Question

is there any way to get an event when I switch between widget on a Stackpanel?
something like an openhandler for stackpanel. I have to know the index of
an open Widget on a Stackpanel.

Answer 1

Using StackLayoutPanel: http://google-web-toolkit.googlecode.com/svn/javadoc/2.0/com/google/gwt/user/client/ui/StackLayoutPanel.html

Use the addSelectionHandler() method. To get the selected widget, you can use getVisibleIndex() or getVisibleWidget().

myStackPanel.addSelectionHandler(new SelectionHandler<Integer>() {
    @Override
    public void onSelection(SelectionEvent<Integer> event) {

        int selectedWidgetIndex = stackPanel.getVisibleIndex());

        //Do stuff with the selectedWidgetIndex
    }
});

EDIT: This is using StackLayoutPanel, the OP uses StackPanel. The difference between them is that StackPanel is for quirks mode and StackLayout Panel for Standards Mode. I would StackLayoutPanel or any Layout related widget as new GWT widgets uses them, like the new DataGrid.