Struct tokio::sync::oneshot::Receiver

source · [−]

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

Expand description

Receives a value from the associated Sender.

A pair of both a Sender and a Receiver are created by the channel function.

This channel has no recv method because the receiver itself implements the Future trait. To receive a value, .await the Receiver object directly.

Examples

use tokio::sync::oneshot;

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

    tokio::spawn(async move {
        if let Err(_) = tx.send(3) {
            println!("the receiver dropped");
        }
    });

    match rx.await {
        Ok(v) => println!("got = {:?}", v),
        Err(_) => println!("the sender dropped"),
    }
}

If the sender is dropped without sending, the receiver will fail with error::RecvError:

use tokio::sync::oneshot;

#[tokio::main]
async fn main() {
    let (tx, rx) = oneshot::channel::<u32>();

    tokio::spawn(async move {
        drop(tx);
    });

    match rx.await {
        Ok(_) => panic!("This doesn't happen"),
        Err(_) => println!("the sender dropped"),
    }
}

To use a Receiver in a tokio::select! loop, add &mut in front of the channel.

use tokio::sync::oneshot;
use tokio::time::{interval, sleep, Duration};

#[tokio::main]
async fn main() {
    let (send, mut recv) = oneshot::channel();
    let mut interval = interval(Duration::from_millis(100));

    tokio::spawn(async move {
        sleep(Duration::from_secs(1)).await;
        send.send("shut down").unwrap();
    });

    loop {
        tokio::select! {
            _ = interval.tick() => println!("Another 100ms"),
            msg = &mut recv => {
                println!("Got message: {}", msg.unwrap());
                break;
            }
        }
    }
}

Implementations

source

impl<T> Receiver<T>

source

pub fn close(&mut self)

Prevents the associated Sender handle from sending a value.

Any send operation which happens after calling close is guaranteed to fail. After calling close, try_recv should be called to receive a value if one was sent before the call to close completed.

This function is useful to perform a graceful shutdown and ensure that a value will not be sent into the channel and never received.

close is no-op if a message is already received or the channel is already closed.

Examples

Prevent a value from being sent

use tokio::sync::oneshot;
use tokio::sync::oneshot::error::TryRecvError;

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

    assert!(!tx.is_closed());

    rx.close();

    assert!(tx.is_closed());
    assert!(tx.send("never received").is_err());

    match rx.try_recv() {
        Err(TryRecvError::Closed) => {}
        _ => unreachable!(),
    }
}

Receive a value sent before calling close

use tokio::sync::oneshot;

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

    assert!(tx.send("will receive").is_ok());

    rx.close();

    let msg = rx.try_recv().unwrap();
    assert_eq!(msg, "will receive");
}

source

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

Attempts to receive a value.

If a pending value exists in the channel, it is returned. If no value has been sent, the current task will not be registered for future notification.

This function is useful to call from outside the context of an asynchronous task.

Return

Ok(T) if a value is pending in the channel.
Err(TryRecvError::Empty) if no value has been sent yet.
Err(TryRecvError::Closed) if the sender has dropped without sending a value.

Examples

try_recv before a value is sent, then after.

use tokio::sync::oneshot;
use tokio::sync::oneshot::error::TryRecvError;

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

    match rx.try_recv() {
        // The channel is currently empty
        Err(TryRecvError::Empty) => {}
        _ => unreachable!(),
    }

    // Send a value
    tx.send("hello").unwrap();

    match rx.try_recv() {
        Ok(value) => assert_eq!(value, "hello"),
        _ => unreachable!(),
    }
}

try_recv when the sender dropped before sending a value

use tokio::sync::oneshot;
use tokio::sync::oneshot::error::TryRecvError;

#[tokio::main]
async fn main() {
    let (tx, mut rx) = oneshot::channel::<()>();

    drop(tx);

    match rx.try_recv() {
        // The channel will never receive a value.
        Err(TryRecvError::Closed) => {}
        _ => unreachable!(),
    }
}