Struct Select

pub struct Select<'a> { /* private fields */ }

Selects from a set of channel operations.

Select allows you to define a set of channel operations, wait until any one of them becomes ready, and finally execute it. If multiple operations are ready at the same time, a random one among them is selected.

An operation is considered to be ready if it doesn’t have to block. Note that it is ready even when it will simply return an error because the channel is disconnected.

The [select!] macro is a convenience wrapper around Select. However, it cannot select over a dynamically created list of channel operations.

Once a list of operations has been built with Select, there are two different ways of proceeding:

• Select an operation with try_select, select, or select_timeout. If successful, the returned selected operation has already begun and must be completed. If we don’t complete it, a panic will occur.

• Wait for an operation to become ready with try_ready, ready, or ready_timeout. If successful, we may attempt to execute the operation, but are not obliged to. In fact, it’s possible for another thread to make the operation not ready just before we try executing it, so it’s wise to use a retry loop. However, note that these methods might return with success spuriously, so it’s a good idea to always double check if the operation is really ready.

Examples

Use select to receive a message from a list of receivers:

use crossbeam_channel::{Receiver, RecvError, Select};

fn recv_multiple<T>(rs: &[Receiver<T>]) -> Result<T, RecvError> {
    // Build a list of operations.
    let mut sel = Select::new();
    for r in rs {
        sel.recv(r);
    }

    // Complete the selected operation.
    let oper = sel.select();
    let index = oper.index();
    oper.recv(&rs[index])
}

Use ready to receive a message from a list of receivers:

use crossbeam_channel::{Receiver, RecvError, Select};

fn recv_multiple<T>(rs: &[Receiver<T>]) -> Result<T, RecvError> {
    // Build a list of operations.
    let mut sel = Select::new();
    for r in rs {
        sel.recv(r);
    }

    loop {
        // Wait until a receive operation becomes ready and try executing it.
        let index = sel.ready();
        let res = rs[index].try_recv();

        // If the operation turns out not to be ready, retry.
        if let Err(e) = res {
            if e.is_empty() {
                continue;
            }
        }

        // Success!
        return res.map_err(|_| RecvError);
    }
}

Implementations

impl<'a> Select<'a>

pub fn new() -> Select<'a>

Creates an empty list of channel operations for selection.

Examples

use crossbeam_channel::Select;

let mut sel = Select::new();

// The list of operations is empty, which means no operation can be selected.
assert!(sel.try_select().is_err());

pub fn send<T>(&mut self, s: &'a Sender<T>) -> usize

Adds a send operation.

Returns the index of the added operation.

Examples

use crossbeam_channel::{unbounded, Select};

let (s, r) = unbounded::<i32>();

let mut sel = Select::new();
let index = sel.send(&s);

pub fn recv<T>(&mut self, r: &'a Receiver<T>) -> usize

Adds a receive operation.

Returns the index of the added operation.

Examples

use crossbeam_channel::{unbounded, Select};

let (s, r) = unbounded::<i32>();

let mut sel = Select::new();
let index = sel.recv(&r);

pub fn remove(&mut self, index: usize)

Removes a previously added operation.

This is useful when an operation is selected because the channel got disconnected and we want to try again to select a different operation instead.

If new operations are added after removing some, the indices of removed operations will not be reused.

Panics

An attempt to remove a non-existing or already removed operation will panic.

Examples

use crossbeam_channel::{unbounded, Select};

let (s1, r1) = unbounded::<i32>();
let (_, r2) = unbounded::<i32>();

let mut sel = Select::new();
let oper1 = sel.recv(&r1);
let oper2 = sel.recv(&r2);

// Both operations are initially ready, so a random one will be executed.
let oper = sel.select();
assert_eq!(oper.index(), oper2);
assert!(oper.recv(&r2).is_err());
sel.remove(oper2);

s1.send(10).unwrap();

let oper = sel.select();
assert_eq!(oper.index(), oper1);
assert_eq!(oper.recv(&r1), Ok(10));

pub fn try_select(&mut self) -> Result<SelectedOperation<'a>, TrySelectError>

Attempts to select one of the operations without blocking.

If an operation is ready, it is selected and returned. If multiple operations are ready at the same time, a random one among them is selected. If none of the operations are ready, an error is returned.

An operation is considered to be ready if it doesn’t have to block. Note that it is ready even when it will simply return an error because the channel is disconnected.

The selected operation must be completed with SelectedOperation::send or SelectedOperation::recv.

Examples

use crossbeam_channel::{unbounded, Select};

let (s1, r1) = unbounded();
let (s2, r2) = unbounded();

s1.send(10).unwrap();
s2.send(20).unwrap();

let mut sel = Select::new();
let oper1 = sel.recv(&r1);
let oper2 = sel.recv(&r2);

// Both operations are initially ready, so a random one will be executed.
let oper = sel.try_select();
match oper {
    Err(_) => panic!("both operations should be ready"),
    Ok(oper) => match oper.index() {
        i if i == oper1 => assert_eq!(oper.recv(&r1), Ok(10)),
        i if i == oper2 => assert_eq!(oper.recv(&r2), Ok(20)),
        _ => unreachable!(),
    }
}

pub fn select(&mut self) -> SelectedOperation<'a>

Blocks until one of the operations becomes ready and selects it.

