pub struct Ref<'a, T> { /* private fields */ }
sync
only.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§
Source§impl<'a, T> Ref<'a, T>
impl<'a, T> Ref<'a, T>
Sourcepub fn has_changed(&self) -> bool
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());
}