Структура Ref

use tokio::sync::watch;

let (tx, mut rx) = watch::channel("hello");

tx.send("goodbye").unwrap();
// The sender does never consider the value as changed.
assert!(!tx.borrow().has_changed());

// Drop the sender immediately, just for testing purposes.
drop(tx);

// Even if the sender has already been dropped...
assert!(rx.has_changed().is_err());
// ...the modified value is still readable and detected as changed.
assert_eq!(*rx.borrow(), "goodbye");
assert!(rx.borrow().has_changed());

// Read the changed value and mark it as seen.
{
    let received = rx.borrow_and_update();
    assert_eq!(*received, "goodbye");
    assert!(received.has_changed());
    // Release the read lock when leaving this scope.
}

// Now the value has already been marked as seen and could
// never be modified again (after the sender has been dropped).
assert!(!rx.borrow().has_changed());