Question

以下几乎是node.js应用程序的所有代码，如果您运行命令后跟艺术家名称，则可以获取艺术家的播放列表

simplay the Beatles

从终端的输出中，我知道._flush方法中的代码（添加到UrlsForNamesTransform的原型中）正在运行，但它从未被明确调用过。 UrlsForNamesTransform扩展了node.js中的Transform流，我提到它是因为我在其他代码中看到它在函数运行之前没有显式调用（至少我可以看到）。它是关于转换还是正在发生什么使得._flush中的代码运行？这是代码https://github.com/thlorenz/simplay

的github仓库

'use strict';

var urlschema = 'http://ws.audioscrobbler.com/2.0/?method=artist.getsimilar&artist={{artist}}&api_key={{apikey}}&format=json';

var hyperquest =  require('hyperquest')
  , table      =  require('text-table')
  , colors     =  require('ansicolors')
  , styles     =  require('ansistyles')

var stream = require('stream');
var util = require('util');

var Transform = stream.Transform;

util.inherits(UrlsForNamesTransform, Transform);

function UrlsForNamesTransform (opts) {
  if (!(this instanceof UrlsForNamesTransform)) return new UrlsForNamesTransform(opts);

  opts = opts || {};

  Transform.call(this, opts);
  this._writableState.decodeStrings = false;
  this.artist = opts.artist;
  this.json = '';
}

UrlsForNamesTransform.prototype._transform = function (chunk, encoding, cb) {
  this.json += chunk.toString();
  cb();
};

UrlsForNamesTransform.prototype._flush = function (cb) {
  var records = [];
  try {
    var o = JSON.parse(this.json);
    var artists = o.similarartists.artist;
    if (!Array.isArray(artists)) {
      this.push('Sorry, no records for "' + this.artist + '" where found, please correct your spelling and/or try another artist.');
      return cb();
    }

    artists.forEach(function (node) {
      var youtubeurl = 'http://www.youtube.com/results?search_query={{artist}},playlist'.replace('{{artist}}', node.name);
      var rdiourl = 'http://www.rdio.com/search/{{artist}}/artists/'.replace('{{artist}}', node.name);
      var lastfmurl = 'http://www.last.fm/music/{{artist}}'.replace('{{artist}}', node.name);
      var lastfmRadioUrl = 'http://www.last.fm/listen/artist/{{artist}}'.replace('{{artist}}', node.name);
      var urls = [
          ''
        , colors.white('  youtube:       ') + styles.underline(colors.brightBlue(encodeURI(youtubeurl)))
        , colors.blue ('  rdio:          ') + styles.underline(colors.brightBlue(encodeURI(rdiourl)))
        , colors.brightRed  ('  last.fm:       ') + styles.underline(colors.brightBlue(encodeURI(lastfmurl)))
        , colors.red  ('  last.fm radio: ') + styles.underline(colors.brightBlue(encodeURI(lastfmRadioUrl)))
        , ''
        , ''].join('\n');

      records.push([ '\n' + colors.brightYellow(node.name), colors.cyan(node.match), urls ]);
    })
    this.push(table(records.reverse()));
    cb();
  } catch (err) {
    cb(err);
  }
}


var go = module.exports = 

/**
 * Retrieves similar artists for the given artist from last.fm using the apikey.
 * Then it converts the information to display youtube.com, last.fm, rdio playlist/artist urls for each artist.
 * 
 * @name simplay
 * @function
 * @param {String} artist the artist to find similar artists for
 * @param {String} apikey the api key to be used with last.fm
 * @return {ReadableStream} that will push the url information
 */
function simplay(artist, apikey) {
  if (!artist) throw new Error('Please provid the artist that you like to get similar artist links for');
  if (!apikey) throw new Error('Please set LASTFM_API env variable to your API key: http://www.last.fm/api/account/create');

  var url = urlschema
    .replace('{{artist}}', artist)
    .replace('{{apikey}}', apikey);

  return hyperquest(url)
    .on('error', console.error)
    .pipe(new UrlsForNamesTransform({ artist: artist }))
    .on('error', console.error)
};

Answer 1

重要的是这一行：

util.inherits(UrlsForNamesTransform, Transform);

这意味着UrlsForNamesTransform是Transform的子类。有关子类Transform的非常好的文档，可以找到on the node.js api site。

基本上，Transform的子类必须实现_transform并且可以实现_flush，但是预计永远不会调用这些函数中的任何一个。 Transform中的方法将根据传入流上的事件调用它们。

没有显式调用的函数运行

1 个答案: