applyMiddleware.js

import compose from "./compose";

/**
 * Creates a store enhancer that applies middleware to the dispatch method
 * of the Redux store. This is handy for a variety of tasks, such as expressing
 * asynchronous actions in a concise manner, or logging every action payload.
 *
 * See `redux-thunk` package as an example of the Redux middleware.
 *
 * Because middleware is potentially asynchronous, this should be the first
 * store enhancer in the composition chain.
 *
 * Note that each middleware will be given the `dispatch` and `getState` functions
 * as named arguments.
 *
 * @param {...Function} middlewares The middleware chain to be applied.
 * @returns {Function} A store enhancer applying the middleware.
 */
// 中间件
export default function applyMiddleware(...middlewares) {
  return createStore => (reducer, preloadedState, enhancer) => {
    const store = createStore(reducer, preloadedState, enhancer);
    let dispatch = store.dispatch;
    let chain = [];

    const middlewareAPI = {
      getState: store.getState,
      dispatch: action => dispatch(action)
    };
    chain = middlewares.map(middleware => middleware(middlewareAPI));
    dispatch = compose(...chain)(store.dispatch);

    return {
      ...store,
      dispatch
    };
  };
}