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 * Open MCT Web, Copyright (c) 2014-2015, United States Government
 * as represented by the Administrator of the National Aeronautics and Space
 * Administration. All rights reserved.
 *
 * Open MCT Web is licensed under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * http://www.apache.org/licenses/LICENSE-2.0.
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations
 * under the License.
 *
 * Open MCT Web includes source code licensed under additional open source
 * licenses. See the Open Source Licenses file (LICENSES.md) included with
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 *****************************************************************************/
/*global define,Promise*/

/**
 * Module defining MutationCapability. Created by vwoeltje on 11/12/14.
 */
define(
    [],
    function () {
        "use strict";

        var TOPIC_PREFIX = "mutation:";

        // Utility function to overwrite a destination object
        // with the contents of a source object.
        function copyValues(destination, source) {
            // First, remove all previously-existing keys
            Object.keys(destination).forEach(function (k) {
                delete destination[k];
            });
            // Second, write all new keys
            Object.keys(source).forEach(function (k) {
                destination[k] = source[k];
            });
        }

        // Utility function to cast to a promise, without waiting
        // for nextTick if a value is non-promise-like.
        function fastPromise(value) {
            return (value || {}).then ? value : {
                then: function (callback) {
                    return fastPromise(callback(value));
                }
            };
        }

        /**
         * The `mutation` capability allows a domain object's model to be
         * modified. Wrapping such modifications in calls made through
         * this capability allows these changes to be tracked (e.g. to
         * ensure that a domain object's `modified` timestamp is kept
         * up-to-date.)
         *
         * Usage:
         *
         * ```
         * domainObject.useCapability("mutation", function (model) {
         *     // make changes to model here...
         * });
         * ```
         *
         * @param {DomainObject} domainObject the domain object
         *        which will expose this capability
         * @constructor
         */
        function MutationCapability(topic, now, domainObject) {
            var t = topic(TOPIC_PREFIX + domainObject.getId());

            function mutate(mutator, timestamp) {
                // Get the object's model and clone it, so the
                // mutator function has a temporary copy to work with.
                var model = domainObject.getModel(),
                    clone = JSON.parse(JSON.stringify(model)),
                    useTimestamp = arguments.length > 1;

                // Function to handle copying values to the actual
                function handleMutation(mutationResult) {
                    // If mutation result was undefined, just use
                    // the clone; this allows the mutator to omit return
                    // values and just change the model directly.
                    var result = mutationResult || clone;

                    // Allow mutators to change their mind by
                    // returning false.
                    if (mutationResult !== false) {
                        // Copy values if result was a different object
                        // (either our clone or some other new thing)
                        if (model !== result) {
                            copyValues(model, result);
                        }
                        model.modified = useTimestamp ? timestamp : now();
                        t.notify(model);
                    }

                    // Report the result of the mutation
                    return mutationResult !== false;
                }

                // Invoke the provided mutator, then make changes to
                // the underlying model (if applicable.)
                return fastPromise(mutator(clone)).then(handleMutation);
            }

            function listen(listener) {
                return t.listen(listener);
            }

            return {
                /**
                 * Alias of `mutate`, used to support useCapability.
                 */
                invoke: mutate,
                /**
                 * Modify the domain object's model, using a provided
                 * function. This function will receive a copy of the
                 * domain object's model as an argument; behavior
                 * varies depending on that function's return value:
                 *
                 * * If no value (or undefined) is returned by the mutator,
                 *   the state of the model object delivered as the mutator's
                 *   argument will become the domain object's new model.
                 *   This is useful for writing code that modifies the model
                 *   directly.
                 * * If a plain object is returned, that object will be used
                 *   as the domain object's new model.
                 * * If boolean `false` is returned, the mutation will be
                 *   cancelled.
                 * * If a promise is returned, its resolved value will be
                 *   handled as one of the above.
                 *
                 *
                 * @param  {function} mutator the function which will make
                 *         changes to the domain object's model.
                 * @param  {number} [timestamp] timestamp to record for
                 *         this mutation (otherwise, system time will be
                 *         used)
                 * @returns {Promise.<boolean>} a promise for the result
                 *         of the mutation; true if changes were made.
                 */
                mutate: mutate,
                /**
                 * Listen for mutations of this domain object's model.
                 * The provided listener will be invoked with the domain
                 * object's new model after any changes. To stop listening,
                 * invoke the function returned by this method.
                 * @param {Function} listener function to call on mutation
                 * @returns {Function} a function to stop listening
                 */
                listen: listen
            };
        }

        return MutationCapability;
    }
);