Struct neon::types::JsFuture

source ·

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

Available on crate features napi-5 and futures only.

Expand description

A type of JavaScript Promise object that acts as a Future.

Unlike typical Future implementations, JsFutures are eagerly executed because they are backed by a Promise.

§Example

This example uses a JsFuture to take asynchronous binary data and perform potentially expensive computations on that data in a Rust thread.

The example uses a Tokio thread pool (allocated and stored on demand with a OnceCell) to run the computations.

use neon::types::buffer::TypedArray;
use once_cell::sync::OnceCell;
use tokio::runtime::Runtime;

// Lazily allocate a Tokio runtime to use as the thread pool.
fn runtime<'a, C: Context<'a>>(cx: &mut C) -> NeonResult<&'static Runtime> {
    static RUNTIME: OnceCell<Runtime> = OnceCell::new();

    RUNTIME
        .get_or_try_init(Runtime::new)
        .or_else(|err| cx.throw_error(&err.to_string()))
}

// async_compute: Promise<Float64Array> -> Promise<number>
//
// Takes a promise that produces a typed array and returns a promise that:
// - awaits the typed array from the original promise;
// - computes a value from the contents of the array in a background thread; and
// - resolves once the computation is completed
pub fn async_compute(mut cx: FunctionContext) -> JsResult<JsPromise> {
    let nums: Handle<JsPromise> = cx.argument(0)?;

    // Convert the JS Promise to a Rust Future for use in a compute thread.
    let nums = nums.to_future(&mut cx, |mut cx, result| {
        // Get the promise's result value (or throw if it was rejected).
        let value = result.or_throw(&mut cx)?;

        // Downcast the result value to a Float64Array.
        let array: Handle<JsFloat64Array> = value.downcast_or_throw(&mut cx)?;

        // Convert the typed array to a Rust vector.
        let vec = array.as_slice(&cx).to_vec();
        Ok(vec)
    })?;

    // Construct a result promise which will be fulfilled when the computation completes.
    let (deferred, promise) = cx.promise();
    let channel = cx.channel();
    let runtime = runtime(&mut cx)?;

    // Perform the computation in a background thread using the Tokio thread pool.
    runtime.spawn(async move {
        // Await the JsFuture, which yields Result<Vec<f64>, JoinError>.
        let result = match nums.await {
            // Perform the computation. In this example, we just calculate the sum
            // of all values in the array; more involved examples might be running
            // compression or decompression algorithms, encoding or decoding media
            // codecs, image filters or other media transformations, etc.
            Ok(nums) => Ok(nums.into_iter().sum::<f64>()),
            Err(err) => Err(err)
        };
     
        // Resolve the result promise with the result of the computation.
        deferred.settle_with(&channel, |mut cx| {
            let result = result.or_throw(&mut cx)?;
            Ok(cx.number(result))
        });
    });

    Ok(promise)
}