textfield实现点击即选择数据

        var fieldForm = new Ext.form.FormPanel({ 
            renderTo : document.body, 
            layout : 'form', 
            items : [{ 
                xtype : 'textfield', 
                fieldLabel : 'TextSelectTest', 
                value : 'SelectContent', 
                listeners  : { 
                   'focus':function(){ 
                       this.selectText(); 
                   } 
               } 
           }]  
       })