前端中的中间件

摘要：场景现有函数，要求在不改写函数的基础上，在执行该函数之前添加检查，检查返回，再执行我们大都会这样写很明显，这样的很不灵活如果现在又有，同样需要在执行之前进行检查，再写一个吗不，修改函数滑水的日子木有几天，又出现了新的需求，在之前，还有一步操

function stepOne(msg) {
    console.log(msg)
}

function checkStepOne(msg) {
    console.log(`check:${msg}`)
    return msg === "success" ? true : false
}

现有函数 stepOne()，要求在不改写函数的基础上，在执行该函数之前添加检查 checkStepOne()，

检查返回 ture，再执行 stepOne()

我们大都会这样写

function flow(msg){
    if(checkStepOne(msg)){
        return stepOne(msg)
    }
    return false
}

很明显，这样的 flow() 很不灵活

如果现在又有 stepTwo()，同样需要在执行之前进行检查 checkStepTwo()，再写一个flowTwo() 吗？

不，修改函数 flow()

function flow(fn, checkFn, msg) {
    if (checkFn(msg)) {
        return fn(msg)
    }
    return false
}

flow(stepOne, checkStepOne, "success")
flow(stepTwo, checkStepTwo, "success")

滑水的日子木有几天，又出现了新的需求，在 checkStepOne() 之前，还有一步操作，beforeCheckStepOne()

function beforeCheckStepOne(msg) {
    console.log(`beforeCheckStepOne is "${msg}"`)
}

修改函数 flow()

function flow(fns, msg) {
    let current = fns.shift()
    let result
    while (current) {
        result = current(msg)
        if (result === false) {
            return false
        }
        current = fns.shift()
    }
    return result
}

flow([beforeCheckStepOne, checkStepOne, stepOne], "fail")
// beforeCheckStepOne is "fail"
// checkMsg is "fail"

flow(fns, msg) 中 fns 用来存储要执行的步骤，如果上一个步骤返回 false，就不继续下面的步骤了

AOP，Aspect-oriented programming，面向切面编程

改写Function的原型

Function.prototype.before = function (fn) {
    let rawFn = this
    return function () {
        if (fn.apply(null, arguments) === false) {
            return false
        }
        rawFn.apply(null, arguments)
    }
}

stepOne.before(checkStepOne).before(beforeCheckStepOne)("success")
// beforeCheckStepOne is "success"
// checkMsg is "success"
// success

再换个花样

Function.prototype.after = function (fn) {
    let rawFn = this
    return function () {
        if (rawFn.apply(null, arguments) === false) {
            return false
        }
        fn.apply(null, arguments)
    }
}

beforeCheckStepOne.after(checkStepOne).after(stepOne)("success")
// beforeCheckStepOne is "success"
// checkMsg is "success"
// success

OS：这样写不会被人打吗？不仅改写了 Function.prototype，看起来还太装逼

滑水的日子木有几天，又出现了新的需求，步骤之间能传递额外的消息

改造完，如下，多个 context 对象，用于传递信息

function stepOne(msg, context) {
    console.log(msg)
    console.log(context.data)
}

function checkStepOne(msg, context) {
    console.log(`checkMsg is "${msg}"`)
    return msg === "success" ? true : false
}

function beforeCheckStepOne(msg, context) {
    console.log(`beforeCheckStepOne is "${msg}"`)
    context.data = "from beforeCheckStepOne"
}

function flow(fns, msg) {
    let currentFn = fns.shift()
    let result
    let context = {}
    while (currentFn) {
        result = currentFn(msg, context)
        if (result === false) {
            return false
        }
        currentFn = fns.shift()
    }
    return result
}

flow([beforeCheckStepOne, checkStepOne, stepOne], "success")

盗图自前端开发中的中间件

function middle1(next) {
    return () => {
        console.log("Enter the middle1")
        next()
        console.log("Exit the middle1")
    }
}

function middle2(next) {
    return () => {
        console.log("Enter the middle2")
        next()
        console.log("Exit the middle2")
    }
}

function middle3(next) {
    return () => {
        console.log("Enter the middle3")
        next()
        console.log("Exit the middle3")
    }
}

function next() {
    console.log("next")
}

middle1(middle2(middle3(next)))()

这还是3个中间件，调用起来就如此丑陋了，当有更多的中间件该是如何

重写个flow()函数好了

function flow(funcs, rawNext) {
    let next = funcs.pop()
    next = next(rawNext)
    let middle
    while (funcs.length > 0) {
        middle = funcs.pop()
        next = middle(next)
    }
    return next
}

flow([middle1, middle2, middle3], next)()
// Enter the middle1
// Enter the middle2
// Enter the middle3
// next
// Exit the middle3
// Exit the middle2
// Exit the middle1

执行 flow() 的过程，就是在拼凑 middle1(middle2(middle3(next))) 的过程

同时，next() 也可以看成是个中间件

function flow(funcs) {
    let next = funcs.pop()
    while (funcs.length > 0) {
        let middle = funcs.pop()
        next = middle(next)
    }
    return next
}

flow([middle1, middle2, middle3, next])()

