如何让 for 语句等待多个猫鼬查询？

Question

我目前正在尝试做的是显示所有包含歌曲的播放列表。为此，我首先找到每个播放列表，然后执行 for 循环遍历所有播放列表（同时我初始化 globalArr 并输入值，然后它将作为 json 发送，因为它是一个 API），问题是当我这样做时循环中的另一个查找（PlaylistSong.find 或 Song.find）很好，因为它是异步的，当 for 结束时将进行查找，我将有 0 个结果，因为当他将采用 increment 的值时在他的最大。我听说过async，我什至用谷歌搜索过，但我真的不明白如何执行这段代码，因为它是 for 循环和异步查询的组合......

感谢您的帮助。

router.get('/', function(req, res, next) {
    Playlist.find(function (err, playlists) {
        if (err) return next(err);
        /* Loop through every playlists */
        var globalArr = [];
        for (var increment = 0; increment < playlists.length; ++increment)
        {
            globalArr[increment] = [];
            globalArr[increment]["name"] = playlists[increment].name;
            /* Loop through every links between Songs and Playlist */
            PlaylistSong.find({idPlaylist: playlists[increment]._id}, function (err, songs) {
                if (err) return next(err);
                for (var songIncrement = 0; songIncrement < songs.length; ++songIncrement) {
                {
                    console.log("increment"+increment);
                    globalArr[increment][songIncrement] = [];
                    /* Getting the actual song by his ID */
                    Song.find({_id: song.idSong}, function (err, song) {
                        if (err) return next(err);
                        globalArr[increment][songIncrement]["name"] = songs[songIncrement].name;
                        globalArr[increment][songIncrement]["artist"] = songs[songIncrement].artist;
                        globalArr[increment][songIncrement]["picture"] = songs[songIncrement].picture;
                        globalArr[increment][songIncrement]["price"] = songs[songIncrement].price;
                        globalArr[increment][songIncrement]["file"] = songs[songIncrement].file;
                        globalArr[increment][songIncrement]["difficulty"] = songs[songIncrement].difficulty;
                        globalArr[increment][songIncrement]["downloaded"] = songs[songIncrement].downloaded;
                    });
                }

            }});
        }
        res.contentType('application/json');
        res.send(JSON.stringify(globalArr));
    });
});

Answer 1

查看这个问题和接受的答案： Simplest way to wait some asynchronous tasks complete, in Javascript?

它基本上说要使用Async module，将所有异步函数调用推送到它上面，然后使用async.parallel()，它会在所有异步函数完成后给你一个回调。

我还没有测试过，但这样的东西似乎可以工作：

var async = require('async');

var calls = [];

router.get('/', function(req, res, next) {
    Playlist.find(function (err, playlists) {
        if (err) return next(err);
        /* Loop through every playlists */
        var globalArr = [];
        for (var increment = 0; increment < playlists.length; ++increment)
        {
            (function() {
                var i = increment;
                calls.push(function(callback) {
                    globalArr[i] = [];
                    globalArr[i]["name"] = playlists[i].name;
                    /* Loop through every links between Songs and Playlist */
                    PlaylistSong.find({idPlaylist: playlists[increment]._id}, function (err, songs) {
                        if (err) return next(err);
                        for (var songIncrement = 0; songIncrement < songs.length; ++songIncrement) {
                        {
                            console.log("increment"+i);
                            globalArr[i][songIncrement] = [];
                            /* Getting the actual song by his ID */
                            Song.find({_id: song.idSong}, function (err, song) {
                                if (err) return next(err);
                                globalArr[i][songIncrement]["name"] = songs[songIncrement].name;
                                globalArr[i][songIncrement]["artist"] = songs[songIncrement].artist;
                                globalArr[i][songIncrement]["picture"] = songs[songIncrement].picture;
                                globalArr[i][songIncrement]["price"] = songs[songIncrement].price;
                                globalArr[i][songIncrement]["file"] = songs[songIncrement].file;
                                globalArr[i][songIncrement]["difficulty"] = songs[songIncrement].difficulty;
                                globalArr[i][songIncrement]["downloaded"] = songs[songIncrement].downloaded;
                            });
                        }
                        callback();
                    }});
                });
            })();
        }
        async.parallel(calls, function(err, result) {
            if (err) {
                // TODO: Handle error here
            }
            res.contentType('application/json');
            res.send(JSON.stringify(globalArr));
        });
    });
});

或者如果您不希望 then 并行执行，您可以改用 async.series()。

有关您的情况的简化示例，请参阅此 jsFiddle...https://jsfiddle.net/bpursley/fj22hf6g/

Answer 2

是的，你应该使用异步。稍后我会更详细地解释这一点（儿子必须去睡觉......）

PlaylistSong.statics.findSongsByPlaylistId = function(id, done) {
  PlaylistSong.find({idPlaylist: id}, function(err, songs) {
    if (err) {
      done(err)
      return
    }
    var getSongsFns = songs.map(function(song) {
      return function(callback) {
        Song.find({_id: song.idSong}, callback)
      }
    })
    async.parallel(getSongsFns, done)
  })
}

router.get('/', function(req, res, next) {
    Playlist.find(function (err, playlists) {
        if (err) return next(err);
        var getSongsFns = playlists.map(function(playlist) {
          return function(callback) {
            PlaylistSong.findSongsByPlaylistId(playlist._id, callback)
          }
        })
        async.parallel(getSongsFns, function(err, songs) {
          if (err) {
            res.status(500).send()
            return
          }
          res.contentType('application/json');
          res.send(JSON.stringify(songs));
        }) 
    });
});