mirror of
https://github.com/fluencelabs/wasm-bindgen
synced 2025-05-31 22:51:22 +00:00
109 lines
3.8 KiB
Rust
109 lines
3.8 KiB
Rust
use std::cell::RefCell;
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use futures::future;
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use std::fmt;
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use std::rc::Rc;
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use futures::prelude::*;
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use futures::sync::oneshot;
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use js_sys::Promise;
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use wasm_bindgen::prelude::*;
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/// A Rust `Future` backed by a JavaScript `Promise`.
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///
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/// This type is constructed with a JavaScript `Promise` object and translates
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/// it to a Rust `Future`. This type implements the `Future` trait from the
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/// `futures` crate and will either succeed or fail depending on what happens
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/// with the JavaScript `Promise`.
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///
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/// Currently this type is constructed with `JsFuture::from`.
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pub struct JsFuture {
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rx: oneshot::Receiver<Result<JsValue, JsValue>>,
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}
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impl fmt::Debug for JsFuture {
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fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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write!(f, "JsFuture {{ ... }}")
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}
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}
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impl From<Promise> for JsFuture {
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fn from(js: Promise) -> JsFuture {
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// Use the `then` method to schedule two callbacks, one for the
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// resolved value and one for the rejected value. We're currently
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// assuming that JS engines will unconditionally invoke precisely one of
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// these callbacks, no matter what.
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//
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// Ideally we'd have a way to cancel the callbacks getting invoked and
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// free up state ourselves when this `JsFuture` is dropped. We don't
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// have that, though, and one of the callbacks is likely always going to
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// be invoked.
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//
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// As a result we need to make sure that no matter when the callbacks
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// are invoked they are valid to be called at any time, which means they
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// have to be self-contained. Through the `Closure::once` and some
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// `Rc`-trickery we can arrange for both instances of `Closure`, and the
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// `Rc`, to all be destroyed once the first one is called.
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let (tx, rx) = oneshot::channel();
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let state = Rc::new(RefCell::new(None));
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let state2 = state.clone();
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let resolve = Closure::once(move |val| finish(&state2, Ok(val)));
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let state2 = state.clone();
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let reject = Closure::once(move |val| finish(&state2, Err(val)));
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js.then2(&resolve, &reject);
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*state.borrow_mut() = Some((tx, resolve, reject));
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return JsFuture { rx };
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fn finish(
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state: &RefCell<
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Option<(
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oneshot::Sender<Result<JsValue, JsValue>>,
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Closure<dyn FnMut(JsValue)>,
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Closure<dyn FnMut(JsValue)>,
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)>,
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>,
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val: Result<JsValue, JsValue>,
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) {
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match state.borrow_mut().take() {
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// We don't have any guarantee that anyone's still listening at this
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// point (the Rust `JsFuture` could have been dropped) so simply
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// ignore any errors here.
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Some((tx, _, _)) => drop(tx.send(val)),
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None => wasm_bindgen::throw_str("cannot finish twice"),
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}
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}
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}
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}
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impl Future for JsFuture {
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type Item = JsValue;
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type Error = JsValue;
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fn poll(&mut self) -> Poll<JsValue, JsValue> {
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match self.rx.poll() {
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Ok(Async::Ready(val)) => val.map(Async::Ready),
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Ok(Async::NotReady) => Ok(Async::NotReady),
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Err(_) => wasm_bindgen::throw_str("cannot cancel"),
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}
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}
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}
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/// Converts a Rust `Future` on a local task queue.
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///
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/// The `future` provided must adhere to `'static` because it'll be scheduled
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/// to run in the background and cannot contain any stack references.
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///
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/// # Panics
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///
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/// This function has the same panic behavior as `future_to_promise`.
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pub fn spawn_local<F>(future: F)
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where
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F: Future<Item = (), Error = ()> + 'static,
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{
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crate::future_to_promise(
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future
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.map(|()| JsValue::undefined())
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.or_else(|()| future::ok::<JsValue, JsValue>(JsValue::undefined())),
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);
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}
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