koa源码阅读之context.js/koa-convert/is-generator-function

hey 老伙计

今天我们来讲一下koa的context.js还有一些tj自己写的库吧。

---

context.js

'use strict';
const createError = require('http-errors');
const httpAssert = require('http-assert');
const delegate = require('delegates');
const statuses = require('statuses');

const proto = module.exports = {

    //前面文章有讲过这个了，这里不描述了
  inspect() {
    if (this === proto) return this;
    return this.toJSON();
  },

  toJSON() {
    return {
      request: this.request.toJSON(),//实际上调用request.js的toJSON(下同)
      response: this.response.toJSON(),
      app: this.app.toJSON(),
      originalUrl: this.originalUrl,
      req: '<original node req>',
      res: '<original node res>',
      socket: '<original node socket>'
    };
  },

  /**
   * Similar to .throw(), adds assertion.
   *
   *    this.assert(this.user, 401, 'Please login!');
   *
   * See: https://github.com/jshttp/http-assert
   *
   * @param {Mixed} test
   * @param {Number} status
   * @param {String} message
   * @api public
   */

  assert: httpAssert,

  /**
   * Throw an error with `msg` and optional `status`
   * defaulting to 500. Note that these are user-level
   * errors, and the message may be exposed to the client.
   *
   *    this.throw(403)
   *    this.throw('name required', 400)
   *    this.throw(400, 'name required')
   *    this.throw('something exploded')
   *    this.throw(new Error('invalid'), 400);
   *    this.throw(400, new Error('invalid'));
   *
   * See: https://github.com/jshttp/http-errors
   *
   * @param {String|Number|Error} err, msg or status
   * @param {String|Number|Error} [err, msg or status]
   * @param {Object} [props]
   * @api public
   */
    //throw方法。上面是使用的方法。我们 常用来在中间件throw发出一些错误状态码。
    //从而使得上级中间件可以try catch这个错误从而响应
    //createError([status], [message], [properties])
    //properties - custom properties to attach to the object
  throw(...args) {
    throw createError(...args);
  },
    //默认的一个错误处理
  onerror(err) {
    // don't do anything if there is no error.
    // this allows you to pass `this.onerror`
    // to node-style callbacks.
    if (null == err) return;
    // 如果error不是Error实例。此时生成一个错误实例给下文处理
    if (!(err instanceof Error)) err = new Error(`non-error thrown: ${err}`);

    let headerSent = false;
    //当然需要可写且没有被发送
    if (this.headerSent || !this.writable) {
      headerSent = err.headerSent = true;
    }

    //触发事件
    this.app.emit('error', err, this);

    //发送了肯定啥都不能干了
    if (headerSent) {
      return;
    }

    const { res } = this;
    //解构一下获得response
    
    //兼容咯
    //首次清除所有的headers
    if (typeof res.getHeaderNames === 'function') {
      res.getHeaderNames().forEach(name => res.removeHeader(name));
    } else {
      res._headers = {}; // Node < 7.7
    }

    // 然后设置为错误的headers标识
    this.set(err.headers);

    //强制text/plain
    this.type = 'text';

    // 支持ENOENT 
    if ('ENOENT' == err.code) err.status = 404;

    // 默认转换成500状态码
    if ('number' != typeof err.status || !statuses[err.status]) err.status = 500;

    //响应
    const code = statuses[err.status];
    const msg = err.expose ? err.message : code;
    this.status = err.status;
    this.length = Buffer.byteLength(msg);
    this.res.end(msg);
    //跟原生的一样嘛。
    //给我们一个提示。我们要使一个连接关闭。那么ctx.res.end(msg);
  }
};

/**
 * Response delegation.
 */
//委托到这个上下文对象里
//委托方法 与属性的getter或者setter
delegate(proto, 'response')
  .method('attachment')
  .method('redirect')
  .method('remove')
  .method('vary')
  .method('set')
  .method('append')
  .method('flushHeaders')
  .access('status')
  .access('message')
  .access('body')
  .access('length')
  .access('type')
  .access('lastModified')
  .access('etag')
  .getter('headerSent')
  .getter('writable');

/**
 * Request delegation.
 */

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')
  .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');

---

因为下一个篇幅准备将最重要的application.js
所以这个接下来准备说几个引入的库源码

koa convert

用于什么？用于将koa1的中间件转化为promise

看到co就应该有这么个想法了= =

'use strict'

const co = require('co')
//引入co
const compose = require('koa-compose')

module.exports = convert

function convert (mw) {
    //进行判断
  if (typeof mw !== 'function') {
    throw new TypeError('middleware must be a function')
  }
  if (mw.constructor.name !== 'GeneratorFunction') {
    // assume it's Promise-based middleware
    return mw
  }
  const converted = function (ctx, next) {
    return co.call(ctx, mw.call(ctx, createGenerator(next)))
  }
  converted._name = mw._name || mw.name
  return converted
}

function * createGenerator (next) {
  return yield next()
}

// convert.compose(mw, mw, mw)
// convert.compose([mw, mw, mw])
// koa-compose 日后再说嘻嘻^v^
convert.compose = function (arr) {
  if (!Array.isArray(arr)) {
    arr = Array.from(arguments)
  }
  return compose(arr.map(convert))
}
//我的天啊。这个疯子还支持回退
//回退方法很是精妙啊
convert.back = function (mw) {
  if (typeof mw !== 'function') {
    throw new TypeError('middleware must be a function')
  }
  if (mw.constructor.name === 'GeneratorFunction') {
    // assume it's generator middleware
    return mw
  }
  const converted = function * (next) {
    let ctx = this
    let called = false
    // no need try...catch here, it's ok even `mw()` throw exception
    yield Promise.resolve(mw(ctx, function () {
        //使得next仅仅被调用一次
      if (called) {
        // guard against multiple next() calls
        // https://github.com/koajs/compose/blob/4e3e96baf58b817d71bd44a8c0d78bb42623aa95/index.js#L36
        return Promise.reject(new Error('next() called multiple times'))
      }
      called = true
      return co.call(ctx, next)
    }))
  }
  converted._name = mw._name || mw.name
  return converted
}

---

is-generator-function

'use strict';
//减少查找引用，常见的优化方法
var toStr = Object.prototype.toString;
var fnToStr = Function.prototype.toString;
//这个正则匹配function *但是好像有点bug
//* function(){}也会受到判定true
var isFnRegex = /^\s*(?:function)?\*/;

var hasToStringTag = typeof Symbol === 'function' && typeof Symbol.toStringTag === 'symbol';
var getProto = Object.getPrototypeOf;
var getGeneratorFunc = function () { // eslint-disable-line consistent-return
    if (!hasToStringTag) {
        return false;
    }
    try {
        return Function('return function*() {}')();
    } catch (e) {
    }
};
var generatorFunc = getGeneratorFunc();
var GeneratorFunction = generatorFunc ? getProto(generatorFunc) : {};

//主要从三点看。
//一点是function toString
//一点是[object GeneratorFunction] Object toString
//一点是从原型看(内部[[Prototype]]属性的值)
module.exports = function isGeneratorFunction(fn) {
    if (typeof fn !== 'function') {
        return false;
    }
    if (isFnRegex.test(fnToStr.call(fn))) {
        return true;
    }
    if (!hasToStringTag) {
        var str = toStr.call(fn);
        return str === '[object GeneratorFunction]';
    }
    return getProto(fn) === GeneratorFunction;
};

---

这次写的好粗糙啊= =
希望在结尾部分能写好。
sry sry sry

看完全文都是好样的！！

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