koa源码中promise的解读

lib
|-- application.js
|-- context.js
|-- request.js
|-- response.js

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;
}
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);
}

// compose源码
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!')
 }
 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)
  }
 }
 }
}

/*
** 中间件应用示例代码
*/
let Koa = require('koa')
let app = new Koa()
app.use(async function ware0 (ctx, next) {
 await setTimeout(function () {
 console.log('ware0 request')
 }, 0)
 next()
 console.log('ware0 response')
})
app.use(function ware1 (ctx, next) {
 console.log('ware1 request')
 next()
 console.log('ware1 response')
})
// 执行结果
ware0 request
ware1 request

ware1 response
ware0 response

function convert (mw) {
 // mw为generator中间件
 if (typeof mw !== 'function') {
 throw new TypeError('middleware must be a function')
 }
 if (mw.constructor.name !== 'GeneratorFunction') {
 // assume it's Promise-based middleware
 return mw
 }
 const converted = function (ctx, next) {
 return co.call(ctx, mw.call(ctx, createGenerator(next)))
 }
 converted._name = mw._name || mw.name
 return converted
}

function * createGenerator (next) {
 return yield next()
}

function co(gen) {
 var ctx = this;
 var args = slice.call(arguments, 1);

 // we wrap everything in a promise to avoid promise chaining,
 // which leads to memory leak errors.
 // see https://github.com/tj/co/issues/180
 return new Promise(function(resolve, reject) {
 if (typeof gen === "function") gen = gen.apply(ctx, args);
 if (!gen || typeof gen.next !== "function") return resolve(gen);

 onFulfilled();

 /**
  * @param {Mixed} res
  * @return {Promise}
  * @api private
  */

 function onFulfilled(res) {
  var ret;
  try {
  ret = gen.next(res);
  } catch (e) {
  return reject(e);
  }
  next(ret);
 }

 /**
  * @param {Error} err
  * @return {Promise}
  * @api private
  */

 function onRejected(err) {
  var ret;
  try {
  ret = gen.throw(err);
  } catch (e) {
  return reject(e);
  }
  next(ret);
 }

 /**
  * Get the next value in the generator,
  * return a promise.
  *
  * @param {Object} ret
  * @return {Promise}
  * @api private
  */

 function next(ret) {
  if (ret.done) return resolve(ret.value);
  // toPromise是一个函数，返回一个promise示例
  var value = toPromise.call(ctx, ret.value);
  if (value && isPromise(value)) return value.then(onFulfilled, onRejected);
  return onRejected(
  new TypeError(
   "You may only yield a function, promise, generator, array, or object, " +
   'but the following object was passed: "' +
   String(ret.value) +
   '"'
  )
  );
 }
 });
}