Struct OnceLock

pub struct OnceLock<T> { /* private fields */ }

The OnceLock is a synchronization primitive that allows for initializing a value once, and allowing others to .await a reference to the value. This is useful for lazy initialization of a static value.

Note: this implementation uses a busy loop to poll the value, which is not as efficient as registering a dedicated Waker. However, if the usecase for it is to initialize a static variable relatively early in the program life cycle, it should be fine.

Example

use futures_executor::block_on;
use embassy_sync::once_lock::OnceLock;

// Define a static value that will be lazily initialized
static VALUE: OnceLock<u32> = OnceLock::new();

let f = async {

// Initialize the value
let reference = VALUE.get_or_init(|| 20);
assert_eq!(reference, &20);

// Wait for the value to be initialized
// and get a static reference it
assert_eq!(VALUE.get().await, &20);

};
block_on(f)

Implementations

impl<T> OnceLock<T>

pub const fn new() -> Self

Create a new uninitialized OnceLock.

pub async fn get(&self) -> &T

Get a reference to the underlying value, waiting for it to be set. If the value is already set, this will return immediately.

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

Try to get a reference to the underlying value if it exists.

pub fn init(&self, value: T) -> Result<(), T>

Set the underlying value. If the value is already set, this will return an error with the given value.

pub fn get_or_init<F>(&self, f: F) -> &T

where F: FnOnce() -> T,

Get a reference to the underlying value, initializing it if it does not exist.

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

Consume the OnceLock, returning the underlying value if it was initialized.

pub fn take(&mut self) -> Option<T>

Take the underlying value if it was initialized, uninitializing the OnceLock in the process.

pub fn is_set(&self) -> bool

Check if the value has been set.

Trait Implementations

impl<T> Sync for OnceLock<T>