Function futures_util::future::select
source · pub fn select<A, B>(future1: A, future2: B) -> Select<A, B> ⓘ
Expand description
Waits for either one of two differently-typed futures to complete.
This function will return a new future which awaits for either one of both futures to complete. The returned future will finish with both the value resolved and a future representing the completion of the other work.
Note that this function consumes the receiving futures and returns a wrapped version of them.
Also note that if both this and the second future have the same
output type you can use the Either::factor_first
method to
conveniently extract out the value at the end.
Examples
A simple example
use futures::{
pin_mut,
future::Either,
future::self,
};
// These two futures have different types even though their outputs have the same type.
let future1 = async {
future::pending::<()>().await; // will never finish
1
};
let future2 = async {
future::ready(2).await
};
// 'select' requires Future + Unpin bounds
pin_mut!(future1);
pin_mut!(future2);
let value = match future::select(future1, future2).await {
Either::Left((value1, _)) => value1, // `value1` is resolved from `future1`
// `_` represents `future2`
Either::Right((value2, _)) => value2, // `value2` is resolved from `future2`
// `_` represents `future1`
};
assert!(value == 2);
A more complex example
use futures::future::{self, Either, Future, FutureExt};
// A poor-man's join implemented on top of select
fn join<A, B>(a: A, b: B) -> impl Future<Output=(A::Output, B::Output)>
where A: Future + Unpin,
B: Future + Unpin,
{
future::select(a, b).then(|either| {
match either {
Either::Left((x, b)) => b.map(move |y| (x, y)).left_future(),
Either::Right((y, a)) => a.map(move |x| (x, y)).right_future(),
}
})
}