title: wicket ajaxformcomponentupdatingbehavior get value of TextField by clicking button
body: <p>I have a form and there is a TextField. I added button to a form, different than the default submit. I want to use Ajaxformcomponentupdatingbehavior to get the value from the TextField after clicking the button next to the TextField.</p>
  
  <p>My code looks like:</p>
  
  <pre><code>private String string;
  ...
  public ..() {   
  Form form = new Form("form") {
              @Override
              protected void onSubmit() { 
           //some code
  };
  
  add(form);
  TextField textField = new TextField("string", new PropertyModel&lt;String&gt;(this,"string"));
   textField.setOutputMarkupId(true);
  form.add(textField);
  Button button = new Button("evalButton");   
  form.add(button);
  button.add(new AjaxFormComponentUpdatingBehavior("onclick") {
              @Override
              protected void onUpdate(AjaxRequestTarget target) {               
                  System.out.print(textField.getValue());
               }
  });
  </code></pre>
  
  <p>the value of the TextField is null, after clicking the button for the second time, I get the right value. How could I get the value of the TextField after one button click?</p>