An unbounded sender that does not prevent the channel from being closed.

If all UnboundedSender instances of a channel were dropped and only WeakUnboundedSender instances remain, the channel is closed.

In order to send messages, the WeakUnboundedSender needs to be upgraded using WeakUnboundedSender::upgrade, which returns Option<UnboundedSender>. It returns None if all UnboundedSenders have been dropped, and otherwise it returns an UnboundedSender.

Examples

use tokio::sync::mpsc::unbounded_channel;

# #[tokio::main(flavor = "current_thread")]
# async fn main() {
let (tx, _rx) = unbounded_channel::<i32>();
let tx_weak = tx.downgrade();

// Upgrading will succeed because `tx` still exists.
assert!(tx_weak.upgrade().is_some());

// If we drop `tx`, then it will fail.
drop(tx);
assert!(tx_weak.clone().upgrade().is_none());
# }