers often ask you to write a logger on the spot.