The source code

    /**
     * @class Ext.CycleButton
     * @extends Ext.SplitButton
     * A specialized SplitButton that contains a menu of {@link Ext.menu.CheckItem} elements.  The button automatically
     * cycles through each menu item on click, raising the button's {@link #change} event (or calling the button's
     * {@link #changeHandler} function, if supplied) for the active menu item. Clicking on the arrow section of the
     * button displays the dropdown menu just like a normal SplitButton.  Example usage:
     *
    var btn = new Ext.CycleButton({
        showText: true,
        prependText: 'View as ',
        items: [{
            text:'text only',
            iconCls:'view-text',
            checked:true
        },{
            text:'HTML',
            iconCls:'view-html'
        }],
        changeHandler:function(btn, item){
            Ext.Msg.alert('Change View', item.text);
        }
    });

     * @constructor
     * Create a new split button
     * @param {Object} config The config object
     * @xtype cycle
     */
    Ext.CycleButton = Ext.extend(Ext.SplitButton, {
        /**
         * @cfg {Array} items An array of {@link Ext.menu.CheckItem} config objects to be used when creating the
         * button's menu items (e.g., {text:'Foo', iconCls:'foo-icon'})
         */
        /**
         * @cfg {Boolean} showText True to display the active item's text as the button text (defaults to false)
         */
        /**
         * @cfg {String} prependText A static string to prepend before the active item's text when displayed as the
         * button's text (only applies when showText = true, defaults to '')
         */
        /**
         * @cfg {Function} changeHandler A callback function that will be invoked each time the active menu
         * item in the button's menu has changed.  If this callback is not supplied, the SplitButton will instead
         * fire the {@link #change} event on active item change.  The changeHandler function will be called with the
         * following argument list: (SplitButton this, Ext.menu.CheckItem item)
         */
        /**
         * @cfg {String} forceIcon A css class which sets an image to be used as the static icon for this button.  This
         * icon will always be displayed regardless of which item is selected in the dropdown list.  This overrides the
         * default behavior of changing the button's icon to match the selected item's icon on change.
         */
        /**
         * @property menu
         * @type Menu
         * The {@link Ext.menu.Menu Menu} object used to display the {@link Ext.menu.CheckItem CheckItems} representing the available choices.
         */

        // private
        getItemText : function(item){
            if(item && this.showText === true){
                var text = '';
                if(this.prependText){
                    text += this.prependText;
                }
                text += item.text;
                return text;
            }
            return undefined;
        },

        /**
         * Sets the button's active menu item.
         * @param {Ext.menu.CheckItem} item The item to activate
         * @param {Boolean} suppressEvent True to prevent the button's change event from firing (defaults to false)
         */
        setActiveItem : function(item, suppressEvent){
            if(typeof item != 'object'){
                item = this.menu.items.get(item);
            }
            if(item){
                if(!this.rendered){
                    this.text = this.getItemText(item);
                    this.iconCls = item.iconCls;
                }else{
                    var t = this.getItemText(item);
                    if(t){
                        this.setText(t);
                    }
                    this.setIconClass(item.iconCls);
                }
                this.activeItem = item;
                if(!item.checked){
                    item.setChecked(true, true);
                }
                if(this.forceIcon){
                    this.setIconClass(this.forceIcon);
                }
                if(!suppressEvent){
                    this.fireEvent('change', this, item);
                }
            }
        },

        /**
         * Gets the currently active menu item.
         * @return {Ext.menu.CheckItem} The active item
         */
        getActiveItem : function(){
            return this.activeItem;
        },

        // private
        initComponent : function(){
            this.addEvents(
                /**
                 * @event change
                 * Fires after the button's active menu item has changed.  Note that if a {@link #changeHandler} function
                 * is set on this CycleButton, it will be called instead on active item change and this change event will
                 * not be fired.
                 * @param {Ext.CycleButton} this
                 * @param {Ext.menu.CheckItem} item The menu item that was selected
                 */
                "change"
            );

            if(this.changeHandler){
                this.on('change', this.changeHandler, this.scope||this);
                delete this.changeHandler;
            }

            this.itemCount = this.items.length;

            this.menu = {cls:'x-cycle-menu', items:[]};
            var checked;
            for(var i = 0, len = this.itemCount; i < len; i++){
                var item = this.items[i];
                item.group = item.group || this.id;
                item.itemIndex = i;
                item.checkHandler = this.checkHandler;
                item.scope = this;
                item.checked = item.checked || false;
                this.menu.items.push(item);
                if(item.checked){
                    checked = item;
                }
            }
            this.setActiveItem(checked, true);
            Ext.CycleButton.superclass.initComponent.call(this);

            this.on('click', this.toggleSelected, this);
        },

        // private
        checkHandler : function(item, pressed){
            if(pressed){
                this.setActiveItem(item);
            }
        },

        /**
         * This is normally called internally on button click, but can be called externally to advance the button's
         * active item programmatically to the next one in the menu.  If the current item is the last one in the menu
         * the active item will be set to the first item in the menu.
         */
        toggleSelected : function(){
            this.menu.render();

            var nextIdx, checkItem;
            for (var i = 1; i < this.itemCount; i++) {
                nextIdx = (this.activeItem.itemIndex + i) % this.itemCount;
                // check the potential item
                checkItem = this.menu.items.itemAt(nextIdx);
                // if its not disabled then check it.
                if (!checkItem.disabled) {
                    checkItem.setChecked(true);
                    break;
                }
            }
        }
    });
    Ext.reg('cycle', Ext.CycleButton);