/*
	 * Creates a callback to be called after multiple functions are done
	 * Usage:
	 * var wait = Q.wait(function (params, subjects) {
	 *   // arguments that were passed are in params.user, params.stream
	 *   // this objects that were passed are in subjects.user, subjects.stream
	 * }, ['user', 'stream]);
	 * mysql("SELECT * FROM user WHERE user_id = 2", wait.fill('user'));
	 * mysql("SELECT * FROM stream WHERE publisher_id = 2", wait.fill('stream'));
	 *
	 * @param callback Function
	 *  The callback to invoke when everything is ready. It is passed
	 *  an array or object, depending on whether the "required" field was
	 *  a number or array, respectively.
	 * @param required Array
	 *  Pass an array of required field names here.
	 * @param defaultReturn
	 *  Defaults to undefined. The value to return if callback is not yet ready.
	 * @return Object
	 *  An object with the following method: fill(field).
	 *  Call this method to return a callback.
	 */
	Q.wait = function(callback, required, defaultReturn) {
		if (Q.typeOf(required) !== 'array') {
			return false;
		}
		var len = required.length;
		var result = {
			callback: callback,
			params: {},
			subjects: {},
			fill: function(field) {
				var t = this;
				return function() {
					t.params[field] = Array.prototype.slice.call(arguments);
					t.subjects[field] = this;
					t.check();
				};
			},
			check: function () {
				var i, k, found;
				for (i=0; i<len; ++i) {
					found=false;
					for (k in this.params) {
						if (k === required[i]) {
							found=true;
							break;
						}
					}
					if (!found) {
						return defaultReturn;
					}
				}
				return this.callback.call(this, this.params, this.subjects);
			}
		};
		return result;
	};