接上篇文章：Redux中间件介绍。

1，applyMiddleware 原理

Redux 应用中间件方式：

const store = createStore(reducer, applyMiddleware(logger1, logger2));

实际会转为下面的方式执行：

const store = applyMiddleware(logger1, logger2)(createStore)(reducer)

• applyMiddleware 会确定用到的中间件；它会返回一个用来创建仓库的函数A，参数是 createStore；
• 函数 A，会返回创建仓库的函数B，和 createStore 函数差不多。
• 函数B，会对中间件函数做处理，并修改原始 dispatch。

大致相当于：

export const applyMiddleware = (...middlewares) => {// 用来创建的仓库的函数return (createStore) => {// 创建仓库的函数return (reducer, defaultState) => {const store = createStore(reducer, defaultState)const dispatch = '经过 middlewares 处理的 store.dispatch'return {...store,dispatch}}}
}

2，实现

2.1，applyMiddleware

上篇文章介绍了 中间件函数 的写法和多个中间件函数的执行顺序。

基于此，实现这个 dispatch 流转的逻辑，并得到最终的 dispatch 即可完成 applyMiddleware。

2.1.1，compose 方法

关键要实现

const resultDispatch = logger1(logger2(logger3(store.dispatch)))

实现：

/*** @param  {...any} funcs* @returns {function}*/
export const compose = (...funcs) => {if (funcs.length === 0) {return (args) => args;} else if (funcs.length === 1) {return funcs[0];}return (...args) => {let lastReturn = null;for (let i = funcs.length - 1; i >= 0; i--) {const func = funcs[i];if (i === funcs.length - 1) {lastReturn = func(...args);} else {lastReturn = func(lastReturn);}}return lastReturn;};
};

// 测试代码
const add = (n) => {return n + n;
};const mult = (n) => {return n * n;
};const b = compose(add, mult);
console.log(b(3)); // 先乘后加，18

可使用 Array.reduce 简化：

export const compose = (...funcs) =>funcs.reduce((prev, next) =>(...args) =>prev(next(...args)));

2.1.2，applyMiddleware

import { compose } from "./compose";
export const applyMiddleware = (...middlewares) => {return (createStore) => {return (reducer, defaultState) => {const store = createStore(reducer, defaultState);let dispatch = () => {throw new Error("目前还不能使用 dispatch");};// 传递给中间件函数的 store 只有这2个属性。const simpleStore = {getState: store.getState,dispatch: (...args) => dispatch(...args), // 每个中间件函数的 dispatch 都是上一个中间件组装后的};// 获取用于创建 dispatch 的函数const dispatchProducts = middlewares.map((m) => m(simpleStore));// 重新组装后的 dispatchdispatch = compose(...dispatchProducts)(store.dispatch);return {...store,dispatch,};};};
};

2.2，修改 createStore

之前实现的 createStore，没有对可能的第3个函数做处理。这里补充下：

• 如果第2个参数是 applyMiddleware，那说明没有 defaultState。

这里就简单判断写第2个参数是不是函数，实际源码中 defaultState 也可以通过一个函数创建。

export const createStore = (reducer, defaultState, enhancer) => {// enhancer 表示 applymiddleware 返回的函数。if (typeof defaultState === 'function') {enhancer = defaultStatedefaultState = undefined}if (typeof enhancer === 'function') {enhancer(createStore)(reducer, defaultState)}// 其他剩余代码没有做变动。// ...
}

完整代码

export const createStore = (reducer, defaultState, enhancer) => {// enhancer 表示 applymiddleware 返回的函数。if (typeof defaultState === "function") {enhancer = defaultState;defaultState = undefined;}if (typeof enhancer === "function") {enhancer(createStore)(reducer, defaultState);}let currentReducer = reducer;let currentState = defaultState;let listeners = [];const dispatch = (action) => {if (typeof action !== "object" || Object.getPrototypeOf(action) !== Object.prototype) {throw new Error("action 必须是一个 plain Object");}if (action.type === undefined) {throw new Error("action 必须有 type 属性");}currentState = currentReducer(currentState, action);// 每次更新时，遍历监听器。for (const listener of listeners) {listener();}};const getState = () => {return currentState;};const subscribe = (listener) => {listeners.push(listener);let isRemove = false;// 取消监听return () => {if (isRemove) {return;} else {isRemove = true;listeners = listeners.filter((f) => f !== listener);}};};// createStore 创建时会调用一次。dispatch({type: `@@redux/INIT${getRandomString}`,});return {dispatch,getState,subscribe,};
};function getRandomString() {return Math.random().toString(36).substring(2, 8).split("").join(".");
}

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以上。