Struct tokio::sync::watch::Ref

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pub struct Ref<'a, T> { /* private fields */ }

Expand description

Returns a reference to the inner value.

Outstanding borrows hold a read lock on the inner value. This means that long-lived borrows could cause the producer half to block. It is recommended to keep the borrow as short-lived as possible. Additionally, if you are running in an environment that allows !Send futures, you must ensure that the returned Ref type is never held alive across an .await point, otherwise, it can lead to a deadlock.

The priority policy of the lock is dependent on the underlying lock implementation, and this type does not guarantee that any particular policy will be used. In particular, a producer which is waiting to acquire the lock in send might or might not block concurrent calls to borrow, e.g.:

Potential deadlock example

// Task 1 (on thread A)    |  // Task 2 (on thread B)
let _ref1 = rx.borrow();   |
                           |  // will block
                           |  let _ = tx.send(());
// may deadlock            |
let _ref2 = rx.borrow();   |

Implementations§

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impl<'a, T> Ref<'a, T>

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pub fn has_changed(&self) -> bool

Indicates if the borrowed value is considered as changed since the last time it has been marked as seen.

Unlike Receiver::has_changed(), this method does not fail if the channel is closed.

When borrowed from the Sender this function will always return false.

Examples

use tokio::sync::watch;

#[tokio::main]
async fn main() {
    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());
}