code: private SignatureFieldExtension initExtension() {
          SignatureFieldExtension ext = new SignatureFieldExtension(this);
          ext.addSignatureChangeListener(new SignatureFieldExtension.SignatureChangeListener() {
              
              private static final long serialVersionUID = 1L;
  
              @Override
              public void signatureChange(SignatureFieldExtension.SignatureChangeEvent event) {
                  changingVariables = true;
                  try {
                      setValue(event.getSignature(), true);
                  } finally {
                      changingVariables = false;
                  }
              }
          });
          return ext;
      }
docs: Creates the javascript extension used to communicate with the
  client-side.
  @return The extension of this field