Koa中间件思想

对于node.js而言，目前接触的比较多的就是Express框架，对于Express框架的中间件理解就是，一层一层逐级往下，类似流水线上的每一个工序，如下

const express = require('express');
const app = new express();

app.use(function(){
  console.log('Here is the first one';)
})

app.use(function(){
  console.log('Here is the second one')
})

//输出结果为
// Here is the first one
// Here is the second one

由于当时理解洋葱模型时候，不够理解透彻，当时认为只是简单的layer through layer，每一个layer只是单单为一个二维平面，但是实际上，Koa的洋葱模型，每一层layer相当于一个球面，当贯穿整个模型时，实际上每一个球面(sophere)会穿透两次，在首先理解这个点时，先熟悉一下ES6的 yield 语法

class Test{
  constructor(){
    this.state = this.fn1();
    this.state.next();
    this.state.next();
  }
  *fn1(){
    console.log('This is fn1, phase 1');
    yield* this.fn2();
    console.log('This is fn1, phase 2');
  }
  *fn2(){
    console.log('This is fn2, phase 1');
    yield* this.fn3();
    console.log('This is fn2, phase 2');
  }
  *fn3(){
    console.log('This is fn3, phase 1');
    console.log('This is fn3, phase 2');
  }
}

const test = new Test();

// 此时的yield* 代表会执行后面的函数
// 输出结果为
//This is fn1, phase 1
//This is fn2, phase 1
//This is fn3, phase 1
//This is fn3, phase 2
//This is fn2, phase 2
//This is fn1, phase 2
//典型的洋葱结构

对于Koa

var koa = require('koa');
var app = koa();

app.use(function* f1(next) {
    console.log('f1: pre next');
    yield next;
    console.log('f1: post next');
});

app.use(function* f2(next) {
    console.log('  f2: pre next');
    yield next;
    console.log('  f2: post next');
});

app.use(function* f3(next) {
    console.log('    f3: pre next');
    this.body = 'hello world';
    console.log('    f3: post next');
});

//执行熟悉如下
f1: pre next
  f2: pre next
    f3: pre next
    f3: post next
  f2: post next
f1: post next

每个中间件会执行两次，这就是Koa中间件的核心思想