Struct async_once_cell::OnceFuture

pub struct OnceFuture<T, F = Pin<Box<dyn Future<Output = T> + Send>>, I = Infallible> { /* private fields */ }

A Future which is executed exactly once, producing an output accessible without locking.

This is primarily used as a building block for Lazy and ConstLazy, but can also be used on its own similar to OnceCell.

use std::sync::Arc;
use async_once_cell::OnceFuture;

let shared = Arc::new(OnceFuture::new());
let value : &i32 = shared.get_or_init_with(|| async {
    4
}).await;
assert_eq!(value, &4);

Implementations

impl<T, F, I> OnceFuture<T, F, I>

pub const fn with_init(init: I) -> Self

Creates a new OnceFuture with an initializing value

pub const fn with_no_init() -> Self

Creates a new OnceFuture without an initializing value

The resulting Future must be produced by the closure passed to Self::get_or_init_with. This function is identical to Self::new but is more likely to need type hints.

pub const fn with_value(value: T) -> Self

Creates a new OnceFuture that is immediately ready

pub fn get(&self) -> Option<&T>

Gets the value without blocking or starting the initialization.

pub fn get_mut(&mut self) -> (Option<&mut I>, Option<&mut T>)

Get mutable access to the initializer or final value.

This requires mutable access to self, so rust’s aliasing rules prevent any concurrent access and allow violating the usual rules for accessing this cell.

pub fn into_inner(self) -> (Option<I>, Option<T>)

Gets the initializer or final value

impl<T, F> OnceFuture<T, F>

pub const fn new() -> Self

Creates a new OnceFuture without an initializing value

The resulting Future must be produced by the closure passed to get_or_init_with

impl<F> OnceFuture<F::Output, F> where
    F: Future + Send + 'static, 

pub fn from_future(future: F) -> Self

Creates a new OnceFuture directly from a Future.

The gen_future or into_future closures will never be called.

impl<T, F, I> OnceFuture<T, F, I> where
    F: Future<Output = T> + Send + 'static, 

pub async fn get_or_init_with(&self, gen_future: impl FnOnce() -> F) -> &T

Create and run the future until it produces a result, then return a reference to that result.

This is a convenience wrapper around OnceFuture::get_or_populate_with for use when the initializer value is not used or not present.

pub async fn get_or_populate_with(
    &self,
    into_future: impl FnOnce(Option<I>) -> F
) -> &T

Create and run the future until it produces a result, then return a reference to that result.

Only one into_future closure will be called per OnceFuture instance, and only if the future was not already set by from_future.

pub fn poll_populate(
    &self,
    cx: &mut Context<'_>,
    into_future: impl FnOnce(Option<I>) -> F
) -> Poll<&T>

Create and run the future until it produces a result, then return a reference to that result.

Only one into_future closure will be called per OnceFuture instance, and only if the future was not already set by from_future.

Trait Implementations

impl<T: Debug, F: Debug, I: Debug> Debug for OnceFuture<T, F, I>

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

Formats the value using the given formatter.

impl<T: RefUnwindSafe + UnwindSafe, F, I: RefUnwindSafe> RefUnwindSafe for OnceFuture<T, F, I>

impl<T: Send, F: Send, I: Send> Send for OnceFuture<T, F, I>

impl<T: Sync + Send, F: Send, I: Send> Sync for OnceFuture<T, F, I>

impl<T, F, I> Unpin for OnceFuture<T, F, I>

impl<T: UnwindSafe, F, I: UnwindSafe> UnwindSafe for OnceFuture<T, F, I>