Function spawn 

pub fn spawn(
    scenario: Scenario,
    deliver: impl FnMut(Packet) -> bool + Send + 'static,
) -> Result<JoinHandle<()>, String>

Starts an absolute timeline; false from the callback stops delivery.

Joining this thread proves only delivery has ended, not that inputs were handled, changed application state, rendered, or appeared on a display.

Errors

Returns an error for an invalid timeline, opted-in gate configuration/source/ descriptor, unsupported opted-in gate platform, or thread creation failure.