The source code

    /*!
     * Ext JS Library 3.3.1
     * Copyright(c) 2006-2010 Sencha Inc.
     * licensing@sencha.com
     * http://www.sencha.com/license
     */
    // We are adding these custom layouts to a namespace that does not
    // exist by default in Ext, so we have to add the namespace first:
    Ext.ns('Ext.ux.layout');

    /**
     * @class Ext.ux.layout.CenterLayout
     * @extends Ext.layout.FitLayout
     * This is a very simple layout style used to center contents within a container.  This layout works within
     * nested containers and can also be used as expected as a Viewport layout to center the page layout.
     * As a subclass of FitLayout, CenterLayout expects to have a single child panel of the container that uses
     * the layout.  The layout does not require any config options, although the child panel contained within the
     * layout must provide a fixed or percentage width.  The child panel's height will fit to the container by
     * default, but you can specify autoHeight:true to allow it to autosize based on its content height.
     * Example usage:
     *
    // The content panel is centered in the container
    var p = new Ext.Panel({
        title: 'Center Layout',
        layout: 'ux.center',
        items: [{
            title: 'Centered Content',
            width: '75%',
            html: 'Some content'
        }]
    });

    // If you leave the title blank and specify no border
    // you'll create a non-visual, structural panel just
    // for centering the contents in the main container.
    var p = new Ext.Panel({
        layout: 'ux.center',
        border: false,
        items: [{
            title: 'Centered Content',
            width: 300,
            autoHeight: true,
            html: 'Some content'
        }]
    });

     */
    Ext.ux.layout.CenterLayout = Ext.extend(Ext.layout.FitLayout, {
    	// private
        setItemSize : function(item, size){
            this.container.addClass('ux-layout-center');
            item.addClass('ux-layout-center-item');
            if(item && size.height > 0){
                if(item.width){
                    size.width = item.width;
                }
                item.setSize(size);
            }
        }
    });

    Ext.Container.LAYOUTS['ux.center'] = Ext.ux.layout.CenterLayout;