没有定义过多变量的 while，总是可以用 reduceRight 修饰一下

function flow(funcs) {
    return funcs.reduceRight((a, b) => b(a))
}

flow([middle1, middle2, middle3, next])()

/**
 * Composes single-argument functions from right to left. The rightmost
 * function can take multiple arguments as it provides the signature for
 * the resulting composite function.
 *
 * @param {...Function} funcs The functions to compose.
 * @returns {Function} A function obtained by composing the argument functions
 * from right to left. For example, compose(f, g, h) is identical to doing
 * (...args) => f(g(h(...args))).
 */

export default function compose(...funcs) {
  if (funcs.length === 0) {
    return arg => arg
  }

  if (funcs.length === 1) {
    return funcs[0]
  }

  return funcs.reduce((a, b) => (...args) => a(b(...args)))
}

如它注释中所说，compose(f, g, h) is identical to doing (...args) => f(g(h(...args)))

// 输入16进制字符串，返回8位2进制字符串
let sixTeenToTen = x => parseInt(x, 16)
let tenToTwo = x => (x).toString(2)
let addZero = x => ("00000000" + x).slice(-8)

let sixTeenToTwo = compose(addZero, tenToTwo, sixTeenToTen)
console.log(sixTeenToTwo("0x62")) // 01100010

当然，你也可以这样写

let sixTeenToTwo2 = x => ("00000000" + (parseInt(x, 16)).toString(2)).slice(-8)
console.log(sixTeenToTwo2("0x62")) // 01100010

开心就好

回到之前的middle1,middle2,middle3 函数那，同时把next改写成middle4

function middle1(next) {
    return (a) => {
        console.log("Enter the middle1")
        next(a)
        console.log("Exit the middle1")
    }
}

function middle2(next) {
    return (a) => {
        console.log("Enter the middle2")
        next(a)
        console.log("Exit the middle2")
    }
}

function middle3(next) {
    return (a) => {
        console.log("Enter the middle3")
        next(a)
        console.log("Exit the middle3")
    }
}

function middle4(next) {
    return (a) => {
        console.log(`middle4:${a}`)
    }
}

let middles = compose(middle1, middle2, middle3, middle4)()
middles("msg")
// Enter the middle1
// Enter the middle2
// Enter the middle3
// middle4:msg
// Exit the middle3
// Exit the middle2
// Exit the middle1

值得一提的是，let middles = compose(middle1, middle2, middle3, middle4)() 最后有一组()，调用函数，相当于middle1(middle2(middle3(middle4()))) 给 middle4 传入空参数

执行 middle4()，返回

(a) => {
    console.log(`middle4:${a}`)
}

这个函数，作为 next 参数，执行 middle3(next)，返回

(a) => {
    console.log("Enter the middle3")
    console.log(`middle4:${a}`)
    console.log("Exit the middle3")
}

这个函数，作为 next 参数，执行 middle2(next)，返回

(a) => {
     console.log("Enter the middle2")
     console.log("Enter the middle3")
     console.log(`middle4:${a}`)
     console.log("Exit the middle3")
     console.log("Exit the middle2")
}

这个函数，作为 next 参数，执行 middle1(next)，返回

(a) => {
     console.log("Enter the middle1")
     console.log("Enter the middle2")
     console.log("Enter the middle3")
     console.log(`middle4:${a}`)
     console.log("Exit the middle3")
     console.log("Exit the middle2")
     console.log("Exit the middle1")
}

所以，最终 middles 就是这样的

let middles = compose(middle1, middle2, middle3, middle4)()
// 相当于
let middles = (a) => {
     console.log("Enter the middle1")
     console.log("Enter the middle2")
     console.log("Enter the middle3")
     console.log(`middle4:${a}`)
     console.log("Exit the middle3")
     console.log("Exit the middle2")
     console.log("Exit the middle1")  
}

class Middle {
    constructor() {
        this.funcs = []
    }

    use(fn) {
        this.funcs.push(fn)
        return this
    }

    work() {
        this.funcs.reduceRight((fn1, fn2) => {
            return () => fn2(fn1)
        }, () => {})()
    }

}

function m1(next) {
    console.log("Enter the middle1")
    next()
    console.log("Exit the middle1")
}

function m2(next) {
    console.log("Enter the middle2")
    next()
    console.log("Exit the middle2")
}

function m3(next) {
    console.log("Enter the middle3")
    next()
    console.log("Exit the middle3")
}

function m4(next) {
    console.log("Enter the middle4")
    console.log("Exit the middle4")
}

let m = new Middle()
m.use(m1)
m.use(m2)
m.use(m3)
m.use(m4)
m.work()

来段小插曲

let fns = [m1, m2, m3, m4, m5]
fns.reduceRight((fn1, fn2) => () => fn2(fn1), () => {})()
// 相当于
fns.reduceRight((fn1, fn2) => {
    return () => fn2(fn1)
}, () => {})()
// 结合之前定义的 m1, m2, m3, m4, m5, 得到结果
// Enter the middle1
// Enter the middle2
// Enter the middle3
// Enter the middle4
// Exit the middle4
// Exit the middle3
// Exit the middle2
// Exit the middle1

