js洋葱模型

洋葱模型指的是方法的执行像洋葱一样，一层一层往里执行，直到中心点后，再一层一层往外出来。

上面的图片取自koa中间件的流程控制图，react-redux的中间件也采用了一样的原理。

const fn1 = (next) => {
  console.log(1)
  next()
  console.log(2)
}

const fn2 = (next) => {
  console.log(3)
  next()
  console.log(4)
}

const fn3 = (next) => {
  console.log(5)
  next()
  console.log(6)
}

const middlewares = [fn1, fn2, fn3];

compose(middlewares)();

执行结果：1 3 5 6 4 2

compose函数执行视图：

        +----------------------------------------------------------------------------------+
        |                                                                                  |
        |                              fn1                                                 |
        |                                                                                  |
        |          +-----------------------------------------------------------+           |
        |          |                                                           |           |
        |          |                    fn2                                    |           |
        |          |                                                           |           |
        |          |            +---------------------------------+            |           |
        |          |            |                                 |            |           |
        | action   |  action    |        fn3                      |    action  |   action  |
        | 1        |    3       |                                 |      4     |     2     |
        |          |            |   action              action    |            |           |
        |          |            |     5                   6       |            |           |
        |          |            |                                 |            |           |
+----------------------------------------------------------------------------------------------->
        |          |            |                                 |            |           |
        |          |            |                                 |            |           |
        |          |            +---------------------------------+            |           |
        |          +-----------------------------------------------------------+           |
        +----------------------------------------------------------------------------------+

分析

compose必须返回的是一个函数，并且每次函数执行，都需要将下一个函数作为参数传给它，这样才能够让方法一层层的执行下去，直到最里面一层：

function compose(middlewarw) {
  return function(args){
    dispatch(0);
    function dispatch(index){
      const fn = middlewarw[index] || args;
      if(typeof fn !== "function") return;
      const next = ()=> dispatch(index+1);
      fn(next);
    }
  }
};

异步函数也能的处理：

function asyncFn() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      console.log("delay...");
      resolve();
    }, 1000);
  });
}

const fn1 = async (next) => {
  console.log(1)
  await next()
  console.log(2)
}

const fn2 = async (next) => {
  console.log(3)
  await asyncFn();
  await next()
  console.log(4)
}

const fn3 = async (next) => {
  console.log(5)
  await next()
  console.log(6)
};

function compose(middlewarw) {
  return function (args) {
    dispatch(0);
    function dispatch(index) {
      const fn = middlewarw[index] || args;
      if (typeof fn !== "function") return Promise.resolve();
      const next = () => dispatch(index + 1);
      
      // 给执行函数添加返回成功的Promise.resolve
      return Promise.resolve(fn(next))
    }
  }
};

compose([fn1,fn2,fn3])();

Redux中间件

redux的中间件compose函数如下：

function compose(middlewarw) {
  return middlewarw.reduce((total, next) => (...args) => total(next(...args)));
}

redux的中间件函数很不好理解，这里可以将它拆开进行分析：

const fn1 = (next) => {
  return ()=>{
    console.log(1)
    next()
    console.log(2)
  }
}

const fn2 = (next) => {
  return ()=>{
    console.log(3)
    next()
    console.log(4)
  }
}

const fn3 = (next) => {
  return ()=>{
    console.log(5)
    next()
    console.log(6)
  }
}

const dispatch = compose([fn1,fn2,fn3])(()=> console.log("dispatch"));

dispatch();

middlewarw经过reduce叠加，每次都将上一次的结果返回给下一个函数作参数：

// 第1次 reduce 的返回值，变成 total 传递到下一次
arg => fn1(() => fn2(arg));

// 第2次 reduce 的返回值，继续作为下一次的 total
arg => (arg => fn1(() => fn2(arg)))(() => fn3(arg));

或者将compose转成比较好理解的函数迭代形式：

function compose(middlewarw) {
  return function(cb) {
    function dispatch(index){
      const fn = middlewarw[index];
      const next = ()=>dispatch(index+1); // 下一次的函数执行
      // 如果不存在下一个函数了，拿到传参里面的函数执行，这里需要保证传参是一个函数，对应的是redux里面的dispatch参数
      fn ? fn(next)() : cb() 
    }
    
    // 最终返回一个函数
    return ()=> dispatch(0);
    
  }
};