Trait TypedExecutor

Source

pub trait TypedExecutor<T> {
    // Required method
    fn spawn(&mut self, future: T) -> Result<(), SpawnError>;

    // Provided method
    fn status(&self) -> Result<(), SpawnError> { ... }
}

Expand description

A value that spawns futures of a specific type.

The trait is generic over T: the type of future that can be spawened. This is useful for implementing an executor that is only able to spawn a specific type of future.

The [spawn] function is used to submit the future to the executor. Once submitted, the executor takes ownership of the future and becomes responsible for driving the future to completion.

This trait is useful as a bound for applications and libraries in order to be generic over futures that are Send vs. !Send.

§Examples

Consider a function that provides an API for draining a Stream in the background. To do this, a task must be spawned to perform the draining. As such, the function takes a stream and an executor on which the background task is spawned.

#[macro_use]
extern crate futures;
extern crate tokio;

use futures::{Future, Stream, Poll};
use tokio::executor::TypedExecutor;
use tokio::sync::oneshot;

pub fn drain<T, E>(stream: T, executor: &mut E)
    -> impl Future<Item = (), Error = ()>
where
    T: Stream,
    E: TypedExecutor<Drain<T>>
{
    let (tx, rx) = oneshot::channel();

    executor.spawn(Drain {
        stream,
        tx: Some(tx),
    }).unwrap();

    rx.map_err(|_| ())
}

// The background task
pub struct Drain<T: Stream> {
    stream: T,
    tx: Option<oneshot::Sender<()>>,
}

impl<T: Stream> Future for Drain<T> {
    type Item = ();
    type Error = ();

    fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
        loop {
            let item = try_ready!(
                self.stream.poll()
                    .map_err(|_| ())
            );

            if item.is_none() { break; }
        }

        self.tx.take().unwrap().send(()).map_err(|_| ());
        Ok(().into())
    }
}

By doing this, the drain fn can accept a stream that is !Send as long as the supplied executor is able to spawn !Send types.

Required Methods§

Source

fn spawn(&mut self, future: T) -> Result<(), SpawnError>

Spawns a future to run on this executor.

future is passed to the executor, which will begin running it. The executor takes ownership of the future and becomes responsible for driving the future to completion.

§Panics

Implementations are encouraged to avoid panics. However, panics are permitted and the caller should check the implementation specific documentation for more details on possible panics.

§Examples

fn example<T>(my_executor: &mut T)
where
    T: TypedExecutor<MyFuture>,
{
    my_executor.spawn(MyFuture).unwrap();
}

struct MyFuture;

impl Future for MyFuture {
    type Item = ();
    type Error = ();

    fn poll(&mut self) -> Poll<(), ()> {
        println!("running on the executor");
        Ok(().into())
    }
}

Provided Methods§

Source

fn status(&self) -> Result<(), SpawnError>

Provides a best effort hint to whether or not spawn will succeed.

This function may return both false positives and false negatives. If status returns Ok, then a call to spawn will probably succeed, but may fail. If status returns Err, a call to spawn will probably fail, but may succeed.

This allows a caller to avoid creating the task if the call to spawn has a high likelihood of failing.

§Panics

This function must not panic. Implementers must ensure that panics do not happen.

§Examples

fn example<T>(my_executor: &mut T)
where
    T: TypedExecutor<MyFuture>,
{
    if my_executor.status().is_ok() {
        my_executor.spawn(MyFuture).unwrap();
    } else {
        println!("the executor is not in a good state");
    }
}

struct MyFuture;

impl Future for MyFuture {
    type Item = ();
    type Error = ();

    fn poll(&mut self) -> Poll<(), ()> {
        println!("running on the executor");
        Ok(().into())
    }
}