为什么在使用 redux-thunk 时，内部函数会返回一些东西？

Question

在使用redux-thunk的时候经常看到代码，内部函数返回一些东西，比如the official Redux async example:

The code:

const fetchPosts = subreddit => dispatch => {
  dispatch(requestPosts(subreddit))
  return fetch(`https://www.reddit.com/r/${subreddit}.json`)
    .then(response => response.json())
    .then(json => dispatch(receivePosts(subreddit, json)))
}

const shouldFetchPosts = (state, subreddit) => {
  // return true or false
}

export const fetchPostsIfNeeded = subreddit => (dispatch, getState) => {
  if (shouldFetchPosts(getState(), subreddit)) {
    return dispatch(fetchPosts(subreddit))
  }
}

所以fetchPostsIfNeeded() 是一个thunk，fetchPosts() 也是。它们都返回一个函数，该函数也返回一些东西。现在这个函数实际上是由中间件调用的，我认为返回的值从来没有被使用过。那么为什么 thunk 中的内部函数不断返回一些东西而不是仅仅调用它呢？代码可能是：

const fetchPosts = subreddit => dispatch => {
  dispatch(requestPosts(subreddit))
  fetch(`https://www.reddit.com/r/${subreddit}.json`)
    .then(response => response.json())
    .then(json => dispatch(receivePosts(subreddit, json)))
}

const shouldFetchPosts = (state, subreddit) => {
  // return true or false
}

export const fetchPostsIfNeeded = subreddit => (dispatch, getState) => {
  if (shouldFetchPosts(getState(), subreddit)) {
    dispatch(fetchPosts(subreddit))
  }
}

Answer 1

它返回一个承诺，允许您在then 链中添加一个回调，并在获取子编辑帖子时执行某些操作，如在示例中。

export const fetchPostsIfNeeded = subreddit => (dispatch, getState) => {
  if (shouldFetchPosts(getState(), subreddit)) {
    dispatch(fetchPosts(subreddit)).then(() => doSomething());
  }
}