node读源码系列---koa2源码分析

koa介绍

koa有1.x和2.x两个版本，koa1.x基于generator,依赖co的包装。koa2.x基于promise,在node v7.6+可以直接运行，低版本可以babel。下面是基于koa2.13源码的介绍。

简单使用

// 1 导包
const Koa = require('koa');
// 2 new一个koa的实例 说明koa是一个类
const app=new Koa();
// 3 挂载中间件
app.use(async function (ctx, next) {
    const startTime = new Date()
    console.log(`${startTime} ${ctx.method} ${ctx.url}`);
    await next();
    const endTime = new Date()
    console.log(`${endTime} ${endTime.getTime() - startTime.getTime()}ms`);
});
app.use(ctx => {
    ctx.body='hello world';
});
// 4 指定监听端口
app.listen('3000', () => {
    console.log('start suc!!');
    console.log('http://127.0.0.1:3000');
});

目录结构和依赖

lib目录下有四个文件:

• application.js: 应用程序。
• context.js：上下文。
• request.js: 请求类。
• response.js: 响应类。

{
  "dependencies": {
    "accepts": "^1.3.5",
    "cache-content-type": "^1.0.0",
    "content-disposition": "~0.5.2",
    "content-type": "^1.0.4",
    "cookies": "~0.8.0",
    "debug": "~3.1.0",
    "delegates": "^1.0.0",
    "depd": "^1.1.2",
    "destroy": "^1.0.4",
    "encodeurl": "^1.0.2",
    "escape-html": "^1.0.3",
    "fresh": "~0.5.2",
    "http-assert": "^1.3.0",
    "http-errors": "^1.6.3",
    "is-generator-function": "^1.0.7",
    "koa-compose": "^4.1.0",
    "koa-convert": "^1.2.0",
    "on-finished": "^2.3.0",
    "only": "~0.0.2",
    "parseurl": "^1.3.2",
    "statuses": "^1.5.0",
    "type-is": "^1.6.16",
    "vary": "^1.1.2"
  },
  "deprecated": false,
  "description": "Koa web app framework",
  "devDependencies": {
    "egg-bin": "^4.13.0",
    "eslint": "^6.5.1",
    "eslint-config-koa": "^2.0.0",
    "eslint-config-standard": "^14.1.0",
    "eslint-plugin-import": "^2.18.2",
    "eslint-plugin-node": "^10.0.0",
    "eslint-plugin-promise": "^4.2.1",
    "eslint-plugin-standard": "^4.0.1",
    "gen-esm-wrapper": "^1.0.6",
    "mm": "^2.5.0",
    "supertest": "^3.1.0"
  },
  "engines": {
    "node": "^4.8.4 || ^6.10.1 || ^7.10.1 || >= 8.1.4"
  },
  "exports": {
    ".": {
      "require": "./lib/application.js",
      "import": "./dist/koa.mjs"
    },
    "./": "./"
  },
  "files": [
    "dist",
    "lib"
  ],
  "main": "lib/application.js",
  "name": "koa",
  "version": "2.13.0"
}

入口文件: lib/application.js

一、 application.js

module.exports = class Application extends Emitter
...
module.exports.HttpError = HttpError;

Application 是继承Emitter的类，并且同时导出了HttpErrorr。为什么会继承Emitter？往下看。

1 constructor

constructor(options) {
    super();
    options = options || {};
    this.proxy = options.proxy || false;
    this.subdomainOffset = options.subdomainOffset || 2;
    this.proxyIpHeader = options.proxyIpHeader || 'X-Forwarded-For';
    this.maxIpsCount = options.maxIpsCount || 0;
    this.env = options.env || process.env.NODE_ENV || 'development';
    if (options.keys) this.keys = options.keys;
    this.middleware = [];
    this.context = Object.create(context);
    this.request = Object.create(request);
    this.response = Object.create(response);
    // util.inspect.custom support for node 6+
    /* istanbul ignore else */
    if (util.inspect.custom) {
      this[util.inspect.custom] = this.inspect;
    }
  }

option配置分以下几种
1 主要配置反向代理,客户端ip相关的配置(proxy subdomainOffset proxyIpHeader maxIpsCount )
2 middleware是中间件数组。
3 响应头、请求头、上下文(request response Context) 通过Object.create继承原有对象的属性并且创建新的对象。
4 util.inspect.custom是一个Symbol类型的值，通过定义对象的[util.inspect.custom]属性为一个函数，简单来说就是自定义对象的查看函数。

