洋葱模型洋葱模型

1. app.use()

app.use(async (ctx, next) => {}); // 将中间件放入中间件数组

源码：

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); // 将中间件放入中间件数组 return this; }

2. app.listen()

module.exports = app.listen(config.port, '0.0.0.0', () => { console.log('进入app'); });

源码：

listen(...args) { debug('listen'); // this.callback()为connection listener, 当客户端连接上服务器时会触发该函数，即handleRequest const server = http.createServer(this.callback()); return server.listen(...args); }/** * Return a request handler callback * for node's native http server. * * @return {Function} * @api public */callback() { // 将中间件数组传入koa-compose, 得到一个返回函数，将该函数传入this.handleRequest const fn = compose(this.middleware); if (!this.listeners('error').length) this.on('error', this.onerror); const handleRequest = (req, res) => { const ctx = this.createContext(req, res); return this.handleRequest(ctx, fn); }; return handleRequest; }/** * Handle request in callback. * * @api private */handleRequest(ctx, fnMiddleware) { const res = ctx.res; res.statusCode = 404; const onerror = err => ctx.onerror(err); const handleResponse = () => respond(ctx); onFinished(res, onerror); // fnMiddleware(ctx) 返回promise /* return Promise.resolve(fn(context, function next () { return dispatch(i + 1) })) 首先执行第一个中间件函数，传入context和next函数，next返回也是返回promise */ return fnMiddleware(ctx).then(handleResponse).catch(onerror); }

3. koa-compose

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) { // next 为undefined // 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, function next () { return dispatch(i + 1) })) } catch (err) { return Promise.reject(err) } } } }

总结
【洋葱模型】整个流程为：
app.use函数将中间件放入中间件数组。当app接收到http请求时，会执行connection listener函数，这个函数中会去执行koa-compose返回的函数(return dispatch(0))，此时会执行第一个中间件函数，传入content和next函数，每个next函数也会返回一个promise。在中间件函数中await next()，即执行下一个中间件函数(return dispatch(i + 1))，直到所有的中间件都执行结束后，将undefined赋值给fn(if (i === middleware.length) fn = next)。