示例
先看看官网示例：
const Koa = require('koa');
const app = new Koa();
app.use(async (ctx, next) => {
await next();
const rt = ctx.response.get('X-Response-Time');
console.log(`${ctx.method} ${ctx.url} - ${rt}`);
});
app.use(async (ctx, next) => {
const start = Date.now();
await next();
const ms = Date.now() - start;
ctx.set('X-Response-Time', `${ms}ms`);
});
app.use(async ctx => {
ctx.body = 'Hello World';
});
app.listen(3000);
初始化
\\lib\applicationjs
module.exports = class Application extends Emitter {
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;
}
}
}
主要是做些初始化，引入一个koa应用里会包含的三大对象
ContextRequestResponse
来看看Request和Response是什么：
\\lib\request.js
module.exports = {
get header() {
return this.req.headers;
},
...
}
\\lib\response.js
module.exports = {
get header() {
const { res } = this;
return typeof res.getHeaders === 'function'
? res.getHeaders()
: res._headers || {}; // Node < 7.7
},
...
}
看起来就是对原始请求和原始响应做了封装而已。再看看context:
const proto = module.exports = {
...
get cookies() {
if (!this[COOKIES]) {
this[COOKIES] = new Cookies(this.req, this.res, {
keys: this.app.keys,
secure: this.request.secure
});
}
return this[COOKIES];
},
set cookies(_cookies) {
this[COOKIES] = _cookies;
}
}
delegate(proto, 'response')
.method('attachment')
.method('redirect')
.method('remove')
.method('vary')
...
delegate(proto, 'request')
.method('acceptsLanguages')
.method('acceptsEncodings')
.method('acceptsCharsets')
.method('accepts')
.method('get')
.method('is')
...
看来cookie的处理被放在了context里面，并且代理了request和response的方法，相当于在context上也挂载了response和request的方法、字段属性等。
由此来看主要的东西还是在res和req里面。
中间件
初始化完成后就是use添加中间件了：
//application.js
use(fn) {
...
this.middleware.push(fn);
return this;
}
use只是把传入的回调函数先存到middleware数组里而已
启动
接下来就是listen方法启动了：
listen(...args) {
const server = http.createServer(this.callback());
return server.listen(...args);
}
callback() {
const fn = compose(this.middleware);
const handleRequest = (req, res) => {
const ctx = this.createContext(req, res);
return this.handleRequest(ctx, fn);
};
return handleRequest;
}
//compose来自于koa-compose
function compose (middleware) {
//...
return function (context, next) {
// last called middleware #
let index = -1
return dispatch(0)
function dispatch (i) {
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 () {
return dispatch(i + 1)
}))
} catch (err) {
return Promise.reject(err)
}
}
}
}
compose返回了一个函数，接受两个参数，context 和 next。context 是 KOA 中的 context，next 是所有中间件执行完后，框架使用者来最后处理请求和返回的回调函数。同时函数是一个闭包函数，存储了所有的中间件，通过递归的方式不断的运行中间件。
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;
}
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);
}
createContext用了Object.create又继承了一层this.context，然后对于Context对象挂载了一些属性。
所以handleRequest中middleware接受的context继承了构造函数创建的context。
整个流程就是handleRequest=>fnMiddleware=>handleResponse，也就是众所周知的洋葱模型了：
结论
KOA在nodejs的基础上进行了context和request&response的封装，采用洋葱模型将扩展性放到了中间件中。