2 use

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;
  }

由于koa最开始支持使用generator函数作为中间件使用，如果是generator函数这里使用convert进行了一次转换，并且push到了middleware数组中。

3 listen

 listen(...args) {
    debug('listen');
    const server = http.createServer(this.callback());
    return server.listen(...args);
  }

callback() {
    const fn = compose(this.middleware);

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

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

    return handleRequest;
  }

listen方法先调用http模块的createServer, 传入回调函数callback,listen方法的入参合http的listen的入参是一致的。创建http服务成功之后会调用callback函数。先调用compose返回fn。之后就是handleRequest生成ctx，并按中间件加载的顺序执行逻辑了。到这里就可以回到继承Emitter，因为这里用到事件触发器(on和emit)完成事件监听和发送。

4 createContext(req, res)

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.state = {};
    return context;
  }

createContext主要做的就是构造一个ctx对象。

5 handleRequest

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

fnMiddleware就是koa-compose函数的返回值

6 compose(koa-compose)函数 ！！重点

function compose (middleware) {
  if (!Array.isArray(middleware)) throw new TypeError('Middleware stack must be an array!')
  for (const fn of middleware) {
    if (typeof fn !== 'function') throw new TypeError('Middleware must be composed of functions!')
  }

  /**
   * @param {Object} context
   * @return {Promise}
   * @api public
   */

  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)
      }
    }
  }
}

按app.use的顺序执行，形成了洋葱模型。compose的返回值是一个匿名函数，有context和next两个函数。首次执行时候context是由fnMiddleware函数传入的，next为undefined。fn为执行当前次的中间件函数，next其实就是为dispatch函数。整体上使用递归+闭包的方式进行了实现。

7 respond(ctx)

  // allow bypassing koa
  if (false === ctx.respond) return;

  if (!ctx.writable) return;

  const res = ctx.res;
  let body = ctx.body;
  const code = ctx.status;

  // ignore body
  if (statuses.empty[code]) {
    // strip headers
    ctx.body = null;
    return res.end();
  }

  if ('HEAD' === ctx.method) {
    if (!res.headersSent && !ctx.response.has('Content-Length')) {
      const { length } = ctx.response;
      if (Number.isInteger(length)) ctx.length = length;
    }
    return res.end();
  }

  // status body
  if (null == body) {
    if (ctx.response._explicitNullBody) {
      ctx.response.remove('Content-Type');
      ctx.response.remove('Transfer-Encoding');
      return res.end();
    }
    if (ctx.req.httpVersionMajor >= 2) {
      body = String(code);
    } else {
      body = ctx.message || String(code);
    }
    if (!res.headersSent) {
      ctx.type = 'text';
      ctx.length = Buffer.byteLength(body);
    }
    return res.end(body);
  }

  // responses
  if (Buffer.isBuffer(body)) return res.end(body);
  if ('string' === typeof body) return res.end(body);
  if (body instanceof Stream) return body.pipe(res);

  // body: json
  body = JSON.stringify(body);
  if (!res.headersSent) {
    ctx.length = Buffer.byteLength(body);
  }
  res.end(body);
}

当所有的中间件都成功执行了，就会进到respond函数里，这里会判断ctx.body是不是有值的，如果没有值则返回404页面。

二、request.js

整体的属性就是对http.req中比较重要的字段重新定义了出来，比如length字段，就是res.header.Content-Length字段。

三、response.js

挑出几个比较常用的。

1 set body

set body(val) {
    const original = this._body;
    this._body = val;

