title: Mark form as invalid
body: <p>On my field(s) in the FormPanel I added the listener <code>blur</code>. If this event occurs I execute the form validation as follows:</p>
  
  <pre><code>listeners:{
      scope: this,
      blur: function(field, value){
          var username = field;
          if (username.getValue() == '') {
              username.markInvalid('This field is required');
          } else {
              Ext.Ajax.request({
                  url: 'users/validateForm',
                  method: 'POST',
                  params: 'username=' + username.getValue(),
                  success: function(response, opts) {
                      var jsonData = Ext.decode(response.responseText);
                      if (jsonData.msg.username != '') {
                          username.markInvalid(jsonData.msg.username);
                      }
                  }
              });
          }
      }
  }
  
  
  buttons: [{
      text: 'Save',
      handler: function() {
          var form = this.up('form').getForm();
          if (form.isValid()) {
              alert('test');
          }
      },
      formBind:true
  },{
      text: 'Cancel'
  }]
  </code></pre>
  
  <p>As the documentation of Ext JS explains, the <a href="http://docs.sencha.com/ext-js/4-0/#!/api/Ext.form.field.Field-method-markInvalid" rel="nofollow">markInvalid()</a> function affects only the field itself. But I want also setting the form as invalid, so that the submit button is disabled when the form is invalid. Currently the function isValid() on the form always returns true, although the field(s) are marked as invalid.</p>
  
  <p>My question now is, how can I achieve so that the form is also set as invalid with the code above?</p>
  
  <p>Thank you very much for your help! </p>