Struct async_std::task::LocalKey

pub struct LocalKey<T: Send + 'static> { /* fields omitted */ }

The key for accessing a task-local value.

Every task-local value is lazily initialized on first access and destroyed when the task completes.

Implementations

impl<T: Send + 'static> LocalKey<T>

pub fn with<F, R>(&'static self, f: F) -> R where
    F: FnOnce(&T) -> R, 

Gets a reference to the task-local value with this key.

The passed closure receives a reference to the task-local value.

The task-local value will be lazily initialized if this task has not accessed it before.

Panics

This function will panic if not called within the context of a task created by block_on, spawn, or Builder::spawn.

Examples

use std::cell::Cell;

use async_std::task;
use async_std::prelude::*;

task_local! {
    static NUMBER: Cell<u32> = Cell::new(5);
}

task::block_on(async {
    let v = NUMBER.with(|c| c.get());
    assert_eq!(v, 5);
});

pub fn try_with<F, R>(&'static self, f: F) -> Result<R, AccessError> where
    F: FnOnce(&T) -> R, 

Attempts to get a reference to the task-local value with this key.

The passed closure receives a reference to the task-local value.

The task-local value will be lazily initialized if this task has not accessed it before.

This function returns an error if not called within the context of a task created by block_on, spawn, or Builder::spawn.

Examples

use std::cell::Cell;

use async_std::task;
use async_std::prelude::*;

task_local! {
    static VAL: Cell<u32> = Cell::new(5);
}

task::block_on(async {
    let v = VAL.try_with(|c| c.get());
    assert_eq!(v, Ok(5));
});

// Returns an error because not called within the context of a task.
assert!(VAL.try_with(|c| c.get()).is_err());

Trait Implementations

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

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

Formats the value using the given formatter.