createContext(req, res)幹了什麼？

前言

最近在一次面試中被問到 koa 裏面的 delegates、request、respone、req、res之間的關係？我當時只回答了 koa-compose遞歸原理,雖然最後經過了面試，可是我以爲仍是有必要追其緣由，由於我沒回答出來。

Koa 源碼 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;
  }

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前提

首先咱們必須先了解一下代碼

this.context = Object.create(context);
this.request = Object.create(request);
this.response = Object.create(response);
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分析 context

1. context

   const context = require('./context');
   this.context = Object.create(context);
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   1.1. context 幹了什麼？

   const delegate = require('delegates');
   const proto = module.exports = {
       ...
   }
   
   delegate(proto, 'response')
     .method('attachment')
     .method('redirect')..
   
   delegate(proto, 'request')
     .method('acceptsLanguages')
     .method('acceptsEncodings')...
   
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   1.2. delegates幹了什麼？

   module.exports = Delegator;
   
   function Delegator(proto, target) {} 
   
   Delegator.auto = function(proto, targetProto, targetProp){}
   
   Delegator.prototype.method = function(name){
       proto[name] = function(){
           return this[target][name].apply(this[target], arguments);
       };
       
       return this;
   }
   
   Delegator.prototype.access = function(name){
       return this.getter(name).setter(name);
   };
   
   Delegator.prototype.getter = function(name){
       proto.__defineGetter__(name, function(){
           return this[target][name];
       });
       return this;
   }
   
   Delegator.prototype.setter = function(name){
    
     proto.__defineSetter__(name, function(val){
       return this[target][name] = val;
     });
   
     return this;
   };
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   經常使用的 method 方法內部實現：

   proto[name] = function(){
       return this[target][name].apply(this[target], arguments);
    };
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   其實就是給context.js 返回的對象proto 對於request 、response 屬性增長增、讀取、設置、改變一些方法，每一個方法：

分析 request.js 和 response.js

```
module.exports  ={
    ...
}
request.js 封裝了req ,response.js封裝了res

```
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分析-1：

const context = Object.create(this.context);
const request = context.request = Object.create(this.request);
const response = context.response = Object.create(this.response);
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先建立context對象、而後爲context上下文增長request、request屬性。如今context除了context.js文件導出的proto對象的方法和屬性外、以及利用delegates在proto.request、proto.response上面定義的方法外，如今又在context.request、context.respons上面增長了分別對應request.js 和 response.js 默認導出的方法,request.js 又封裝了req ,response.js封裝了res。

分析-2：

context.app = request.app = response.app = this;
context.req = request.req = response.req = req;
context.res = request.res = response.res = res;
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1. 整個context返回的內容：

2. 咱們發現context對象裏面有app這個屬性,而且request、response都有app、req、res

分析-3

request.ctx = response.ctx = context;
request.response = response;
response.request = request;
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從上面的圖咱們能夠看出 context.request.ctx response 、context.response.ctx request 都有對應的屬性

分析-4

context.originalUrl = request.originalUrl = req.url;
context.state = {};

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在context上下文中咱們能夠查看originalUrl、state

總結:

const Koa = require("koa");

const app = new Koa();

app.use(async ctx => {
  ctx.body = 'Hello World';
});

app.listen(3000);

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在koa 源碼createContext函數裏面console.log(context) 就能夠看出整個函數返回context是什麼了。

context關係