async_cell

The key type of this crate is AsyncCell which can be found in both thread-safe and single-threaded variants. It is intended as a useful async primitive which can replace more expensive channels in a fair number of cases.

AsyncCell<T> behaves a lot like a Cell<Option<T>> that you can await on.

This is used to create futures from a callbacks:

use async_cell::sync::AsyncCell;

let cell = AsyncCell::shared();
let future = cell.take_shared();

thread::spawn(move || cell.set("Hello, World!"));

println!("{}", future.await);

You can also use an async_cell for static variable initialization, wherever the blocking behavior of a OnceCell is unacceptable:

use async_cell::sync::AsyncCell;

// AsyncCell::new() is const!
static GREETING: AsyncCell<String> = AsyncCell::new();

// Read the file on a background thread,
// setting a placeholder value if the thread panics.
thread::spawn(|| {
    let greeting = GREETING.guard("ERROR".to_string());
    let hello = std::fs::read_to_string("tests/hello.txt").unwrap();
    greeting.set(hello);
});

// Do some work while waiting for the file.

// And greet the user!
println!("{}", &GREETING.get().await);

Async cells can also be used to react to the latest value of a variable, since the same cell can be reused as many times as desired. This is one way AsyncCell differs from a one-shot channel:

use async_cell::sync::AsyncCell;

// Allocate space for our timer.
let timer = AsyncCell::<i32>::shared();

// Try to print out the time as fast as it updates.
// Some ticks will be skipped if this loop runs too slowly!
let watcher = timer.take_weak();
spawn(async move {
    while let Some(time) = (&watcher).await {
        println!("Launch in T-{} ticks!", time);
    }
});

// Begin counting down!
for i in (0..60).rev() {
    timer.set(i);
}

Although this crate contains a number of utility functions, you can often make due with just AsyncCell::new, AsyncCell::set, and AsyncCell::take.

Limitations

Cells are not channels! Channels will queue all sent values until a receiver can process them. Readers of a cell will only ever see the most recently written value. As an example, imagine a GUI with a text box. An AsyncCell would be perfect to watch the text content of the box, since it is not necessary to send the whole thing on every keystroke. But the keystrokes themselves must be sent to the box via a channel to avoid any getting lost.

Also avoid using AsyncCell in situations with high contention. Cells block momentarily while cloning values, allocating async callbacks, etc. As a rule of thumb, try to fill cells from one thread or task and empty from one other. Although multiple futures can wait on the same cell, that case is not highly optimized.