    // no content
    if (null == val) {
      if (!statuses.empty[this.status]) this.status = 204;
      if (val === null) this._explicitNullBody = true;
      this.remove('Content-Type');
      this.remove('Content-Length');
      this.remove('Transfer-Encoding');
      return;
    }

    // set the status
    if (!this._explicitStatus) this.status = 200;

    // set the content-type only if not yet set
    const setType = !this.has('Content-Type');

    // string
    if ('string' === typeof val) {
      if (setType) this.type = /^\s*</.test(val) ? 'html' : 'text';
      this.length = Buffer.byteLength(val);
      return;
    }

    // buffer
    if (Buffer.isBuffer(val)) {
      if (setType) this.type = 'bin';
      this.length = val.length;
      return;
    }

    // stream
    if (val instanceof Stream) {
      onFinish(this.res, destroy.bind(null, val));
      if (original != val) {
        val.once('error', err => this.ctx.onerror(err));
        // overwriting
        if (null != original) this.remove('Content-Length');
      }

      if (setType) this.type = 'bin';
      return;
    }

    // json
    this.remove('Content-Length');
    this.type = 'json';
  }

当ctx.body = ''时候调用。有下面几种场景。
1 ctx.body = null 或者undefined时候，相当于状态码是204
2 ctx.body非null 则 状态码为200
3 ctx.body为字符串类型，则Type为html或text
4 ctx.body为buffer类型，则Type为bin
5 ctx.body为Stream类型，则Type为bin
5 ctx.body其他，则Type为json

2 status

set status(code) {
    if (this.headerSent) return;

    assert(Number.isInteger(code), 'status code must be a number');
    assert(code >= 100 && code <= 999, `invalid status code: ${code}`);
    this._explicitStatus = true;
    this.res.statusCode = code;
    if (this.req.httpVersionMajor < 2) this.res.statusMessage = statuses[code];
    if (this.body && statuses.empty[code]) this.body = null;
  }

四、context.js

重点看下别名怎么实现的。如ctx.body是ctx.response.body的别名

delegate(proto, 'response')
  .method('attachment')
  .method('redirect')
  .method('remove')
  .method('vary')
  .method('has')
  .method('set')
  .method('append')
  .method('flushHeaders')
  .access('status')
  .access('message')
  .access('body')
  .access('length')
  .access('type')
  .access('lastModified')
  .access('etag')
  .getter('headerSent')
  .getter('writable');

delegate(proto, 'request')
  .method('acceptsLanguages')
  .method('acceptsEncodings')
  .method('acceptsCharsets')
  .method('accepts')
  .method('get')
  .method('is')
  .access('querystring')
  .access('idempotent')
  .access('socket')
  .access('search')
  .access('method')
  .access('query')
  .access('path')
  .access('url')
  .access('accept')
  .getter('origin')
  .getter('href')
  .getter('subdomains')
  .getter('protocol')
  .getter('host')
  .getter('hostname')
  .getter('URL')
  .getter('header')
  .getter('headers')
  .getter('secure')
  .getter('stale')
  .getter('fresh')
  .getter('ips')
  .getter('ip');

可以看出来是通过delegate，深入到delegate内部看下

Delegator.prototype.method = function(name){
  var proto = this.proto;
  var target = this.target;
  this.methods.push(name);

  proto[name] = function(){
    return this[target][name].apply(this[target], arguments);
  };

  return this;
};
Delegator.prototype.getter = function(name){
  var proto = this.proto;
  var target = this.target;
  this.getters.push(name);

  proto.__defineGetter__(name, function(){
    return this[target][name];
  });

  return this;
};

/**
 * Delegator setter `name`.
 *
 * @param {String} name
 * @return {Delegator} self
 * @api public
 */

Delegator.prototype.setter = function(name){
  var proto = this.proto;
  var target = this.target;
  this.setters.push(name);

  proto.__defineSetter__(name, function(val){
    return this[target][name] = val;
  });

  return this;
};

get、set其实就是通过defineGetter和defineSetter进行实现的。

五、总结

koa中间件执行流程图

image.png