CAPI: How to define a graph-pane with all nodes collapsed?

I'm a newbie in CAPI programming. I wrote a CAPI program which contains a graph-pane with thousands of nodes.

When applications started, all nodes are expanded (by default), how can I do this instead:

1. Let all nodes in graph pane be collapsed, leave only root nodes displayed in graph pane.

2. Let one of the root nodes be selected.

Can anyone show me a example? Thanks.

Chun TIAN (binghe)