Once an operation becomes ready, it is selected and returned. If multiple operations are ready at the same time, a random one among them is selected.

An operation is considered to be ready if it doesn’t have to block. Note that it is ready even when it will simply return an error because the channel is disconnected.

The selected operation must be completed with SelectedOperation::send or SelectedOperation::recv.

Panics

Panics if no operations have been added to Select.

Examples

use std::thread;
use std::time::Duration;
use crossbeam_channel::{unbounded, Select};

let (s1, r1) = unbounded();
let (s2, r2) = unbounded();

thread::spawn(move || {
    thread::sleep(Duration::from_secs(1));
    s1.send(10).unwrap();
});
thread::spawn(move || s2.send(20).unwrap());

let mut sel = Select::new();
let oper1 = sel.recv(&r1);
let oper2 = sel.recv(&r2);

// The second operation will be selected because it becomes ready first.
let oper = sel.select();
match oper.index() {
    i if i == oper1 => assert_eq!(oper.recv(&r1), Ok(10)),
    i if i == oper2 => assert_eq!(oper.recv(&r2), Ok(20)),
    _ => unreachable!(),
}

pub fn select_timeout( &mut self, timeout: Duration, ) -> Result<SelectedOperation<'a>, SelectTimeoutError>

Blocks for a limited time until one of the operations becomes ready and selects it.

If an operation becomes ready, it is selected and returned. If multiple operations are ready at the same time, a random one among them is selected. If none of the operations become ready for the specified duration, an error is returned.

An operation is considered to be ready if it doesn’t have to block. Note that it is ready even when it will simply return an error because the channel is disconnected.

The selected operation must be completed with SelectedOperation::send or SelectedOperation::recv.

Examples

use std::thread;
use std::time::Duration;
use crossbeam_channel::{unbounded, Select};

let (s1, r1) = unbounded();
let (s2, r2) = unbounded();

thread::spawn(move || {
    thread::sleep(Duration::from_secs(1));
    s1.send(10).unwrap();
});
thread::spawn(move || s2.send(20).unwrap());

let mut sel = Select::new();
let oper1 = sel.recv(&r1);
let oper2 = sel.recv(&r2);

// The second operation will be selected because it becomes ready first.
let oper = sel.select_timeout(Duration::from_millis(500));
match oper {
    Err(_) => panic!("should not have timed out"),
    Ok(oper) => match oper.index() {
        i if i == oper1 => assert_eq!(oper.recv(&r1), Ok(10)),
        i if i == oper2 => assert_eq!(oper.recv(&r2), Ok(20)),
        _ => unreachable!(),
    }
}

pub fn select_deadline( &mut self, deadline: Instant, ) -> Result<SelectedOperation<'a>, SelectTimeoutError>

Blocks until a given deadline, or until one of the operations becomes ready and selects it.

If an operation becomes ready, it is selected and returned. If multiple operations are ready at the same time, a random one among them is selected. If none of the operations become ready before the given deadline, an error is returned.

An operation is considered to be ready if it doesn’t have to block. Note that it is ready even when it will simply return an error because the channel is disconnected.

The selected operation must be completed with SelectedOperation::send or SelectedOperation::recv.

Examples

use std::thread;
use std::time::{Instant, Duration};
use crossbeam_channel::{unbounded, Select};

let (s1, r1) = unbounded();
let (s2, r2) = unbounded();

thread::spawn(move || {
    thread::sleep(Duration::from_secs(1));
    s1.send(10).unwrap();
});
thread::spawn(move || s2.send(20).unwrap());

let mut sel = Select::new();
let oper1 = sel.recv(&r1);
let oper2 = sel.recv(&r2);

let deadline = Instant::now() + Duration::from_millis(500);

// The second operation will be selected because it becomes ready first.
let oper = sel.select_deadline(deadline);
match oper {
    Err(_) => panic!("should not have timed out"),
    Ok(oper) => match oper.index() {
        i if i == oper1 => assert_eq!(oper.recv(&r1), Ok(10)),
        i if i == oper2 => assert_eq!(oper.recv(&r2), Ok(20)),
        _ => unreachable!(),
    }
}

pub fn try_ready(&mut self) -> Result<usize, TryReadyError>

Attempts to find a ready operation without blocking.

If an operation is ready, its index is returned. If multiple operations are ready at the same time, a random one among them is chosen. If none of the operations are ready, an error is returned.

An operation is considered to be ready if it doesn’t have to block. Note that it is ready even when it will simply return an error because the channel is disconnected.

Note that this method might return with success spuriously, so it’s a good idea to always double check if the operation is really ready.

