玩转Koa之核心原理分析

const http = require('http')

const server = http.createServer((req, res) => {
 // 每一次请求处理的方法
 console.log(req.url)
 res.writeHead(200, { 'Content-Type': 'text/plain' })
 res.end('Hello NodeJS')
})

server.listen(8080)

// application.js
listen(...args) {
 const server = http.createServer(this.callback());
 return server.listen(...args);
}

callback() {
 // 组织中间件
 const fn = compose(this.middleware);

 // 未监听异常处理，则采用默认的异常处理方法
 if (!this.listenerCount('error')) this.on('error', this.onerror);

 const handleRequest = (req, res) => {
  // 生成context上下文对象
  const ctx = this.createContext(req, res);
  return this.handleRequest(ctx, fn);
 };

 return handleRequest;
}

handleRequest(ctx, fnMiddleware) {
 const res = ctx.res;
 // 默认状态码为404
 res.statusCode = 404;
 // 中间件执行完毕之后 采用默认的 错误 与 成功 的处理方式
 const onerror = err => ctx.onerror(err);
 const handleResponse = () => respond(ctx);
 onFinished(res, onerror);
 return fnMiddleware(ctx).then(handleResponse).catch(onerror);
}

Object.defineProperties(http.IncomingMessage.prototype, {
 query: {
  get () {
   return querystring.parse(url.parse(this.url).query)
  }
 }
})

Object.defineProperties(http.ServerResponse.prototype, {
 json: {
  value: function (obj) {
   if (typeof obj === 'object') {
    obj = JSON.stringify(obj)
   }
   this.end(obj)
  }
 }
})

// application.js
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; // 保存原生req对象
  context.res = request.res = response.res = res; // 保存原生res对象
  request.ctx = response.ctx = context;
  request.response = response;
  response.request = request;
  context.originalUrl = request.originalUrl = req.url;
  context.state = {};
  // 最终返回完整的context上下文对象
  return context;
}

// request.js
get header() {
 return this.req.headers;
},
set header(val) {
 this.req.headers = val;
},

ctx.request.header

function access (proto, target, name) {
 Object.defineProperty(proto, name, {
  get () {
   return target[name]
  },
  set (value) {
   target[name] = value
  }
 })
}

access(context, request, 'header')

function method (proto, target, name) {
 proto[name] = function () {
  return target[name].apply(target, arguments)
 }
}

 function use(route, fn) {
 var handle = fn;
 var path = route;

 // 不传入route则默认为'/'，这种基本是框架处理参数的一种套路
 if (typeof route !== 'string') {
  handle = route;
  path = '/';
 }

 ...
 // 存储中间件
 this.stack.push({ route: path, handle: handle });
 
 // 以便链式调用
 return this;
}

function handle(req, res, out) {
 var index = 0;
 var stack = this.stack;
 ...
 function next(err) {
  ...
  // 依次取出中间件
  var layer = stack[index++]

  // 终止条件
  if (!layer) {
   defer(done, err);
   return;
  }

  var path = parseUrl(req).pathname || '/';
  var route = layer.route;

  // 路由匹配规则
  if (path.toLowerCase().substr(0, route.length) !== route.toLowerCase()) {
   return next(err);
  }
  ...
  call(layer.handle, route, err, req, res, next);
 }

 next();
}

function call(handle, route, err, req, res, next) {
 var arity = handle.length;
 var error = err;
 var hasError = Boolean(err);

 try {
  if (hasError && arity === 4) {
   // 错误处理中间件
   handle(err, req, res, next);
   return;
  } else if (!hasError && arity < 4) {
   // 请求处理中间件
   handle(req, res, next);
   return;
  }
 } catch (e) {
  // 记录错误
  error = e;
 }

 // 将错误传递下去
 next(error);
}

use(fn) {
 this.middleware.push(fn);
 return this;
}

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) {
  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 {
    // 递归调用下一个中间件
    return Promise.resolve(fn(context, dispatch.bind(null, i + 1))); 
   } catch (err) {
    return Promise.reject(err)
   }
  }
 }
}

 this.app.emit('error', err, this);

// 也可以这样自定义错误处理
app.use(async (ctx, next) => {
 try {
  await next();
 } catch (err) {
  ctx.status = err.status || 500
  ctx.body = err
 }
})