产生的价值不能被视为承诺

Question

我正在使用Koa和Bluebird并正在尝试协同程序.我发现下面给出了一个错误.但如果我替换yield initRouters()它的内容initRouters()工作.这是为什么？

'use strict';

const fs = require('fs');
const Promise = require('bluebird');
const readdir = Promise.promisify(fs.readdir);

let app = require('koa')();

let initRouters = function *() {
  const routerDir = `${__dirname}/routers`;
  let routerFiles = yield readdir(routerDir);
  return Promise.map(routerFiles, function *(file) {
    var router = require(`${routerDir}/${file}`);

    app
      .use(router.routes())
      .use(router.allowedMethods());
  });
}

Promise.coroutine(function *() {
  yield initRouters();

  const PORT = process.env.PORT || 8000;
  app.listen(PORT);
  console.log(`Server listening on ${PORT}`);
})();

我得到的错误是

Unhandled rejection TypeError: A value [object Generator] was yielded that could not be treated as a promise

    See http://goo.gl/MqrFmX

From coroutine:
    at Function.Promise.coroutine (/home/jiewmeng/Dropbox/expenses-app/node_modules/bluebird/js/release/generators.js:176:17)
    at Object.<anonymous> (/home/jiewmeng/Dropbox/expenses-app/src/app.js:21:9)
    at PromiseSpawn._continue (/home/jiewmeng/Dropbox/expenses-app/node_modules/bluebird/js/release/generators.js:145:21)
    at PromiseSpawn._promiseFulfilled (/home/jiewmeng/Dropbox/expenses-app/node_modules/bluebird/js/release/generators.js:92:10)
    at /home/jiewmeng/Dropbox/expenses-app/node_modules/bluebird/js/release/generators.js:183:15
    at Object.<anonymous> (/home/jiewmeng/Dropbox/expenses-app/src/app.js:27:3)
    at Module._compile (module.js:413:34)
    at Object.Module._extensions..js (module.js:422:10)
    at Module.load (module.js:357:32)
    at Function.Module._load (module.js:314:12)
    at Function.Module.runMain (module.js:447:10)
    at startup (node.js:141:18)
    at node.js:933:3

Answer 1

你有鸡蛋/鸡肉问题.

Promise.coroutine 让我们承诺:

返回一个可以使用yield来产生promise的函数.

这里的问题是你试图产生一个生成器函数,而不是一个承诺.这会导致错误:

Unhandled rejection TypeError: A value [object Generator] was yielded that could not be treated as a promise

为什么？

好吧,生成器函数返回一个promise,但是当你产生它时它只是一个promise.而且因为你不能产生生成器函数,所以你永远不会得到价值(承诺).

你可以(而且应该)使用coroutine和yield initRouters();.但是,这initRouters()应该是一个返回promise而不是生成器函数的普通函数.

作为旁注,你不需要,Promise.map()因为你在里面做的一切.map都是同步的,所以你可以完美地做到:

let initRouters = function() {
  const routerDir = `${__dirname}/routers`;

  return readdir(routerDir).then(function (files) {
    files.map(function (file) {
      var router = require(`${routerDir}/${file}`);

      app
        .use(router.routes())
        .use(router.allowedMethods());
    })
  })
}

在这里,您将返回一个承诺,当您屈服时它会解决.但正如你所看到的,你已经回复了一个承诺,这就是为什么Promise.coroutine不给你任何错误.对于生成器,您不会返回承诺.