My approach was not that bad, but instead of working with the DragContext, I have to work with the getSelectedWidgets().iterator() from the DragController.
edit
I switched my code to onDragEnd(), in this case I don't screw up the moving widget. And I changed the Draggable widget to a Panel. I just have to add and remove widgets from it.
answer
Samplecode:
dragController.addDragHandler(new DragHandler(){
public void onDragEnd(DragEndEvent event) {
Iterable<Widget> myiterable = myDragCTRL_subcont.getSelectedWidgets();
Widget mywidget = myiterable.iterator().next();
FlowPanel flowtemp = ((FlowPanel)mywidget);
flowtemp.add(new HTML("label"));
...
...
}
public void onDragStart(DragStartEvent event) {
...
}
public void onPreviewDragEnd(DragEndEvent event)
throws VetoDragException {
...
}
public void onPreviewDragStart(DragStartEvent event)
throws VetoDragException {
...
}
});