Multi-page doPost() w/ Client Handlers & State in TextBox

// Muliple page form using Google Apps Script

function doGet(eventInfo)  {return GUI(eventInfo)};
function doPost(eventInfo) {return GUI(eventInfo)};

function GUI (eventInfo) {
  var n = (eventInfo.parameter.HiddenState == void(0) ? 0 : parseInt(eventInfo.parameter.HiddenState));
  var ui = ((n == 0)? UiApp.createApplication() : UiApp.getActiveApplication());
  var Form;
  switch(n){
    case -1: {
      Form = ui.createFormPanel().add(ui.createHTML("<H3>Exited by submission of form "+eventInfo.parameter.formId+"</h3>"));
    } break;
    case 0: {
//      setStyleAttributes(ui,css.app);// UiApp does not support the .setStyleAttributes() method
//      var MastHead = ui.createAbsolutePanel().setWidth('auto').setHeight('190').setId('MastHead')
//        .add(ui.createHTML(title).setStyleAttributes(css.title), 265,75)
//      ui.add(MastHead);
      Form = getForm(eventInfo,n); // Use identical forms for demo purpose only
    } break;
    case 1: {
      Form = getForm(eventInfo,n); // In reality, each form would differ but...
    } break;
    default: {
      Form = getForm(eventInfo,n) // each form must abide by (implement) the hidden HiddenState variable
    } break;
  }
  return ui.add(Form);
};

function getForm(eventInfo,n) {
  var ui = UiApp.getActiveApplication();

  // Increment the ID stored in a hidden text-box
  var HiddenState = ui.createTextBox().setId('HiddenState').setName('HiddenState').setValue(1+n).setVisible(true).setEnabled(false);
  var H1 = ui.createHTML("<H1>Form "+n+"</H1>");
  var H2 = ui.createHTML(
    "<h2>"+(eventInfo.parameter.formId==void(0)?"":"Created by submission of form "+eventInfo.parameter.formId)+"</h2>");

  // Add four submit buttons to go forward, backward, to stay/validate and to exit the form
  var Btn = {};
  Btn.Next = ui.createSubmitButton("Next").setEnabled(true).setVisible(true);
  Btn.Back = ui.createSubmitButton("Back").setEnabled(n>1).setVisible(true);
  Btn.Stay = ui.createSubmitButton("Stay").setEnabled(n>0).setVisible(true);
  Btn.Exit = ui.createSubmitButton("Exit").setEnabled(n>0).setVisible(true);
  var Buttons = ui.createHorizontalPanel().add(Btn.Back).add(Btn.Stay).add(Btn.Next).add(Btn.Exit);
  var Body = ui.createVerticalPanel().add(HiddenState).add(H1).add(H2).add(Buttons).add(getParameters(eventInfo));
  var Form = ui.createFormPanel().setId((n>0?'doPost[':'doGet[')+n+']').add(Body);

  // Add client handlers using setText() to adjust HiddenState prior to form submission
  // NB: Use of the .setValue(val) and .setValue(val,bool) methods give runtime errors!
  Btn.Stay.addClickHandler(ui.createClientHandler().forTargets(HiddenState).setText(''+(parseInt(n))));
  Btn.Back.addClickHandler(ui.createClientHandler().forTargets(HiddenState).setText(''+(parseInt(n)-1)));
  Btn.Exit.addClickHandler(ui.createClientHandler().forTargets(HiddenState).setText('-1'));

  // Add an event handler executed prior to form submission
  var onFormSubmit = ui.createClientHandler()
  .forTargets(HiddenState).setEnabled(true) // Enable so value gets included in post parameters
  .forTargets(Body).setStyleAttribute("backgroundColor","#EEE");
  Form.addSubmitHandler(onFormSubmit);

  return Form;
}

function getParameters(eventInfo) {
  var ui = UiApp.getActiveApplication();
  var panel = ui.createVerticalPanel().add(ui.createLabel("Parameters: "));
  for( p in eventInfo.parameter)
    panel.add(ui.createLabel(" - " + p + " = " + eventInfo.parameter[p]));
  return panel;
}