[−][src]Trait async_std::future::Future

pub trait Future {
    type Output;
    fn poll(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output>;

    fn delay(self, dur: Duration) -> ImplFuture<Self::Output>
    where
        Self: Future + Sized,
    { ... }

    fn flatten(self) -> ImplFuture<<Self::Output as IntoFuture>::Output>
    where
        Self: Future + Sized,
        Self::Output: IntoFuture,
    { ... }

    fn race<F>(self, other: F) -> ImplFuture<Self::Output>
    where
        Self: Future + Sized,
        F: Future<Output = Self::Output>,
    { ... }

    fn try_race<F: Future, T, E>(self, other: F) -> ImplFuture<Self::Output>
    where
        Self: Future<Output = Result<T, E>> + Sized,
        F: Future<Output = Self::Output>,
    { ... }
}

A future represents an asynchronous computation.

A future is a value that may not have finished computing yet. This kind of "asynchronous value" makes it possible for a thread to continue doing useful work while it waits for the value to become available.

The provided methods do not really exist in the trait itself, but they become available when FutureExt from the prelude is imported:

use async_std::prelude::*;

The poll method

The core method of future, poll, attempts to resolve the future into a final value. This method does not block if the value is not ready. Instead, the current task is scheduled to be woken up when it's possible to make further progress by polling again. The context passed to the poll method can provide a Waker, which is a handle for waking up the current task.

When using a future, you generally won't call poll directly, but instead .await the value.

Associated Types

type Output

The type of value produced on completion.

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Required methods

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

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

Return value

This function returns:

Once a future has finished, clients should not poll it again.

When a future is not ready yet, poll returns Poll::Pending and stores a clone of the Waker copied from the current Context. This Waker is then woken once the future can make progress. For example, a future waiting for a socket to become readable would call .clone() on the Waker and store it. When a signal arrives elsewhere indicating that the socket is readable, Waker::wake is called and the socket future's task is awoken. Once a task has been woken up, it should attempt to poll the future again, which may or may not produce a final value.

Note that on multiple calls to poll, only the Waker from the Context passed to the most recent call should be scheduled to receive a wakeup.

Runtime characteristics

Futures alone are inert; they must be actively polled to make progress, meaning that each time the current task is woken up, it should actively re-poll pending futures that it still has an interest in.

The poll function is not called repeatedly in a tight loop -- instead, it should only be called when the future indicates that it is ready to make progress (by calling wake()). If you're familiar with the poll(2) or select(2) syscalls on Unix it's worth noting that futures typically do not suffer the same problems of "all wakeups must poll all events"; they are more like epoll(4).

An implementation of poll should strive to return quickly, and should not block. Returning quickly prevents unnecessarily clogging up threads or event loops. If it is known ahead of time that a call to poll may end up taking awhile, the work should be offloaded to a thread pool (or something similar) to ensure that poll can return quickly.

Panics

Once a future has completed (returned Ready from poll), calling its poll method again may panic, block forever, or cause other kinds of problems; the Future trait places no requirements on the effects of such a call. However, as the poll method is not marked unsafe, Rust's usual rules apply: calls must never cause undefined behavior (memory corruption, incorrect use of unsafe functions, or the like), regardless of the future's state.

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Provided methods

fn delay(self, dur: Duration) -> ImplFuture<Self::Output> where
    Self: Future + Sized, 

This is supported on unstable only.

Returns a Future that delays execution for a specified time.

Examples

use async_std::prelude::*;
use async_std::future;
use std::time::Duration;

let a = future::ready(1).delay(Duration::from_millis(2000));
dbg!(a.await);

fn flatten(self) -> ImplFuture<<Self::Output as IntoFuture>::Output> where
    Self: Future + Sized,
    Self::Output: IntoFuture, 

This is supported on unstable only.

Flatten out the execution of this future when the result itself can be converted into another future.

Examples

use async_std::prelude::*;

let nested_future = async { async { 1 } };
let future = nested_future.flatten();
assert_eq!(future.await, 1);

fn race<F>(self, other: F) -> ImplFuture<Self::Output> where
    Self: Future + Sized,
    F: Future<Output = Self::Output>, 

This is supported on unstable only.

Waits for one of two similarly-typed futures to complete.

Awaits multiple futures simultaneously, returning the output of the first future that completes.

This function will return a new future which awaits for either one of both futures to complete. If multiple futures are completed at the same time, resolution will occur in the order that they have been passed.

Note that this function consumes all futures passed, and once a future is completed, all other futures are dropped.

Examples

use async_std::prelude::*;
use async_std::future;

let a = future::pending();
let b = future::ready(1u8);
let c = future::ready(2u8);

let f = a.race(b).race(c);
assert_eq!(f.await, 1u8);

fn try_race<F: Future, T, E>(self, other: F) -> ImplFuture<Self::Output> where
    Self: Future<Output = Result<T, E>> + Sized,
    F: Future<Output = Self::Output>, 

This is supported on unstable only.

Waits for one of two similarly-typed fallible futures to complete.

Awaits multiple futures simultaneously, returning all results once complete.

try_race is similar to race, but keeps going if a future resolved to an error until all futures have been resolved. In which case an error is returned.

The ordering of which value is yielded when two futures resolve simultaneously is intentionally left unspecified.

Examples

use async_std::prelude::*;
use async_std::future;
use std::io::{Error, ErrorKind};

let a = future::pending::<Result<_, Error>>();
let b = future::ready(Err(Error::from(ErrorKind::Other)));
let c = future::ready(Ok(1u8));

let f = a.try_race(b).try_race(c);
assert_eq!(f.await?, 1u8);
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Implementations on Foreign Types

impl<F: Future + Unpin + ?Sized> Future for Box<F>[src]

type Output = F::Output

impl<'_, F: Future + Unpin + ?Sized> Future for &'_ mut F[src]

type Output = F::Output

impl<F: Future> Future for AssertUnwindSafe<F>[src]

type Output = F::Output

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Implementors

impl<P> Future for Pin<P> where
    P: DerefMut + Unpin,
    <P as Deref>::Target: Future, [src]

type Output = <<P as Deref>::Target as Future>::Output

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