Struct Receiver

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

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 Result<T, error::RecvError>, .await the Receiver object directly.

The poll method on the Future trait is allowed to spuriously return Poll::Pending even if the message has been sent. If such a spurious failure happens, then the caller will be woken when the spurious failure has been resolved so that the caller can attempt to receive the message again. Note that receiving such a wakeup does not guarantee that the next call will succeed — it could fail with another spurious failure. (A spurious failure does not mean that the message is lost. It is just delayed.)

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

impl<T> Receiver<T>

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");
}

pub fn is_terminated(&self) -> bool

Checks if this receiver is terminated.

This function returns true if this receiver has already yielded a Poll::Ready result. If so, this receiver should no longer be polled.

Examples

Sending a value and polling it.

use tokio::sync::oneshot;

use std::task::Poll;

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

    // A receiver is not terminated when it is initialized.
    assert!(!rx.is_terminated());

    // A receiver is not terminated it is polled and is still pending.
    let poll = futures::poll!(&mut rx);
    assert_eq!(poll, Poll::Pending);
    assert!(!rx.is_terminated());

    // A receiver is not terminated if a value has been sent, but not yet read.
    tx.send(0).unwrap();
    assert!(!rx.is_terminated());

    // A receiver *is* terminated after it has been polled and yielded a value.
    assert_eq!((&mut rx).await, Ok(0));
    assert!(rx.is_terminated());
}

Dropping the sender.

use tokio::sync::oneshot;

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

    // A receiver is not immediately terminated when the sender is dropped.
    drop(tx);
    assert!(!rx.is_terminated());

    // A receiver *is* terminated after it has been polled and yielded an error.
    let _ = (&mut rx).await.unwrap_err();
    assert!(rx.is_terminated());
}

pub fn is_empty(&self) -> bool

Checks if a channel is empty.

This method returns true if the channel has no messages.

It is not necessarily safe to poll an empty receiver, which may have already yielded a value. Use is_terminated() to check whether or not a receiver can be safely polled, instead.

Examples

Sending a value.

use tokio::sync::oneshot;

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

    tx.send(0).unwrap();
    assert!(!rx.is_empty());

    let _ = (&mut rx).await;
    assert!(rx.is_empty());
}

Dropping the sender.

use tokio::sync::oneshot;

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

    // A channel is empty if the sender is dropped.
    drop(tx);
    assert!(rx.is_empty());

    // A closed channel still yields an error, however.
    (&mut rx).await.expect_err("should yield an error");
    assert!(rx.is_empty());
}

Terminated channels are empty.

use tokio::sync::oneshot;

#[tokio::main]
async fn main() {
    let (tx, mut rx) = oneshot::channel();
    tx.send(0).unwrap();
    let _ = (&mut rx).await;

    // NB: an empty channel is not necessarily safe to poll!
    assert!(rx.is_empty());
    let _ = (&mut rx).await;
}

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.

Note that unlike the poll method, the try_recv method cannot fail spuriously. Any send or close event that happens before this call to try_recv will be correctly returned to the caller.

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, or if the message has already been received.

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!(),
    }
}

pub fn blocking_recv(self) -> Result<T, RecvError>

Blocking receive to call outside of asynchronous contexts.

Panics

This function panics if called within an asynchronous execution context.

Examples

use std::thread;
use tokio::sync::oneshot;

#[tokio::main]
async fn main() {
    let (tx, rx) = oneshot::channel::<u8>();

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

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

Trait Implementations

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.

impl<T> Future for Receiver<T>

type Output = Result<T, RecvError>

The type of value produced on completion.

fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>

Attempts to resolve the future to a final value, registering the current task for wakeup if the value is not yet available.