使用 promises Nodejs 链接异步函数

Question

var fs = require('fs');
var node_dir = require('node-dir');
var bluebird = require('bluebird');
var moviesClient = new ApiClient(...)
var musicClient = new ApiClient(...)
var lib = require('./index.js');

var generateMovieMetaData = async function(){
  var json = { movies: [] };

  node_dir.files(path, function(err, files) {

    bluebird.mapSeries(files, function(file){

    return moviesClient.send(new lib.requests.Movie(file))
       .then((movie) => {
      // movie is json
      /* do some loops and work with the movie json*/
      json.movies.push(movie);
       });
    })
    .then(function(movies){
      fs.writeFile('./movies.json', JSON.stringify(json), 'utf8', (err)=>{
         if(err) console.log(err)
         else { 
          console.log('File saved');
          }
      })
      return json; // go to the next function if any
    })
    .catch(function(err){
      console.log("Movie metadata could not be generated due to some error", err);
    });
  });
};

var generateMusicMetaData = async function(){
  var json = { music: [] };

  node_dir.subdirs(config.music.path, function(err, subdirs) {
    if (err) throw err;

    bluebird.mapSeries(subdirs, function(dir){

    return musicClient.send(new lib.requests.Album(dir))
      .then((album) => {
      // album is json
      /* do some loops and work with the album json*/
      json.music.push(album);
      });
    })
    .then(function(music){
      fs.writeFile('./music.json', JSON.stringify(json), 'utf8', (err)=>{
         if(err) console.log(err)
         else { 
          console.log('File saved');
          }
      })
      return json; // go to the next function if any
    })
    .catch(function(err){
      console.log("Album metadata could not be generated due to some error", err);
    });
  });
};

上面，我有两个异步函数 generateMovieMetaData 和 generateMusicMetaData 它们每个都有 Promise.mapSeries 逻辑

当我自己调用它们时，它们可以正常工作而不会抛出错误。

我想像这样在复合函数中链接这两个函数

var generateMetaData = function(){
  generateMusicMetaData()
   .then(() => generateMovieMetaData());
}
generateMetaData();

从第一个函数调用 generateMetaData returns TypeError: Cannot read property 'push' of undefined 错误：

Album metadata could not be generated due to some error TypeError: Cannot read property 'push' of undefined
[0]     at musicClient.send.then (/mnt/c/Users/ridhwaan/Source/homehost/server.js:179:23)
[0]     at tryCatcher (/mnt/c/Users/ridhwaan/Source/homehost/node_modules/bluebird/js/release/util.js:16:23)
[0]     at Promise._settlePromiseFromHandler (/mnt/c/Users/ridhwaan/Source/homehost/node_modules/bluebird/js/release/promise.js:51
2:31)
[0]     at Promise._settlePromise (/mnt/c/Users/ridhwaan/Source/homehost/node_modules/bluebird/js/release/promise.js:569:18)
[0]     at Promise._settlePromise0 (/mnt/c/Users/ridhwaan/Source/homehost/node_modules/bluebird/js/release/promise.js:614:10)
[0]     at Promise._settlePromises (/mnt/c/Users/ridhwaan/Source/homehost/node_modules/bluebird/js/release/promise.js:693:18)
[0]     at Async._drainQueue (/mnt/c/Users/ridhwaan/Source/homehost/node_modules/bluebird/js/release/async.js:133:16)
[0]     at Async._drainQueues (/mnt/c/Users/ridhwaan/Source/homehost/node_modules/bluebird/js/release/async.js:143:10)
[0]     at Immediate.Async.drainQueues [as _onImmediate] (/mnt/c/Users/ridhwaan/Source/homehost/node_modules/bluebird/js/release/a
sync.js:17:14)
[0]     at runCallback (timers.js:756:18)
[0]     at tryOnImmediate (timers.js:717:5)
[0]     at processImmediate [as _immediateCallback] (timers.js:697:5)

编辑 1： 好的，我更改了代码以使其看起来更简单。我收到同样的错误不知道为什么
日志记录显示两个函数中的 bluebird.mapSeries 同时发生，然后抛出错误

Answer 1

我没有解决每个函数中的承诺，我只有一个 return。我将 return json; 所在的行更改为 resolve(json);

var generateMusicMetaData = function() {
  return new Promise(function(resolve, reject) {
  ...
  r̶e̶t̶u̶r̶n̶ ̶j̶s̶o̶n̶;̶
  resolve(json);
  ...
  });
};

var generateMovieMetaData = function() {
  return new Promise(function(resolve, reject) {
  ...
  r̶e̶t̶u̶r̶n̶ ̶j̶s̶o̶n̶;̶
  resolve(json);
  ...
  });
};

那我有

var generateMetaData = function(){
  generateMusicMetaData()
    .then(function(result) { 
      return generateMovieMetaData();
    });
}

Answer 2

原始实现存在许多问题，因此它们不会 return 当每个函数中的所有异步操作完全完成时，承诺就会得到解决。因此，您无法将它们与其他异步操作协调。

所以首先让我们修复你的两个函数的实现。只使用 ES6 async/await 来实现要简单得多：

const Promise = require('bluebird');
const fs = Promise.promisifyAll(require('fs'));
const node_dir = Promise.promisifyAll(require('node-dir'));
let moviesClient = new ApiClient(...)
let musicClient = new ApiClient(...)
let lib = require('./index.js');


async function generateMovieMetaData(path) {
    var json = { movies: [] };

    let files = await node_dir.filesAsync(path);

    for (let f of files) {
        let movie = await moviesClient.send(new lib.requests.Movie(f));
        json.movies.push(movie);
    }

    await fs.writeFileAsync('./movies.json', JSON.stringify(json), 'utf8').catch(err => {
        console.log("Movie metadata could not be generated due to some error", err);
        throw err;
    });
    return json;
}

async generateMusicMetaData function(path) {
    var json = { music: [] };

    let files = await node_dir.subdirsAsync(path);

    for (let d of dirs) {
        let album = await musicClient.send(new lib.requests.Album(d));
        json.music.push(album);
    }

    await fs.writeFileAsync('./music.json', JSON.stringify(json), 'utf8').catch(err => {
        console.log("Album metadata could not be generated due to some error", err);
        throw err;
    });
    return json;
}

请注意，此实现中根本没有简单的回调。一切都是使用承诺和异步操作完成的，这些承诺和异步操作自然不会 return 承诺是 "promisified"。我在这里使用了 Bluebird，因为你已经展示了你正在使用它，但你也可以使用现在内置于 node.js 中的 util.promisify()。

现在，我们有了它，所以这些函数中的每一个 return 都是一个用 json 解决的承诺，并且只有在内部的所有异步操作完成时才解决。这为使用 async/await:

进行排序做好了准备

function async generateMetaData(path){
    let music = await generateMusicMetaData(path);
    let movie = await generateMovieMetaData(path);
    return {music, movie};
}

generateMetaData(path).then(results => {
    console.log(results);
}).catch(err => {
    console.log(err);
});

但是，由于这些似乎是两个完全不相关的操作，您可以并行执行它们：

function generateMetaData(path){
    return Promise.all([generateMusicMetaData(path), generateMovieMetaData(path)]).then(([music, movie]) => {
        return {music, movie};
    });
}

generateMetaData(path).then(results => {
    console.log(results);
}).catch(err => {
    console.log(err);
});

使用 promises Nodejs 链接异步函数

Chaining async functions with promises Nodejs

javascript

asynchronous

callback

promise

bluebird