其实那段 reduceRight，本来是写成 while 的

let fns = [m1, m2, m3, m4, m5]
let next = () => {}
while(fns.length > 0){
    let fn = fns.pop()
    next = () => fn(next)
}
next()
// 一直输出 Enter the middle1 

所以做了些调整

let fns = [m1, m2, m3, m4, m5]
let next = () => {}
while (fns.length > 0) {
    let fn = fns.pop()
    next = function (fn, next) {
        return () => fn(next)
    }(fn, next)
}
next()
// 输出结果符合预期

来自网上的套路是这样的

class Middle {
    constructor() {
        this.funcs = []
        this.middlewares = []
    }

    use(fn) {
        this.funcs.push(fn)
        return this
    }

    next(fn) {
        if (this.middlewares && this.middlewares.length > 0) {
            let ware = this.middlewares.shift()
            ware.call(this, this.next.bind(this))
        }
    }

    work() {
        this.middlewares = this.funcs.map(f => f)
        this.next()
    }
}

感觉大概就是这个意思

m4 = m4.bind(null, m5)
m3 = m3.bind(null, m4)
m2 = m2.bind(null, m3)
m1 = m1.bind(null, m2)
m1()
// 或者
m1.call(null, m2.bind(null, m3.bind(null, m4.bind(null, m5))))

再啰嗦地解释下，因为我一开始是看半天没能理解

let m = new Middle()
m.use(m1)
m.use(m2)
m.use(m3)
m.use(m4)
m.use(m5)
m.work()

执行 m.work() 后，

执行 m.next()

从 m.middlewares 中取出 m1

执行 m1.call(m, m.next)

执行 m1 函数体内

console.log("Enter the middle1")

然后遇到 next()

实际上执行了 m.next()

从 m.middlewares 中取出 m2

执行 m2.call(m, m.next)

执行 m2 函数体内

console.log("Enter the middle2")

然后遇到 next()

实际上执行了 m.next()

从 m.middlewares 中取出 m3

执行 m3.call(m, m.next)

执行 m3 函数体内

console.log("Enter the middle3")

...

直至结束

class Middle {
    constructor() {
        this.funcs = []
        this.middlewares = []
        this.options = null
    }

    use(fn) {
        this.funcs.push(fn)
        return this
    }

    next(fn) {
        if (this.middlewares && this.middlewares.length > 0) {
            let ware = this.middlewares.shift()
            ware.call(this, this.options, this.next.bind(this))
        }
    }

    work(options) {
        this.middlewares = this.funcs.map(f => f)
        this.options = options
        this.next()
    }
}

使用样例

function m1(options, next) {
    console.log("Enter the middle1")
    console.log(options.name)
    next()
    console.log("Exit the middle1")
}

function m2(options, next) {
    options.name = "m2"
    console.log("Enter the middle2")
    console.log(options.name)
    next()
    console.log("Exit the middle2")
}

function m3(options, next) {
    options.name = "m3"
    console.log("Enter the middle3")
    console.log(options.name)
    next()
    console.log("Exit the middle3")
}

function m4(options, next) {
    console.log("Enter the middle4")
    console.log(options.name)
    console.log("Exit the middle4")
}

function m5(options, next) {
    console.log("Enter the middle5")
    next()
    console.log("Exit the middle5")
}

let m = new Middle()
m.use(m1)
m.use(m2)
m.use(m3)
m.use(m4)
m.use(m5)
m.work({
    name: "m"
})

// Enter the middle1
// m
// Enter the middle2
// m2
// Enter the middle3
// m3
// Enter the middle4
// Exit the middle4
// Exit the middle3
// Exit the middle2
// Exit the middle1

同样功能的代码

let fns = [m1, m2, m3, m4, m5]
let next = () => {}
let options = {
    name: "m"
}
while (fns.length > 0) {
    let fn = fns.pop()
    next = function (fn, options, next) {
        return () => fn(options, next)
    }(fn, options, next)
}
next()

同样功能的代码

let options = {
    name: "m"
}
m4 = m4.bind(null, options, m5)
m3 = m3.bind(null, options, m4)
m2 = m2.bind(null, options, m3)
m1 = m1.bind(null, options, m2)
m1()
// 相当于
fns.reduceRight((fn1, fn2) => fn2.bind(null, options, fn1))()

同样功能的代码

let options = {
    name: "m"
}

m44 = () => m4(options, m5)
m33 = () => m3(options, m44)
m22 = () => m2(options, m33)
m11 = () => m1(options, m22)
m11()
// 相当于
fns.reduceRight((fn1, fn2) => {
    return () => fn2(options, fn1)
}, () => {})()
// 再精炼的话
fns.reduceRight((fn1, fn2) => () => fn2(options, fn1), () => {})()
// 感觉我3min以后就不认得自己写的代码了

fn.bind(null, args) 和 return () => fn(args) 在一些场合，功能相同

编写可维护代码之“中间件模式”

前端开发中的中间件