Koa中间件next实现

Koa中间件next实现

学习Koa模仿一下中间件原理，与Express比较类似

const http = require("http")

// 处理中间件，实现next机制
function middlewareHandler(middlewareList) {
    
  return function (ctx) {
    
    function dispatchNext(i) {
    
      const fn = middlewareList[i]
      try {
    
        return Promise.resolve(fn(ctx, dispatchNext.bind(null, i + 1)))
      } catch (error) {
    
        return Promise.reject(error)
      }
    }
    dispatchNext(0)
  }
}

class Koa {
    
  constructor() {
    
    this.middlewareList = []
  }

  use(middleware) {
    
    this.middlewareList.push(middleware)
    return this
  }

  createContext(req, res) {
    
    return {
    
      res,
      req,
    }
  }

  executeMiddleware(ctx, fn) {
    
    return fn(ctx)
  }

  callback() {
    
    const fn = middlewareHandler(this.middlewareList)

    return (req, res) => {
    
      const ctx = this.createContext(req, res)
      return this.executeMiddleware(ctx, fn)
    }
  }

  listen(...args) {
    
    const server = http.createServer(this.callback())
    server.listen(...args)
  }
}

module.exports = Koa