define('core/engagement/page',[
    'jquery',
    'underscore',
    'backbone',
    'core/bus',
    'core/utils',
    'core/engagement/vendor/visibly',
], function (
    $,
    _,
    Backbone,
    bus,
    utils,
    visibly
) {
    'use strict';
 
    /**
     * This module reports if the user is engaged on the current
     * page through an amalgamation of various HTML standards and
     * best guesses.
     *
     * Do not use this module for UX methods. We are intentionally
     * conservative when marking a user as engaged for reporting reasons.
     *
     * Inspiration from:
     * - https://chartbeat.com/public-release-methodology/
     * - http://upworthy.github.io/2014/06/implementing-attention-minutes-part-1/
     * - http://jsfiddle.net/zanes/mbGBr/
     * - http://static.chartbeat.com/js/chartbeat.js
     * - https://github.com/serkanyersen/ifvisible.js
     */
 
    // Exported Event object w/ initial defaults
    var exports = _.extend(Backbone.Events, {
        _initialized: false,
        isEngaged: false,
        resetTimeout: null,
        ACTIVE_TIMEOUT: 60 * 1000,
    });
 
    exports._markNotEngaged = function () {
        if (!exports.isEngaged)
            return;
 
        exports.isEngaged = false;
        exports.trigger('change:isEngaged', exports.isEngaged, exports);
    };
 
    exports._markIsEngaged = function () {
        // Restart the interaction timeout clock.
        if (exports.resetTimeout)
            exports.resetTimeout();
 
        if (exports.isEngaged)
            return;
 
        exports.isEngaged = true;
        exports.trigger('change:isEngaged', exports.isEngaged, exports);
    };
 
    exports.start = function () {
        if (exports._initialized)
            return;
        exports._initialized = true;
 
        // Page Visibility API
 
        // If the window is currently visible, mark the user as
        // engaged, just to start things off. If it's not, we'll
        // wait for an interaction event to fire first.
        if (!visibly.hidden())
            exports._markIsEngaged();
 
        // On mobile-like user agents all we need is the Page Visibility API.
        visibly.onVisible(exports._markIsEngaged);
        visibly.onHidden(exports._markNotEngaged);
 
        if (utils.isMobileUserAgent())
            return;
 
        // On desktop-like user agents, we'll additionally use interaction events
        // and a timeout clock to more closely track if a user is still
        // engaged on the page. Each time we record an interaction on the page
        // we reset the clock. If the clock reaches it's`ACTIVE_TIMEOUT` time, we
        // mark the user as no longer engaged with the page (see `_markIsEngaged` method).
        exports.resetTimeout = _.debounce(exports._markNotEngaged, exports.ACTIVE_TIMEOUT);
        exports.resetTimeout();
 
        // Bind into interaction events on the current page.
        $(window.document).on('mousemove keyup', exports._markIsEngaged);
        $(window).on('scroll', exports._markIsEngaged);
 
        // Then bind into interaction events from the parent page.
        exports.listenTo(bus.frame, 'window.mousemove window.scroll window.click', exports._markIsEngaged);
    };
 
    exports.stop = function () {
        if (!exports._initialized)
            return;
        exports._initialized = false;
 
        // Clean up the resetTimeout method.
        exports.resetTimeout = null;
 
        // Turn off listeners before changing
        // the state to off.
        exports.stopListening();
        exports.off();
 
        visibly.visibleCallbacks = [];
        visibly.hiddenCallbacks = [];
 
        $(window.document).off('mousemove keyup', exports._markIsEngaged);
        $(window).off('scroll', exports._markIsEngaged);
 
        exports._markNotEngaged();
    };
 
    return exports;
});
 
// https://c.disquscdn.com/next/next-core/core/engagement/page.js