query-synchronizer.js

/**
 * Module allowing to use Azure Mobile Services database on server side
 * in a synchronous manner.
 *
 * Singleton based on http://simplapi.wordpress.com/2012/05/14/node-js-singleton-structure/
 *
 * @example
 *      var synchronizer = require('query-synchronizer');
 *      synchronizer.read(table, function(results) {
 *          // Do something with the results
 *      });
 *      synchronizer.read(second_table, function(results) {
 *          // Do something with the second results
 *          
 *          synchronizer.read(third_table, function(inner_results) {
 *              // Do something with nested results
 *          }
 *      });
 *      synchronizer.done(function(){
 *          // This will be executed only when previous reads are done
 *      });
 *
 * @author Wojtek Siudzinski <admin@suda.pl>
 *
 */
var singleton = function singleton(){
    //defining a var instead of this (works for variable & function) will create a private definition
    var requestsToEnd = 0;

    this.read = function(query, callback){
        requestsToEnd++;
        query.read({
            success: function(results) {
                callback(results);
                requestsToEnd--;
            }
        });

        return this;
    };

    this.done = function(callback){
        var interval = setInterval(function() {
            if (requestsToEnd <= 0) {
                clearInterval(interval);
                callback();
            }
        }, 1);
        return this;
    };

    if(singleton.caller != singleton.getInstance){
        throw new Error("This object cannot be instanciated");
    }
}

singleton.instance = null;

/**
 * Singleton getInstance definition
 * @return singleton class
 */
singleton.getInstance = function(){
    if(this.instance === null){
        this.instance = new singleton();
    }
    return this.instance;
}

module.exports = singleton.getInstance();