The source code

    /**
     * @class Ext.LoadMask
     * A simple utility class for generically masking elements while loading data.  If the {@link #store}
     * config option is specified, the masking will be automatically synchronized with the store's loading
     * process and the mask element will be cached for reuse.  For all other elements, this mask will replace the
     * element's Updater load indicator and will be destroyed after the initial load.
     * <p>Example usage:</p>
     *<pre><code>
    // Basic mask:
    var myMask = new Ext.LoadMask(Ext.getBody(), {msg:"Please wait..."});
    myMask.show();
    </code></pre>
     * @constructor
     * Create a new LoadMask
     * @param {Mixed} el The element or DOM node, or its id
     * @param {Object} config The config object
     */
    Ext.LoadMask = function(el, config){
        this.el = Ext.get(el);
        Ext.apply(this, config);
        if(this.store){
            this.store.on({
                scope: this,
                beforeload: this.onBeforeLoad,
                load: this.onLoad,
                exception: this.onLoad
            });
            this.removeMask = Ext.value(this.removeMask, false);
        }else{
            var um = this.el.getUpdater();
            um.showLoadIndicator = false; // disable the default indicator
            um.on({
                scope: this,
                beforeupdate: this.onBeforeLoad,
                update: this.onLoad,
                failure: this.onLoad
            });
            this.removeMask = Ext.value(this.removeMask, true);
        }
    };

    Ext.LoadMask.prototype = {
        /**
         * @cfg {Ext.data.Store} store
         * Optional Store to which the mask is bound. The mask is displayed when a load request is issued, and
         * hidden on either load sucess, or load fail.
         */
        /**
         * @cfg {Boolean} removeMask
         * True to create a single-use mask that is automatically destroyed after loading (useful for page loads),
         * False to persist the mask element reference for multiple uses (e.g., for paged data widgets).  Defaults to false.
         */
        /**
         * @cfg {String} msg
         * The text to display in a centered loading message box (defaults to 'Loading...')
         */
        msg : 'Loading...',
        /**
         * @cfg {String} msgCls
         * The CSS class to apply to the loading message element (defaults to "x-mask-loading")
         */
        msgCls : 'x-mask-loading',

        /**
         * Read-only. True if the mask is currently disabled so that it will not be displayed (defaults to false)
         * @type Boolean
         */
        disabled: false,

        /**
         * Disables the mask to prevent it from being displayed
         */
        disable : function(){
           this.disabled = true;
        },

        /**
         * Enables the mask so that it can be displayed
         */
        enable : function(){
            this.disabled = false;
        },

        // private
        onLoad : function(){
            this.el.unmask(this.removeMask);
        },

        // private
        onBeforeLoad : function(){
            if(!this.disabled){
                this.el.mask(this.msg, this.msgCls);
            }
        },

        /**
         * Show this LoadMask over the configured Element.
         */
        show: function(){
            this.onBeforeLoad();
        },

        /**
         * Hide this LoadMask.
         */
        hide: function(){
            this.onLoad();
        },

        // private
        destroy : function(){
            if(this.store){
                this.store.un('beforeload', this.onBeforeLoad, this);
                this.store.un('load', this.onLoad, this);
                this.store.un('exception', this.onLoad, this);
            }else{
                var um = this.el.getUpdater();
                um.un('beforeupdate', this.onBeforeLoad, this);
                um.un('update', this.onLoad, this);
                um.un('failure', this.onLoad, this);
            }
        }
    };