Node.js 实现串行化流程控制

http://feed.cnblogs.com/blog/u/376823/rss
http://lambda-the-ultimate.org/rss.xml

npm install request
npm install htmlparser

var fs          = require(‘fs‘);
var request     = require(‘request‘);
var htmlparser  = require(‘htmlparser‘);
var configFilename = ‘./rss_feeds.txt‘;

function checkForRSSFile () {
    fs.exists(configFilename, function (exists) {
        if (!exists)
            return next(new Error(‘Missing RSS file: ‘ + configFilename));
        next(null, configFilename);
    });
}

function readRSSFile (configFilename) {
    fs.readFile(configFilename, function(err, feedList) {
        if (err) return next(err);

        feedList = feedList
                    .toString()
                    .replace(/~\s+|\s+$/g, ‘‘)
                    .split("\n");
        
        var random = Math.floor(Math.random() * feedList.length);
        next(null, feedList[random]);
    });
}

function downloadRSSFeed (feedUrl) {
    request({uri: feedUrl}, function(err, res, body) {
        if (err) return next(err);
        if (res.statusCode != 200)
            return next(new Error(‘Abnormal response status code‘));
        next(null, body);
    });
}

function parseRSSFeed (rss) {
    var handler = new htmlparser.RssHandler();
    var parser = new htmlparser.Parser(handler);
    parser.parseComplete(rss);
    if (!handler.dom.items.length)
        return next(new Error(‘No RSS items found‘));
    
    var item = handler.dom.items.shift();
    console.log(item.title);
    console.log(item.link);
}

var tasks = [ checkForRSSFile,
                readRSSFile,
                downloadRSSFeed,
                parseRSSFeed ];

function next(err, result) {
    if (err) throw err;

    var currentTask = tasks.shift();
    if (currentTask) {
        currentTask(result);
    }
}

next();