Examples

use crossbeam_channel::{unbounded, Select};

let (s1, r1) = unbounded();
let (s2, r2) = unbounded();

s1.send(10).unwrap();
s2.send(20).unwrap();

let mut sel = Select::new();
let oper1 = sel.recv(&r1);
let oper2 = sel.recv(&r2);

// Both operations are initially ready, so a random one will be chosen.
match sel.try_ready() {
    Err(_) => panic!("both operations should be ready"),
    Ok(i) if i == oper1 => assert_eq!(r1.try_recv(), Ok(10)),
    Ok(i) if i == oper2 => assert_eq!(r2.try_recv(), Ok(20)),
    Ok(_) => unreachable!(),
}

pub fn ready(&mut self) -> usize

Blocks until one of the operations becomes ready.

Once an operation becomes ready, its index is returned. If multiple operations are ready at the same time, a random one among them is chosen.

An operation is considered to be ready if it doesn’t have to block. Note that it is ready even when it will simply return an error because the channel is disconnected.

Note that this method might return with success spuriously, so it’s a good idea to always double check if the operation is really ready.

Panics

Panics if no operations have been added to Select.

Examples

use std::thread;
use std::time::Duration;
use crossbeam_channel::{unbounded, Select};

let (s1, r1) = unbounded();
let (s2, r2) = unbounded();

thread::spawn(move || {
    thread::sleep(Duration::from_secs(1));
    s1.send(10).unwrap();
});
thread::spawn(move || s2.send(20).unwrap());

let mut sel = Select::new();
let oper1 = sel.recv(&r1);
let oper2 = sel.recv(&r2);

// The second operation will be selected because it becomes ready first.
match sel.ready() {
    i if i == oper1 => assert_eq!(r1.try_recv(), Ok(10)),
    i if i == oper2 => assert_eq!(r2.try_recv(), Ok(20)),
    _ => unreachable!(),
}

pub fn ready_timeout( &mut self, timeout: Duration, ) -> Result<usize, ReadyTimeoutError>

Blocks for a limited time until one of the operations becomes ready.

If an operation becomes ready, its index is returned. If multiple operations are ready at the same time, a random one among them is chosen. If none of the operations become ready for the specified duration, an error is returned.

An operation is considered to be ready if it doesn’t have to block. Note that it is ready even when it will simply return an error because the channel is disconnected.

Note that this method might return with success spuriously, so it’s a good idea to double check if the operation is really ready.

Examples

use std::thread;
use std::time::Duration;
use crossbeam_channel::{unbounded, Select};

let (s1, r1) = unbounded();
let (s2, r2) = unbounded();

thread::spawn(move || {
    thread::sleep(Duration::from_secs(1));
    s1.send(10).unwrap();
});
thread::spawn(move || s2.send(20).unwrap());

let mut sel = Select::new();
let oper1 = sel.recv(&r1);
let oper2 = sel.recv(&r2);

// The second operation will be selected because it becomes ready first.
match sel.ready_timeout(Duration::from_millis(500)) {
    Err(_) => panic!("should not have timed out"),
    Ok(i) if i == oper1 => assert_eq!(r1.try_recv(), Ok(10)),
    Ok(i) if i == oper2 => assert_eq!(r2.try_recv(), Ok(20)),
    Ok(_) => unreachable!(),
}

pub fn ready_deadline( &mut self, deadline: Instant, ) -> Result<usize, ReadyTimeoutError>

Blocks until a given deadline, or until one of the operations becomes ready.

If an operation becomes ready, its index is returned. If multiple operations are ready at the same time, a random one among them is chosen. If none of the operations become ready before the deadline, an error is returned.

An operation is considered to be ready if it doesn’t have to block. Note that it is ready even when it will simply return an error because the channel is disconnected.

Note that this method might return with success spuriously, so it’s a good idea to double check if the operation is really ready.

Examples

use std::thread;
use std::time::{Duration, Instant};
use crossbeam_channel::{unbounded, Select};

let deadline = Instant::now() + Duration::from_millis(500);

let (s1, r1) = unbounded();
let (s2, r2) = unbounded();

thread::spawn(move || {
    thread::sleep(Duration::from_secs(1));
    s1.send(10).unwrap();
});
thread::spawn(move || s2.send(20).unwrap());

let mut sel = Select::new();
let oper1 = sel.recv(&r1);
let oper2 = sel.recv(&r2);

// The second operation will be selected because it becomes ready first.
match sel.ready_deadline(deadline) {
    Err(_) => panic!("should not have timed out"),
    Ok(i) if i == oper1 => assert_eq!(r1.try_recv(), Ok(10)),
    Ok(i) if i == oper2 => assert_eq!(r2.try_recv(), Ok(20)),
    Ok(_) => unreachable!(),
}

Trait Implementations

impl<'a> Clone for Select<'a>

fn clone(&self) -> Select<'a>

Returns a copy of the value.

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

Performs copy-assignment from source.

impl Debug for Select<'_>

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

Formats the value using the given formatter.

impl<'a> Default for Select<'a>

fn default() -> Select<'a>

Returns the “default value” for a type.

impl Send for Select<'_>

impl Sync for Select<'_>