The source code

    /**
     * @class Ext.menu.CheckItem
     * @extends Ext.menu.Item
     * Adds a menu item that contains a checkbox by default, but can also be part of a radio group.
     * @constructor
     * Creates a new CheckItem
     * @param {Object} config Configuration options
     * @xtype menucheckitem
     */
    Ext.menu.CheckItem = function(config){
        Ext.menu.CheckItem.superclass.constructor.call(this, config);
        this.addEvents(
            /**
             * @event beforecheckchange
             * Fires before the checked value is set, providing an opportunity to cancel if needed
             * @param {Ext.menu.CheckItem} this
             * @param {Boolean} checked The new checked value that will be set
             */
            "beforecheckchange" ,
            /**
             * @event checkchange
             * Fires after the checked value has been set
             * @param {Ext.menu.CheckItem} this
             * @param {Boolean} checked The checked value that was set
             */
            "checkchange"
        );
        /**
         * A function that handles the checkchange event.  The function is undefined by default, but if an implementation
         * is provided, it will be called automatically when the checkchange event fires.
         * @param {Ext.menu.CheckItem} this
         * @param {Boolean} checked The checked value that was set
         * @method checkHandler
         */
        if(this.checkHandler){
            this.on('checkchange', this.checkHandler, this.scope);
        }
        Ext.menu.MenuMgr.registerCheckable(this);
    };
    Ext.extend(Ext.menu.CheckItem, Ext.menu.Item, {
        /**
         * @cfg {String} group
         * All check items with the same group name will automatically be grouped into a single-select
         * radio button group (defaults to '')
         */
        /**
         * @cfg {String} itemCls The default CSS class to use for check items (defaults to "x-menu-item x-menu-check-item")
         */
        itemCls : "x-menu-item x-menu-check-item",
        /**
         * @cfg {String} groupClass The default CSS class to use for radio group check items (defaults to "x-menu-group-item")
         */
        groupClass : "x-menu-group-item",

        /**
         * @cfg {Boolean} checked True to initialize this checkbox as checked (defaults to false).  Note that
         * if this checkbox is part of a radio group (group = true) only the last item in the group that is
         * initialized with checked = true will be rendered as checked.
         */
        checked: false,

        // private
        ctype: "Ext.menu.CheckItem",

        // private
        onRender : function(c){
            Ext.menu.CheckItem.superclass.onRender.apply(this, arguments);
            if(this.group){
                this.el.addClass(this.groupClass);
            }
            if(this.checked){
                this.checked = false;
                this.setChecked(true, true);
            }
        },

        // private
        destroy : function(){
            Ext.menu.MenuMgr.unregisterCheckable(this);
            Ext.menu.CheckItem.superclass.destroy.apply(this, arguments);
        },

        /**
         * Set the checked state of this item
         * @param {Boolean} checked The new checked value
         * @param {Boolean} suppressEvent (optional) True to prevent the checkchange event from firing (defaults to false)
         */
        setChecked : function(state, suppressEvent){
            if(this.checked != state && this.fireEvent("beforecheckchange", this, state) !== false){
                if(this.container){
                    this.container[state ? "addClass" : "removeClass"]("x-menu-item-checked");
                }
                this.checked = state;
                if(suppressEvent !== true){
                    this.fireEvent("checkchange", this, state);
                }
            }
        },

        // private
        handleClick : function(e){
           if(!this.disabled && !(this.checked && this.group)){// disable unselect on radio item
               this.setChecked(!this.checked);
           }
           Ext.menu.CheckItem.superclass.handleClick.apply(this, arguments);
        }
    });
    Ext.reg('menucheckitem', Ext.menu.CheckItem);