Async/Future support

UniFFI supports exposing async Rust functions over the FFI. It can convert a Rust Future/async fn to and from foreign native futures (async/await in Python/Swift, suspend fun in Kotlin etc.)

Check out the examples or the more terse and thorough fixtures.


This is a short "async sleep()" example:

use std::time::Duration;
use async_std::future::{timeout, pending};

/// Async function that says something after a certain time.
pub async fn say_after(ms: u64, who: String) -> String {
    let never = pending::<()>();
    timeout(Duration::from_millis(ms), never).await.unwrap_err();
    format!("Hello, {who}!")

This can be called by the following Python code:

import asyncio
from uniffi_example_futures import *

async def main():
    print(await say_after(20, 'Alice'))

if __name__ == '__main__':

Async functions can also be defined in UDL:

namespace example {
    string say_after(u64 ms, string who);

This code uses asyncio to drive the future to completion, while our exposed function is used with await.

In Rust Future terminology this means the foreign bindings supply the "executor" - think event-loop, or async runtime. In this example it's asyncio. There's no requirement for a Rust event loop.

There are some great API docs on the implementation that are well worth a read.

See the foreign-executor fixture for more implementation details.