[Question] Calling dart code from sync rust

[gabrielDevlog]

First of all, thank you for your work — flutter_rust_bridge is fantastic! I have a question about how to call a Dart function from Rust code. I suspect there's something I'm not understanding correctly about async behavior in Rust.

I have an async Rust function that downloads a file and reports its progress via a synchronous callback:

pub async fn download<F>(callback: F) where F: FnMut(u32, u32);

In my Flutter Rust app, I'm trying to call it like this:

pub async fn download(
    dart_callback: impl Fn(u32, u32) -> DartFnFuture<()>,
) {
    let progress_callback = |current: u32, total: u32| {
        dart_callback(current, total).await; // <- this is where I'm stuck
    };

    myRustPackage
        .download(progress_callback)
        .await?;
}

This is pseudo-code, but it shows the core idea. The download itself works, but dart_callback is never called. How can I call this async Dart function from within synchronous Rust code? Or is there a better/different way to handle this when using flutter_rust_bridge?

Thanks in advance!

[fzyzcjy]

You are welcome!

For progress_callback, it seems it accepts a sync function instead of async function. So dart_callback(current, total).await; will not trigger the real operation (which is async).

You may need to search "how to make async function become sync" etc.

[anlumo]

You could use flutter_rust_bridge::spawn to run the async function within the sync closure. Just keep in mind that things might get messed up when it's called rapidly (like calls going out of order).

[fzyzcjy]

Close since there seems to be an answer, but feel free to reopen if there are any questions!

[fzyzcjy]

Btw it is impossible to call Dart from sync Rust and require sync Dart output, b/c suppose Rust is on a thread other than main thread, then Dart is on the main thread - a different thread, busy doing some other task

[github-actions[bot]]

This thread has been automatically locked since there has not been any recent activity after it was closed. If you are still experiencing a similar issue, please open a new issue.