Node.js 中间件模式详解：从基础实现到 Koa2 洋葱模型

class ZmqMiddlewareManager {
 constructor(socket) {
  this.socket = socket;
  // 两个列表分别保存两类中间件函数：接受到的信息和发送的信息。
  this.inboundMiddleware = [];
  this.outboundMiddleware = [];
  socket.on('message', message => {
   this.executeMiddleware(this.inboundMiddleware, {
    data: message
   });
  });
 }
 
 send(data) {
  const message = { data };
  
  this.excuteMiddleware(this.outboundMiddleware, message, () => {
   this.socket.send(message.data);
  });
 }
 
 use(middleware) {
  if(middleware.inbound) {
   this.inboundMiddleware.push(middleware.inbound);
  }
  if(middleware.outbound) {
   this.outboundMiddleware.push(middleware.outbound);
  }
 }
 
 exucuteMiddleware(middleware, arg, finish) {
  function iterator(index) {
   if(index === middleware.length) {
    return finish && finish();
   }
   middleware[index].call(this, arg, err => {
    if(err) {
     return console.log('There was an error: ' + err.message);
    }
    iterator.call(this, ++index);
   });
  }
  iterator.call(this, 0);
 }
}

const zmqm = new ZmqMiddlewareManager();

zmqm.use({
 inbound: function(message, next) {
  console.log('input message: ', message.data);
  next();
 },
 outbound: function(message, next) {
  console.log('output message: ', message.data);
  next();
 }
});

function(req, res, next) { ... }

constructor() {
 super();

 this.proxy = false;
 this.middleware = [];
 this.subdomainOffset = 2;
 this.env = process.env.NODE_ENV || 'development';
 this.context = Object.create(context);
 this.request = Object.create(request);
 this.response = Object.create(response);
}

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;
}

// 封装之前 http.createServer(app.callback()).listen(...)
listen(...args) {
 debug('listen');
 const server = http.createServer(this.callback());
 return server.listen(...args);
}

callback() {
 const fn = compose(this.middleware);
 
 if (!this.listenerCount('error')) this.on('error', this.onerror);
 
 const handleRequest = (req, res) => {
  const ctx = this.createContext(req, res);
  return this.handleRequest(ctx, fn);
 };
 
 return handleRequest;
}

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)));
   } catch (err) {
    return Promise.reject(err)
   }
  }
 }
}

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);
}

app.use((ctx, next) => {
 const start = new Date();
 return next().then(() => {
  const ms = new Date() - start;
  console.log(`${ctx.method} ${ctx.url} - ${ms}ms`);
 });
});

app.use((ctx, next) => {
 const start = new Date();
 await next();
 const ms = new Date() - start;
 console.log(`${ctx.method} ${ctx.url} - ${ms}ms`);
});

const convert = require('koa-convert');

app.use(convert(function *(next) {
 const start = new Date();
 yield next;
 const ms = new Date() - start;
 console.log(`${this.method} ${this.url} - ${ms}ms`);
}));