code: public OnTouchListener createDragToOpenListener(View src) {
          return new ForwardingListener(src) {
              @Override
              public ListPopupWindow getPopup() {
                  return ListPopupWindow.this;
              }
          };
      }
docs: Returns an {@link OnTouchListener} that can be added to the source view
  to implement drag-to-open behavior. Generally, the source view should be
  the same view that was passed to {@link #setAnchorView}.
  <p>
  When the listener is set on a view, touching that view and dragging
  outside of its bounds will open the popup window. Lifting will select the
  currently touched list item.
  <p>
  Example usage:
  <pre>
  ListPopupWindow myPopup = new ListPopupWindow(context);
  myPopup.setAnchor(myAnchor);
  OnTouchListener dragListener = myPopup.createDragToOpenListener(myAnchor);
  myAnchor.setOnTouchListener(dragListener);
  </pre>
  
  @param src the view on which the resulting listener will be set
  @return a touch listener that controls drag-to-open behavior