ndk-glue: impossible to poll events in a non-blocking fashion

[nwessing]

it's impossible to poll events in ndk-glue without blocking the thread. In the NDK samples I have seen ALooper is used to poll events with a timeout value that can be configured to block or not. Where the ndk-glue module will read the file descriptor directly with a blocking call. This makes it impossible to program games, or any application that wants to keep running without having to block for events/input.

[MarijnS95]

@nwessing When used with winit you have the option to set ControlFlow to Poll instead of Wait, this will disable blocking on the looper (the one that is set up by ndk-glue before calling main(), and retrieved in winit here).

If you are not using winit you can still do the same, call ThreadLooper::for_thread() within main() to get the looper set up by ndk-glue - the one that contains the reading end of PIPE (the same that is used for blocking reads in poll_events). From there you can call ThreadLooper::poll_once_timeout with a timeout of 0: if it returns Ok(Poll::Event { ident: 0 }) an event is ready to be collected with poll_events().

[MarijnS95]

Related side-note:

@dvc94ch You added the event loop logic to winit, and it seems it's erroneously using the data field - which should be a pointer to user-data - to distinguish the event pipe from the INPUT_QUEUE:

https://github.com/rust-windowing/winit/blob/d1a7749df56656e356840112ebcbb3c9f34de248/src/platform_impl/android/mod.rs#L33-L37

Instead ident should be used there which is also set to 0 for the event pipe and 1 for the INPUT_QUEUE.

Shall I clean that up and send some PRs? Important is to get winit updated first, so that we can properly set this field to std::ptr::null() in ndk-glue after.

[nwessing]

I see, I was using the glue module directly, I am experimenting with using the Oculus Mobile SDK in Rust, and it doesn't make too much sense to use winit since I need to hook into a lot of android specific functionality directly anyways. I will use the winit/android implementation as guide then for how to use the ndk-glue crate. Thanks for pointing me in the right direction!

[dvc94ch]

@MarijnS95 ups, it is always 0 because it was set to std::ptr::null().

[MarijnS95]

@dvc94ch it's actually set to 1 for the input queue which is why it works. Have a commit written up, will submit that in a bit ;)

@nwessing using the links provided in the comment and the winit source should help you on your way, but don't hesitate to ask more questions: ndk-glue is largely undocumented and it'd be great to get some usage feedback!

[MarijnS95]

For reference, this is taken care of in rust-windowing/winit#1826.

[kanerogers]

Chiming in to say that I'm also mucking about with the Oculus Mobile SDK in a similar manner to @nwessing, and some documentation on an easy, non-blocking way to get these events would be marvelous. 😄

[MarijnS95]

@kanerogers Does my comment above and the docstrings introduced in #112 provide that? Let us know if anything is missing, then I guess someone has to work that into a minimal looper example.

[nwessing]

@kanerogers I have this working pretty well at this point. I've defined the following function:

pub const LOOPER_ID_MAIN: u32 = 0;
pub const LOOPER_ID_INPUT: u32 = 1;

pub fn poll_all_ms(block: bool) -> Option<ndk_glue::Event> {
    let looper = ThreadLooper::for_thread().unwrap();
    let result = if block {
        let result = looper.poll_all();
        result
    } else {
        looper.poll_all_timeout(std::time::Duration::from_millis(0u64))
    };

    match result {
        Ok(Poll::Event { ident, .. }) => {
            let ident = ident as u32;
            if ident == LOOPER_ID_MAIN {
                ndk_glue::poll_events()
            } else if ident == LOOPER_ID_INPUT {
                if let Some(input_queue) = ndk_glue::input_queue().as_ref() {
                    while let Some(event) = input_queue.get_event() {
                        if let Some(event) = input_queue.pre_dispatch(event) {
                            input_queue.finish_event(event, false);
                        }
                    }
                }

                None
            } else {
                unreachable!("Unrecognized looper identifer");
            }
        }
        _ => None,
    }
}

And it is called in my game loop like this:

while !app.destroy_requested { // Main game loop
        loop { // event pump loop
            let block = !app.destroy_requested && app.ovr.is_none();
            if let Some(event) = poll_all_ms(block) {
                trace!("event: {:?}", event);
                app.handle_lifecycle_event(&event);
                app.update_vr_mode();
            } else {
                break;
            }
        }    
     // update and render
}

app.ovr is an Option<*mut ovr_sys::ovrMobile> (which is mutated in update_vr_mode) that is set to None when the window is destroyed or the app is in a paused state. (It follows the logic from the VrCubeWorld_NativeActivity Sample.

[kanerogers]

@MarijnS95 your documentation definitely helped! Thanks so much! You are a fantastic maintainer of a fantastic project. ❤️

@nwessing Thanks so much for this! 😄 It looks like we are doing the exact same thing (porting VrCubeWorld_NativeActivity to rust) at the same time - lovely to know I'm not alone.

[MarijnS95]

@kanerogers Neat! I'm hoping to get some remaining PRs reviewed/merged before merging #112 and bumping the minor version this weekend 😁. Will try to stuff in a minimal looper example similar to the code pasted above 🎉!

[kanerogers]

Just chiming in to say this is working perfectly for me! :)

…

On Wed, 27 Jan 2021 at 7:23 pm, Marijn Suijten ***@***.***> wrote: @kanerogers <https://github.com/kanerogers> Neat! I'm hoping to get some remaining PRs reviewed/merged before merging #112 <#112> and bumping the minor version this weekend 😁. Will try to stuff in a minimal looper example similar to the code pasted above 🎉! — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#111 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAPNXZ6ZUIOTD4ABJMJE3F3S37EOZANCNFSM4WBG46IA> .