    /**
     * @class Ext.util.History

    History management component that allows you to register arbitrary tokens that signify application
    history state on navigation actions.  You can then handle the history {@link #change} event in order
    to reset your application UI to the appropriate state when the user navigates forward or backward through
    the browser history stack.

    __Initializing__
    The {@link #init} method of the History object must be called before using History. This sets up the internal
    state and must be the first thing called before using History.

    __Setup__
    The History objects requires elements on the page to keep track of the browser history. For older versions of IE,
    an IFrame is required to do the tracking. For other browsers, a hidden field can be used. The history objects expects
    these to be on the page before the {@link #init} method is called. The following markup is suggested in order
    to support all browsers:

        <form id="history-form" class="x-hide-display">
            <input type="hidden" id="x-history-field" />
            <iframe id="x-history-frame"></iframe>
        </form>

     * @markdown
     * @singleton
     */
    Ext.define('Ext.util.History', {
        singleton: true,
        alternateClassName: 'Ext.History',
        mixins: {
            observable: 'Ext.util.Observable'
        },

        constructor: function() {
            var me = this;
            me.oldIEMode = Ext.isIE6 || Ext.isIE7 || !Ext.isStrict && Ext.isIE8;
            me.iframe = null;
            me.hiddenField = null;
            me.ready = false;
            me.currentToken = null;
        },

        getHash: function() {
            var href = window.location.href,
                i = href.indexOf("#");

            return i >= 0 ? href.substr(i + 1) : null;
        },

        doSave: function() {
            this.hiddenField.value = this.currentToken;
        },

        handleStateChange: function(token) {
            this.currentToken = token;
            this.fireEvent('change', token);
        },

        updateIFrame: function(token) {
            var html = '<html><body><div id="state">' +
                        Ext.util.Format.htmlEncode(token) +
                        '</div></body></html>';

            try {
                var doc = this.iframe.contentWindow.document;
                doc.open();
                doc.write(html);
                doc.close();
                return true;
            } catch (e) {
                return false;
            }
        },

        checkIFrame: function () {
            var me = this,
                contentWindow = me.iframe.contentWindow;

            if (!contentWindow || !contentWindow.document) {
                Ext.Function.defer(this.checkIFrame, 10, this);
                return;
            }

            var doc = contentWindow.document,
                elem = doc.getElementById("state"),
                oldToken = elem ? elem.innerText : null,
                oldHash = me.getHash();

            Ext.TaskManager.start({
                run: function () {
                    var doc = contentWindow.document,
                        elem = doc.getElementById("state"),
                        newToken = elem ? elem.innerText : null,
                        newHash = me.getHash();

                    if (newToken !== oldToken) {
                        oldToken = newToken;
                        me.handleStateChange(newToken);
                        window.top.location.hash = newToken;
                        oldHash = newToken;
                        me.doSave();
                    } else if (newHash !== oldHash) {
                        oldHash = newHash;
                        me.updateIFrame(newHash);
                    }
                },
                interval: 50,
                scope: me
            });
            me.ready = true;
            me.fireEvent('ready', me);
        },

        startUp: function () {
            var me = this;

            me.currentToken = me.hiddenField.value || this.getHash();

            if (me.oldIEMode) {
                me.checkIFrame();
            } else {
                var hash = me.getHash();
                Ext.TaskManager.start({
                    run: function () {
                        var newHash = me.getHash();
                        if (newHash !== hash) {
                            hash = newHash;
                            me.handleStateChange(hash);
                            me.doSave();
                        }
                    },
                    interval: 50,
                    scope: me
                });
                me.ready = true;
                me.fireEvent('ready', me);
            }

        },

        /**
         * The id of the hidden field required for storing the current history token.
         * @type String
         * @property
         */
        fieldId: Ext.baseCSSPrefix + 'history-field',
        /**
         * The id of the iframe required by IE to manage the history stack.
         * @type String
         * @property
         */
        iframeId: Ext.baseCSSPrefix + 'history-frame',

        /**
         * Initialize the global History instance.
         * @param {Boolean} onReady (optional) A callback function that will be called once the history
         * component is fully initialized.
         * @param {Object} scope (optional) The scope (<code>this</code> reference) in which the callback is executed. Defaults to the browser window.
         */
        init: function (onReady, scope) {
            var me = this;

            if (me.ready) {
                Ext.callback(onReady, scope, [me]);
                return;
            }

            if (!Ext.isReady) {
                Ext.onReady(function() {
                    me.init(onReady, scope);
                });
                return;
            }

            me.hiddenField = Ext.getDom(me.fieldId);

            if (me.oldIEMode) {
                me.iframe = Ext.getDom(me.iframeId);
            }

            me.addEvents(
                /**
                 * @event ready
                 * Fires when the Ext.util.History singleton has been initialized and is ready for use.
                 * @param {Ext.util.History} The Ext.util.History singleton.
                 */
                'ready',
                /**
                 * @event change
                 * Fires when navigation back or forwards within the local page's history occurs.
                 * @param {String} token An identifier associated with the page state at that point in its history.
                 */
                'change'
            );

            if (onReady) {
                me.on('ready', onReady, scope, {single: true});
            }
            me.startUp();
        },

        /**
         * Add a new token to the history stack. This can be any arbitrary value, although it would
         * commonly be the concatenation of a component id and another id marking the specifc history
         * state of that component.  Example usage:
         * <pre><code>
    // Handle tab changes on a TabPanel
    tabPanel.on('tabchange', function(tabPanel, tab){
    Ext.History.add(tabPanel.id + ':' + tab.id);
    });
    </code></pre>
         * @param {String} token The value that defines a particular application-specific history state
         * @param {Boolean} preventDuplicates When true, if the passed token matches the current token
         * it will not save a new history step. Set to false if the same state can be saved more than once
         * at the same history stack location (defaults to true).
         */
        add: function (token, preventDup) {
            var me = this;

            if (preventDup !== false) {
                if (me.getToken() === token) {
                    return true;
                }
            }

            if (me.oldIEMode) {
                return me.updateIFrame(token);
            } else {
                window.top.location.hash = token;
                return true;
            }
        },

        /**
         * Programmatically steps back one step in browser history (equivalent to the user pressing the Back button).
         */
        back: function() {
            window.history.go(-1);
        },

        /**
         * Programmatically steps forward one step in browser history (equivalent to the user pressing the Forward button).
         */
        forward: function(){
            window.history.go(1);
        },

        /**
         * Retrieves the currently-active history token.
         * @return {String} The token
         */
        getToken: function() {
            return this.ready ? this.currentToken : this.getHash();
        }
    });