ko.transform.js

/**
 * Transform for Knockout 3+
 * (c) Denis Gulin
 * License: MIT (http://www.opensource.org/licenses/mit-license.php)
 */
!function (factory) {
    // Support three module loading scenarios
    if (typeof require === 'function' && typeof exports === 'object' && typeof module === 'object') {
        // [1] CommonJS or Node: hard-coded dependency on "knockout"
        factory(require("knockout"));
    } else if (typeof define === 'function' && define['amd']) {
        // [2] AMD anonymous module with hard-coded dependency on "knockout"
        define(["knockout"], factory);
    } else {
        // [3] No module loader (plain <script> tag) - use the global `ko` object
        factory((this || (0, eval)("this"))["ko"]);
    }
}(function (ko) {
    /**
     * @param {Array} observables
     * @param {Function} evaluator
     * @param {ko.observable|ko.observableArray|?} observable
     * @returns {ko.subscribable}
     */
    ko.transform = function(observables, evaluator, observable){
        if (!observable){
            observable = ko.observable();
        }

        // Arguments for evaluator.
        var values = [],

            // last value (do not use peek)
            lastValue,

            // Routine for observable update.
            update = function(){
                if (lastValue !== (lastValue = evaluator.apply(null, values))){
                    observable(lastValue);
                }
            },

            // List of subscriptions.
            subscriptions = ko.utils.arrayMap(observables, function(one, i){
                values[i] = one.peek();
                return one.subscribe(function(value){
                    values[i] = value;
                    update();
                });
            });

        // Perform initial notification.
        update();

        // Override dispose.
        observable.dispose = (function(dispose){
            return function(){
                dispose && dispose();
                ko.utils.arrayForEach(subscriptions, function(one){
                    one.dispose();
                });
            };
        })(observable.dispose);

        // Return created observable.
        return observable;
    };

    /**
     * Shortcut for single observable.
     * @param {Function} evaluator
     * @returns {*|ko.subscribable}
     */
    ko.subscribable.fn.transform = function(evaluator){
        return ko.transform([this], evaluator);
    };

    return ko;
});