Struct tokio::sync::watch::Receiver

pub struct Receiver<T> { /* private fields */ }

Receives values from the associated Sender.

Instances are created by the channel function.

To turn this receiver into a Stream, you can use the WatchStream wrapper.

Implementations

impl<T> Receiver<T>

pub fn borrow(&self) -> Ref<'_, T>

Returns a reference to the most recently sent value.

This method does not mark the returned value as seen, so future calls to changed may return immediately even if you have already seen the value with a call to borrow.

Outstanding borrows hold a read lock. This means that long lived borrows could cause the send half to block. It is recommended to keep the borrow as short lived as possible.

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();   |

Examples

use tokio::sync::watch;

let (_, rx) = watch::channel("hello");
assert_eq!(*rx.borrow(), "hello");

pub fn borrow_and_update(&mut self) -> Ref<'_, T>

Returns a reference to the most recently sent value and mark that value as seen.

This method marks the value as seen, so changed will not return immediately if the newest value is one previously returned by borrow_and_update.

Outstanding borrows hold a read lock. This means that long lived borrows could cause the send half to block. It is recommended to keep the borrow as short lived as possible.

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 = rx1.borrow_and_update();   |
                                       |  // will block
                                       |  let _ = tx.send(());
// may deadlock                        |
let _ref2 = rx2.borrow_and_update();   |

pub fn has_changed(&self) -> Result<bool, RecvError>

Checks if this channel contains a message that this receiver has not yet seen. The new value is not marked as seen.

Although this method is called has_changed, it does not check new messages for equality, so this call will return true even if the new message is equal to the old message.

Returns an error if the channel has been closed.

Examples

use tokio::sync::watch;

#[tokio::main]
async fn main() {
    let (tx, mut rx) = watch::channel("hello");

    tx.send("goodbye").unwrap();

    assert!(rx.has_changed().unwrap());
    assert_eq!(*rx.borrow_and_update(), "goodbye");

    // The value has been marked as seen
    assert!(!rx.has_changed().unwrap());

    drop(tx);
    // The `tx` handle has been dropped
    assert!(rx.has_changed().is_err());
}

pub async fn changed(&mut self) -> Result<(), RecvError>

Waits for a change notification, then marks the newest value as seen.

If the newest value in the channel has not yet been marked seen when this method is called, the method marks that value seen and returns immediately. If the newest value has already been marked seen, then the method sleeps until a new message is sent by the Sender connected to this Receiver, or until the Sender is dropped.

This method returns an error if and only if the Sender is dropped.

Cancel safety

This method is cancel safe. If you use it as the event in a tokio::select! statement and some other branch completes first, then it is guaranteed that no values have been marked seen by this call to changed.

Examples

use tokio::sync::watch;

#[tokio::main]
async fn main() {
    let (tx, mut rx) = watch::channel("hello");

    tokio::spawn(async move {
        tx.send("goodbye").unwrap();
    });

    assert!(rx.changed().await.is_ok());
    assert_eq!(*rx.borrow(), "goodbye");

    // The `tx` handle has been dropped
    assert!(rx.changed().await.is_err());
}

Trait Implementations

impl<T> Clone for Receiver<T>

fn clone(&self) -> Self

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<T: Debug> Debug for Receiver<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T> Drop for Receiver<T>

fn drop(&mut self)

Executes the destructor for this type.