koa2源码解读

最近在复习node的基础知识，因而看了看koa2的源码，写此文分享一下包括了Koa2的使用、中间件及上下文对象的大体实现原理。

koa的github地址：https://github.com/koajs/koa.git

Koa2的安装和简单使用

须要 nodev7.6.0 或者更高的版本，为了支持 ES2015 and async

安装

npm install koa

Hello koa

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

// response
app.use(ctx => {
  ctx.body = 'Hello Koa';
});

app.listen(3000);

中文的api文档：https://github.com/guo-yu/koa-guide

简单分析koa的代码

打开koa的源码，核心文件共四个在lib目录下，application.js,context.js,request.js,response.js

application.js

app的入口文件，就是一个构造函数

简洁的代码
 module.exports = class Application extends Emitter {
  constructor() {
    super();
    //定义下面的属性
    this.proxy = false;
    this.middleware = [];
    this.subdomainOffset = 2;
    this.env = process.env.NODE_ENV || 'development';
    this.context = Object.create(context);
    this.request = Object.create(request);
    this.response = Object.create(response);
  }
  //listen端口方法
  listen(...args) {
    debug('listen');
    const server = http.createServer(this.callback());
    return server.listen(...args);
  }


  toJSON() {
    return only(this, [
      'subdomainOffset',
      'proxy',
      'env'
    ]);
  }

  inspect() {
    return this.toJSON();
  }

  //中间件使用的use方法
  use(fn) {
    if (typeof fn !== 'function') throw new TypeError('middleware must be a function!');
    if (isGeneratorFunction(fn)) {
      deprecate('Support for generators will be removed in v3. ' +
                'See the documentation for examples of how to convert old middleware ' +
                'https://github.com/koajs/koa/blob/master/docs/migration.md');
      fn = convert(fn);
    }
    debug('use %s', fn._name || fn.name || '-');
    this.middleware.push(fn);
    return this;
  }

  //上下文等关键代码
  callback() {
    const fn = compose(this.middleware);

    if (!this.listeners('error').length) this.on('error', this.onerror);

    const handleRequest = (req, res) => {
      res.statusCode = 404;
      const ctx = this.createContext(req, res);
      const onerror = err => ctx.onerror(err);
      const handleResponse = () => respond(ctx);
      onFinished(res, onerror);
      return fn(ctx).then(handleResponse).catch(onerror);
    };

    return handleRequest;
  }

  //建立上下文
  createContext(req, res) {
    const context = Object.create(this.context);
    const request = context.request = Object.create(this.request);
    const response = context.response = Object.create(this.response);
    context.app = request.app = response.app = this;
    context.req = request.req = response.req = req;
    context.res = request.res = response.res = res;
    request.ctx = response.ctx = context;
    request.response = response;
    response.request = request;
    context.originalUrl = request.originalUrl = req.url;
    context.cookies = new Cookies(req, res, {
      keys: this.keys,
      secure: request.secure
    });
    request.ip = request.ips[0] || req.socket.remoteAddress || '';
    context.accept = request.accept = accepts(req);
    context.state = {};
    return context;
  }

  //处理报错
  onerror(err) {
    assert(err instanceof Error, `non-error thrown: ${err}`);

    if (404 == err.status || err.expose) return;
    if (this.silent) return;

    const msg = err.stack || err.toString();
    console.error();
    console.error(msg.replace(/^/gm, '  '));
    console.error();
  }
};

开始的流程：

const app = new Koa();

而后经过 listen来启动服务：

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

看一下原生的启动方法：

// http server 例子
var server = http.createServer(function(serverReq, serverRes){
    var url = serverReq.url;
    serverRes.end( '您访问的地址是：' + url );
});
server.listen(3000);

对比发现this.callback()就是用来建立上下文和处理req和res的，接着看this.callback那个方法：

//处理中间件的使用，后面详细说明
    const fn = compose(this.middleware);

    if (!this.listeners('error').length) this.on('error', this.onerror);
   
    const handleRequest = (req, res) => {
      res.statusCode = 404;
      // 建立上下文
      const ctx = this.createContext(req, res)；
      const onerror = err => ctx.onerror(err);
      
      const handleResponse = () => respond(ctx);
      onFinished(res, onerror);
      //中间件返回promise对象，成功执行handleResponese,错误用onerror处理，
      return fn(ctx).then(handleResponse).catch(onerror);
    };
    返回callback函数
    return handleRequest;

启动服务：

server.listen(...args);

到此服务就起来了。在来看看中间件的使用原理：

use(fn) {
    //判断作兼容处理
    if (typeof fn !== 'function') throw new TypeError('middleware must be a function!');
    if (isGeneratorFunction(fn)) {
      deprecate('Support for generators will be removed in v3. ' +
                'See the documentation for examples of how to convert old middleware ' +
                'https://github.com/koajs/koa/blob/master/docs/migration.md');
      fn = convert(fn);
    }
    debug('use %s', fn._name || fn.name || '-');
    //使用把中间件推送到middleware中保存
    this.middleware.push(fn);
    //返回this，为了连续调用
    return this;
  }

保存到this.middleware,在this.callback进程了处理:

const fn = compose(this.middleware);

看一下compose是怎么处理middleware，代码在const compose = require('koa-compose');

'use strict'


module.exports = compose

function compose (middleware) {
//判断是参数是否为组数
  if (!Array.isArray(middleware)) throw new TypeError('Middleware stack must be an array!')
  //判断单个中间件是否为函数
  for (const fn of middleware) {
    if (typeof fn !== 'function') throw new TypeError('Middleware must be composed of functions!')
  }

  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, function next () {
        //递归调用，直到所有中间件执行完
          return dispatch(i + 1)
        }))
      } catch (err) {
      //有错误
        return Promise.reject(err)
      }
    }
  }
}

经过上面巧妙的递归调用，执行完全部的中间件函数，返回继续启动流程，建立上下文，处理res，req等。