koa源码阅读之application.js

constructor

作用：初始化

  /**
    *
    * @param {object} [options] Application options
    * @param {string} [options.env='development'] Environment
    * @param {string[]} [options.keys] Signed cookie keys
    * @param {boolean} [options.proxy] Trust proxy headers
    * @param {number} [options.subdomainOffset] Subdomain offset
    * @param {boolean} [options.proxyIpHeader] proxy ip header, default to X-Forwarded-For
    * @param {boolean} [options.maxIpsCount] max ips read from proxy ip header, default to 0 (means infinity)
    *
    */

  constructor(options) {
    
    
    super();
    //这个就是new Koa(options)中的options
    options = options || {
    
    };
    //把options参数复制到实例上
    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;
    //:point_down:中间件队列
    this.middleware = [];
    //:point_down:复制一份到自己身上
    this.context = Object.create(context); // const context = require('./context');
    this.request = Object.create(request); // const request = require('./request');
    this.response = Object.create(response);// const response = require('./response');
    // util.inspect.custom support for node 6+
    /* istanbul ignore else */
    if (util.inspect.custom) {
    
    
      this[util.inspect.custom] = this.inspect;
    }
  }

listen

作用：启动服务器监听端口

  /**
   * Shorthand for:
   *
   *    http.createServer(app.callback()).listen(...)
   *
   * @param {Mixed} ...
   * @return {Server}
   * @api public
   */

  listen(...args) {
    
    
    debug('listen');
    //node的创建一个服务器 
    const server = http.createServer(this.callback());
    return server.listen(...args);
  }

理解这一段代码重点在于this.callback

callback

  /**
   * Return a request handler callback
   * for node's native http server.
   *
   * @return {Function}
   * @api public
   */

  callback() {
    
    
    //这一步处理中间件 最后返回一个promise
    const fn = compose(this.middleware);

    if (!this.listenerCount('error')) this.on('error', this.onerror);
    
    // 因为callback返回handleRequest 所以这里的req，res实际上http.createServer的回调函数的参数
    const handleRequest = (req, res) => {
    
     
      //创建上下文 就是把req res request response app 组合在一起 具体的在context.js源码阅读中
      const ctx = this.createContext(req, res);
      //ctx 是上下文 fn是上面执行之后返回的Promise
      return this.handleRequest(ctx, fn);
    };
    //返回的就是上面定义的函数
    return handleRequest;
  }

compose

这个函数返回的是一个promise
作用：中间件数组里的方法依次执行传递context上下文，但是执行下一次的契机是主动触发next 其实就是dispatch.bind(null, i + 1)这个函数

/**
 * Compose `middleware` returning
 * a fully valid middleware comprised
 * of all those which are passed.
 *
 * @param {Array} middleware
 * @return {Function}
 * @api public
 */

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))); //这里是关键点
      } catch (err) {
    
    
        return Promise.reject(err)
      }
    }
  }
}

this.handleRequest

作用：等待中间件执行结束，然后处理响应,处理错误

  handleRequest(ctx, fnMiddleware) {
    
    
    const res = ctx.res;
    res.statusCode = 404; //:point_right:先预设一个404
    const onerror = err => ctx.onerror(err); 
    const handleResponse = () => respond(ctx);//:point_down: 响应处理帮助函数
    onFinished(res, onerror); //http请求结束或者完成或者出错时 执行回调函数 -->这里应该是错误处理？
    return fnMiddleware(ctx).then(handleResponse).catch(onerror);
  }

use

作用:中间件的使用函数，把fn push到数组里顺便检查了一下是不是generator function，v3将不再支持

  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;
  }