addListenerButton.setText("addImageViewerListener");
addListenerButton.addSelectionListener(new SelectionAdapter() {
@Override
public void widgetSelected(SelectionEvent e) {
if (imageViewerListener == null) {
imageViewerListener = new IImageViewerListener() {
@Override
public void imageViewerChanged(ImageViewerEvent event) {
showMessage("imageViewerChanged");
}