koa 功能

  • 封装 http 模块【lib/application 下的 listen 方法】
  • 构建中间件模型【通过 koa-compose 包】
  • 整合了request,response,context【lib/application 下的 createContext 方法】
  • 错误处理【lib/application 下的 onerror 方法】

一个基础模板代码如下：

const Koa = require('koa');
const app = new Koa();

// logger
app.use(async (ctx, next) => {
await next();
const rt = ctx.response.get(‘X-Response-Time’);
console.log(</span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>ctx<span class="token punctuation">.</span>method<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string"> </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>ctx<span class="token punctuation">.</span>url<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string"> - </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>rt<span class="token interpolation-punctuation punctuation">}</span></span><span class="token template-punctuation string">);
});


// x-response-time
app.use(async (ctx, next) => {
const start = Date.now();
await next();
const ms = Date.now() - start;
ctx.set(‘X-Response-Time’, </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>ms<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string">ms</span><span class="token template-punctuation string">);
});


// response
app.use(async ctx => {
ctx.body = ‘Hello World’;
});


app.listen(3000);

koa 里，中间件函数接收两个参数 context, next， 通过 use 方法，将函数添加到 middleware 数组里【 this.middleware.push(fn) 】， 在 listen 里

http.createServer(this.callback()).listen(...args);

调用了 callback，在 callback 里，通过 compose(this.middleware)， 处理之前添加过的中间件，返回 this.handleRequest(ctx, fn) 来执行中间件， handleRequest 是返回 Promise 函数的，所以在编写中间件时，往往通过 async await 来构建，这里主要来解析下 compose，它是引用了 koa-compose 包，主要源码如下：

function compose () {
  return function (context, next) {
    // last called middleware
    let index = -1
    return dispatch(0)
    function dispatch (i) {
      if (i <= index) return Promise.reject(new Error('next() called multiple times'))
      index = i
      let fn = middleware[i]
      if (i === middleware.length) fn = next
      if (!fn) return Promise.resolve()
      try {
        return Promise.resolve(fn(context, dispatch.bind(null, i + 1)));
      } catch (err) {
        return Promise.reject(err)
      }
    }
  }
}

compose 里会返回 dispatch 方法，dispatch 里会先执行第一个fn，并将 middleware 队列里下一个 fn 作为 next 参数，且 dispatch 返回的为 Promise，从而形成了先进后出洋葱式模型。 koa-middleware