Struct future_local_storage::FutureOnceCell

pub struct FutureOnceCell<T>(/* private fields */);

An init-once-per-future cell for thread-local values.

It uses thread local storage to ensure that the each polled future has its own local storage key. Unlike the std::thread::LocalKey this cell will not lazily initialize the value on first access. Instead, the value is first initialized when the future containing the future-local is first polled by an executor.

After the execution finished the value moves from the future local cell to the future output.

Implementations

impl<T> FutureOnceCell<T>

pub const fn new() -> Self

Creates an empty future once cell.

impl<T: Send + 'static> FutureOnceCell<T>

pub fn with<F, R>(&'static self, f: F) -> R

where F: FnOnce(&T) -> R,

Acquires a reference to the value in this future local storage.

Unlike the std::thread::LocalKey::with this method does not initialize the value when called.

Panics

• This method will panic if the future local doesn’t have a value set.

• If you the returned future inside the a call to Self::with on the same cell, then the call to poll will panic.

pub fn get(&'static self) -> T

where T: Copy,

Returns a copy of the contained value.

Panics

This method will panic if the future local doesn’t have a value set.

pub fn scope<F>( &'static self, value: T, future: F ) -> ScopedFutureWithValue<T, F>

where F: Future,

Sets a value T as the future-local value for the future F.

On completion of scope, the future-local value will be returned by the scoped future.

use std::cell::Cell;

use future_local_storage::FutureOnceCell;

static VALUE: FutureOnceCell<Cell<u64>> = FutureOnceCell::new();

#[tokio::main]
async fn main() {
    let (output, answer) = VALUE.scope(Cell::from(0), async {
        VALUE.with(|x| {
            let value = x.get();
            x.set(value + 1);
        });

        42
    }).await;
}

Trait Implementations

impl<T: Debug + Send + 'static> Debug for FutureOnceCell<T>

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

Formats the value using the given formatter.