Using egui with async functions

[Speak2Erase]

I've been working on an egui app for a few days now and I've run into an annoying hitch, So far, I've been targeting both native and wasm32 using eframe, with the goal of using the new JavaScript File System Access API to handle IO. Unfortunately for me, all the functions in it are async, which poses a problem for me since all my code is synchronous.

I've also noticed that WASI has functions for filesystem, which would be amazing to use, but I don't know how I could get that to work with trunk or the eframe tamplate.

I'd ideally like to keep it synchronous, but sadly it seems that isn't possible. I saw this thread which suggests using Pollster but that ends up panicking with condvar wait not supported.

What should I do?

[notgull]

If you enable target_feature = "atomics", that should ensure condvars work, at least on web environments.

[Speak2Erase]

I tried that actually, and it didn't work. It panics with another error Atomics.wait cannot be called in this context which is presumably from blocking the main thread.

[emilk]

Have you tried wasm_bindgen_futures ?

[emilk]

Use Promise::spawn_async with the web feature as covered by the docs: https://docs.rs/poll-promise/latest/poll_promise/#web

[Speak2Erase]

Bit late, but that's what I've been doing!

[jonboh]

@emilk did you mean Promise::spawn_local? Promise::spawn_async is only available on async-std, tokio and smol

[Lubba-64]

Ran into this in 2024. ouch.

[Lubba-64]

Nevermind, for anyone stumbling across this thread, the best I could do to extract the value out of the spawn_local context was to use this pattern:

#[cfg(target_arch = "wasm32")]
pub struct Task<T>(Rc<Cell<Option<thread::Result<T>>>>);

#[cfg(target_arch = "wasm32")]
impl<T: 'static> Task<T> {
    pub fn spawn<F: 'static + Future<Output = T>>(future: F) -> Self {
        let sender = Rc::new(Cell::new(None));
        let receiver = sender.clone();
        wasm_bindgen_futures::spawn_local(async move {
            let future = panic::AssertUnwindSafe(future).catch_unwind();
            sender.set(Some(future.await));
        });
        Self(receiver)
    }
    pub fn take_output(&self) -> Option<thread::Result<T>> {
        self.0.take()
    }
}

This sucks but it works so I am moving on with my life.

[ar37-rs]

👍