A Koa application is an object containing an array of middleware functions which are composed and executed in a stack-like manner upon request. Koa is similar to many other middleware systems that you may have encountered such as Ruby's Rack, Connect, and so on - however a key design decision was made to provide high level "sugar" at the otherwise low-level middleware layer. This improves interoperability, robustness, and makes writing middleware much more enjoyable.

Koa 在2017-02-25 發(fā)布了2.0 版本，詳情請(qǐng)參照git hub 上history.md

1.0 與2.0 版本關(guān)于中間件的異同

1.0 版本是通過(guò)組合不同的 generator，可以免除重復(fù)繁瑣的回調(diào)函數(shù)嵌套，并極大地提升錯(cuò)誤處理的效率。
2.0版本Koa放棄了generator，采用Async 函數(shù)實(shí)現(xiàn)組件數(shù)組瀑布流式（Cascading）的開發(fā)模式。

先來(lái)看看Koa 2.0 版本Hello word的demo，

const Koa = require('koa');
const app = new Koa();
app.use(ctx => {
  ctx.body = 'Hello World';
});

app.listen(3000);

Koa中間件的方式

Common function

/* Middleware normally takes two parameters (ctx, next), ctx is the context for one request,
 next is a function that is invoked to execute the downstream middleware. 
It returns a Promise with a then function for running code after completion. */

app.use((ctx, next) => {
  const start = new Date();
  return next().then(() => {
    const ms = new Date() - start;
    console.log(`${ctx.method} ${ctx.url} - ${ms}ms`);
  });
});

Async function

app.use(async (ctx, next) => {
  const start = new Date();
  await next();
  const ms = new Date() - start;
  console.log(`${ctx.method} ${ctx.url} - ${ms}ms`);
});

async與common function 的區(qū)別主要在于語(yǔ)法糖面前是否存在async，如果需要同步的執(zhí)行中間件或是函數(shù)，只需要在中間件或函數(shù)前加關(guān)鍵字await。

// 同步執(zhí)行中間件
app.use(async (ctx, next) => { await next(); });

Koa Cascading 中間件值的傳遞

Middleware1 中間件中的函數(shù)next()返回的是一個(gè)promise 對(duì)像

const Koa = require('koa');
const app = new Koa();
//Middleware 1
app.use(async function (ctx, next) {
    console.log("1.1")
    await next().then(function (data) {
        console.log(data)               // "Spursyy"
    }).catch(function (err) {
        console.log(err)
    });
    console.log("1.2")

})
//Middleware 2
app.use(async function (ctx, next) {
   return new Promise (function (resolve, reject) {
       resolve("Spursyy")
   })
})

app.listen(3000);
console.log("port 3000 was start!")