The source code

    /*!
     * Ext JS Library 3.4.0
     * Copyright(c) 2006-2011 Sencha Inc.
     * licensing@sencha.com
     * http://www.sencha.com/license
     */
    /**
     * @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 = Ext.extend(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 first item in the group that is
         * initialized with checked = true will be rendered as checked.
         */
        checked: false,

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

        initComponent : function(){
            Ext.menu.CheckItem.superclass.initComponent.call(this);
    	    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);
        },

        // 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){
            var suppress = suppressEvent === true;
            if(this.checked != state && (suppress || this.fireEvent("beforecheckchange", this, state) !== false)){
                Ext.menu.MenuMgr.onCheckChange(this, state);
                if(this.container){
                    this.container[state ? "addClass" : "removeClass"]("x-menu-item-checked");
                }
                this.checked = state;
                if(!suppress){
                    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);