get value on dialogDefinition

hi

i want to get txtdlgGenStyle value (txtdyllgGenStyle=Image Dialog And advanved tab and style elmment)

i think yet dialogDefinition event is not set value in txtdyllgGenStyle

how i can getValue?

CKEDITOR.on('dialogDefinition', function (ev) {

    var dialogName = ev.data.name,
        dialogDefinition = ev.data.definition,
        editor = ev.editor,
        dialog = ev.data.definition.dialog;

    dialog.on('show', function () {
        if (dialogName == 'image') {
            alert(dialog.getValueOf('advanced', 'txtdlgGenStyle');
        }
    });

},null,null);