Async.each最终回调当它在延迟的promise回调函数中时不会触发

Question

根据我迄今为止的研究，我相信我正确地使用了async.each。底部函数addToPlaylist调用顶级函数getPerformances。我试图弄清楚为什么没有调用addToPlaylist中的最终回调以及如何修复它。谢谢！

var getPerformances = function(playlist, performances, add){
  var deferred = Q.defer();
  var tracks = [];

  async.each(performances.performance, function(performance, cb) {
    getSpotifyArtistId(performance.artist.displayName).then(function (artistId) {
      if (add == true) {
        getTopTracks(artistId).then(function (trackstoAdd) {
          tracks.push.apply(tracks, trackstoAdd);
          cb();
        });
      } else {
        //Other stuff... only looking at the true option at this point
      }
    });
  }, function(err){
    if( err ) {
      console.log(err);
      deferred.resolve(tracks);
    } else {
      deferred.resolve(tracks);
    }
  });
  return deferred.promise;
}

问题在于 - 如果cb（）在then（）内，则不会调用最终回调。如果CB被移到promise回调之外，则调用finalCallback，但当然，它被调用得太快了。

var AddToPlaylist = function (playlist, results) {
  var allSongs = [];
  var handler = function (performances, cb) {

    getPerformances(playlist, performances, true)
      .then(
      function (trackstoAdd) { // all is well!
        allSongs.push.apply(allSongs, trackstoAdd);
        //console.log(allSongs) is as expected
        cb();
      }
      ,
      function (err) { // something bad happened!
        cb(err);
      }
    );
  }
  var finalCallback = function () {
    console.log('test');
  }

  async.each(results, handler, finalCallback);
}