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

This count includes messages that have already been overwritten in the ring buffer and are no longer readable. If len is greater than the channel's effective capacity (the provided capacity rounded up to the next power of two), the next call to recv returns Err(RecvError::Lagged) and the next call to try_recv returns Err(TryRecvError::Lagged). For example, with channel(10) the buffer length is 16, so lagging begins once len is larger than 16.

After a successful receive (including after handling Lagged and then reading retained messages), len decreases accordingly.

Examples

use tokio::sync::broadcast;

# #[tokio::main(flavor = "current_thread")]
# 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);
# }

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(flavor = "current_thread")]
# 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());
# }

Returns true if receivers belong to the same channel.

Examples

use tokio::sync::broadcast;

# #[tokio::main(flavor = "current_thread")]
# async fn main() {
let (tx, rx) = broadcast::channel::<()>(16);
let rx2 = tx.subscribe();

assert!(rx.same_channel(&rx2));

let (_tx3, rx3) = broadcast::channel::<()>(16);

assert!(!rx3.same_channel(&rx2));
# }

Returns the number of Sender handles.

Returns the number of WeakSender handles.

Checks if a channel is closed.

This method returns true if the channel has been closed. The channel is closed when all Sender have been dropped.

Examples

use tokio::sync::broadcast;

# #[tokio::main(flavor = "current_thread")]
# async fn main() {
let (tx, rx) = broadcast::channel::<()>(10);
assert!(!rx.is_closed());

drop(tx);

assert!(rx.is_closed());
# }

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(flavor = "current_thread")]
# 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);
# }

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 overwrite old values in the ring buffer. The next call to recv then returns Err(RecvError::Lagged(n)), where n is the number of overwritten messages the receiver missed. The receiver stays subscribed; its internal cursor is advanced to the oldest value still held by the channel. A subsequent call to recv returns that value, unless further sends overwrite it before the receiver reads it. See lagging for details.

Cancel safety

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

Examples

use tokio::sync::broadcast;

# #[tokio::main(flavor = "current_thread")]
# 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;
use tokio::sync::broadcast::error::RecvError;

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

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

// One message was overwritten before this receiver could read it.
assert!(matches!(rx.recv().await, Err(RecvError::Lagged(1))));

// Resume from the oldest retained message, or abort the task instead.
assert_eq!(20, rx.recv().await.unwrap());
assert_eq!(30, rx.recv().await.unwrap());
# }

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 overwrite old values in the ring buffer. The next call to try_recv then returns Err(TryRecvError::Lagged(n)), where n is the number of overwritten messages the receiver missed. The receiver stays subscribed; its internal cursor is advanced to the oldest value still held by the channel. A subsequent call to try_recv returns that value, unless further sends overwrite it before the receiver reads it. If there are no values to receive, Err(TryRecvError::Empty) is returned. See lagging for details.

Examples

use tokio::sync::broadcast;

# #[tokio::main(flavor = "current_thread")]
# 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);
# }

Blocking receive to call outside of asynchronous contexts.

Panics

This function panics if called within an asynchronous execution context.

Examples

# #[cfg(not(target_family = "wasm"))]
# {
use std::thread;
use tokio::sync::broadcast;

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

    let sync_code = thread::spawn(move || {
        assert_eq!(rx.blocking_recv(), Ok(10));
    });

    let _ = tx.send(10);
    sync_code.join().unwrap();
}
# }