import javax.swing.tree.*;

static <A, B extends Collection<A>> JTree jDynamicTree_concurrent(final A root, final F1<A, B> makeChildren, fO... optParams) {
  ret swing(func -> JTree {
    final Q queue = startQ();
    DefaultMutableTreeNode rootNode = new(root);
    rootNode.add(new DefaultMutableTreeNode(jDynamicTree_dummy()));
    final JTree tree = new JTree(rootNode);
    tree.setShowsRootHandles(true);
    tree.collapseRow(0);
    final bool keepChildrenOnCollapse = optParam(optParams, 'keepChildrenOnCollapse, false);
    //final bool makeChildrenIsFast = optParam(optParams, 'makeChildrenIsFast, false);
    final bool debug = boolPar debug(optParams, true);
    
    tree.addTreeWillExpandListener(new TreeWillExpandListener {
      public void treeWillCollapse(TreeExpansionEvent e) {}
      public void treeWillExpand(TreeExpansionEvent e) {
        if (debug) print("treeWillExpand");
        final DefaultMutableTreeNode node = cast e.getPath().getLastPathComponent();
        if (keepChildrenOnCollapse && !jDynamicTree_isInDummyState(node)) ret;
        
        queue.add(r {
          print("Calling: " + makeChildren);
          B raw = callF(makeChildren, (A) node.getUserObject());
          print("raw => " + className(raw));
          fL<A> l = asList(raw);
          if (debug) print("Have children: " + l(l));
          swing {
            if (replaceTreeNodeChildren_withDummyChild(node, l, jDynamicTree_dummy()))
              ((DefaultTreeModel) tree.getModel()).nodeStructureChanged(node);
          }
        });
      }
    });
    
    ret tree;
  });
}

Began life as a copy of #1016044