Struct tokio::sync::broadcast::Receiver

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

Available on crate feature sync only.

Expand description

Receiving-half of the broadcast channel.

Must not be used concurrently. Messages may be retrieved using recv.

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

Examples

use tokio::sync::broadcast;

#[tokio::main]
async fn main() {
    let (tx, mut rx1) = broadcast::channel(16);
    let mut rx2 = tx.subscribe();

    tokio::spawn(async move {
        assert_eq!(rx1.recv().await.unwrap(), 10);
        assert_eq!(rx1.recv().await.unwrap(), 20);
    });

    tokio::spawn(async move {
        assert_eq!(rx2.recv().await.unwrap(), 10);
        assert_eq!(rx2.recv().await.unwrap(), 20);
    });

    tx.send(10).unwrap();
    tx.send(20).unwrap();
}

Implementations

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impl<T> Receiver<T>

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pub fn len(&self) -> usize

Returns the number of messages that were sent into the channel and that this Receiver has yet to receive.

If the returned value from len is larger than the next largest power of 2 of the capacity of the channel any call to recv will return an Err(RecvError::Lagged) and any call to try_recv will return an Err(TryRecvError::Lagged), e.g. if the capacity of the channel is 10, recv will start to return Err(RecvError::Lagged) once len returns values larger than 16.

Examples

use tokio::sync::broadcast;

#[tokio::main]
async fn main() {
    let (tx, mut rx1) = broadcast::channel(16);

    tx.send(10).unwrap();
    tx.send(20).unwrap();

    assert_eq!(rx1.len(), 2);
    assert_eq!(rx1.recv().await.unwrap(), 10);
    assert_eq!(rx1.len(), 1);
    assert_eq!(rx1.recv().await.unwrap(), 20);     
    assert_eq!(rx1.len(), 0);
}

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pub fn is_empty(&self) -> bool

Returns true if there aren’t any messages in the channel that the Receiver has yet to receive.

Examples

use tokio::sync::broadcast;

#[tokio::main]
async fn main() {
    let (tx, mut rx1) = broadcast::channel(16);

    assert!(rx1.is_empty());

    tx.send(10).unwrap();
    tx.send(20).unwrap();

    assert!(!rx1.is_empty());
    assert_eq!(rx1.recv().await.unwrap(), 10);
    assert_eq!(rx1.recv().await.unwrap(), 20);     
    assert!(rx1.is_empty());
}

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impl<T: Clone> Receiver<T>

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pub fn resubscribe(&self) -> Self

Re-subscribes to the channel starting from the current tail element.

This Receiver handle will receive a clone of all values sent after it has resubscribed. This will not include elements that are in the queue of the current receiver. Consider the following example.

Examples

use tokio::sync::broadcast;

#[tokio::main]
async fn main() {
  let (tx, mut rx) = broadcast::channel(2);

  tx.send(1).unwrap();
  let mut rx2 = rx.resubscribe();
  tx.send(2).unwrap();

  assert_eq!(rx2.recv().await.unwrap(), 2);
  assert_eq!(rx.recv().await.unwrap(), 1);
}

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

Receives the next value for this receiver.

Each Receiver handle will receive a clone of all values sent after it has subscribed.

Err(RecvError::Closed) is returned when all Sender halves have dropped, indicating that no further values can be sent on the channel.

If the Receiver handle falls behind, once the channel is full, newly sent values will overwrite old values. At this point, a call to recv will return with Err(RecvError::Lagged) and the Receiver’s internal cursor is updated to point to the oldest value still held by the channel. A subsequent call to recv will return this value unless it has been since overwritten.

Cancel safety

This method is cancel safe. If recv is used as the event in a tokio::select! statement and some other branch completes first, it is guaranteed that no messages were received on this channel.

Examples

use tokio::sync::broadcast;

#[tokio::main]
async fn main() {
    let (tx, mut rx1) = broadcast::channel(16);
    let mut rx2 = tx.subscribe();

    tokio::spawn(async move {
        assert_eq!(rx1.recv().await.unwrap(), 10);
        assert_eq!(rx1.recv().await.unwrap(), 20);
    });

    tokio::spawn(async move {
        assert_eq!(rx2.recv().await.unwrap(), 10);
        assert_eq!(rx2.recv().await.unwrap(), 20);
    });

    tx.send(10).unwrap();
    tx.send(20).unwrap();
}

Handling lag

use tokio::sync::broadcast;

#[tokio::main]
async fn main() {
    let (tx, mut rx) = broadcast::channel(2);

    tx.send(10).unwrap();
    tx.send(20).unwrap();
    tx.send(30).unwrap();

    // The receiver lagged behind
    assert!(rx.recv().await.is_err());

    // At this point, we can abort or continue with lost messages

    assert_eq!(20, rx.recv().await.unwrap());
    assert_eq!(30, rx.recv().await.unwrap());
}

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pub fn try_recv(&mut self) -> Result<T, TryRecvError>

Attempts to return a pending value on this receiver without awaiting.

This is useful for a flavor of “optimistic check” before deciding to await on a receiver.

Compared with recv, this function has three failure cases instead of two (one for closed, one for an empty buffer, one for a lagging receiver).

Err(TryRecvError::Closed) is returned when all Sender halves have dropped, indicating that no further values can be sent on the channel.

If the Receiver handle falls behind, once the channel is full, newly sent values will overwrite old values. At this point, a call to recv will return with Err(TryRecvError::Lagged) and the Receiver’s internal cursor is updated to point to the oldest value still held by the channel. A subsequent call to try_recv will return this value unless it has been since overwritten. If there are no values to receive, Err(TryRecvError::Empty) is returned.

Examples

use tokio::sync::broadcast;

#[tokio::main]
async fn main() {
    let (tx, mut rx) = broadcast::channel(16);

    assert!(rx.try_recv().is_err());

    tx.send(10).unwrap();

    let value = rx.try_recv().unwrap();
    assert_eq!(10, value);
}