详解Redux框架之applyMiddleware

export default function applyMiddleware(...middlewares) {
  return createStore => (...args) => {
    // 利用传入的createStore和reducer和创建一个store
    const store = createStore(...args)
    let dispatch = () => {
      throw new Error(
      )
    }
    const middlewareAPI = {
      getState: store.getState,
      dispatch: (...args) => dispatch(...args)
    }
    // 让每个 middleware 带着 middlewareAPI 这个参数分别执行一遍
    const chain = middlewares.map(middleware => middleware(middlewareAPI))
    // 接着 compose 将 chain 中的所有匿名函数，组装成一个新的函数，即新的 dispatch
    dispatch = compose(...chain)(store.dispatch)
    return {
      ...store,
      dispatch
    }
  }
}