Custom loop runner

[codec-abc]

Bevy version

0.4.0

Operating system & version

Windows 10 and building for WASM

What you did

I tried to use a custom runner for Bevy under WASM. I had a simple program that worked until I tried to modify the code use a custom runner as shown in this sample. A repro project is here. Be cautious: the master branch compile and run just fine while the custom_loop branches contains the change to use a custom runner. And while it compiles it crashes at runtime.

What you expected to happen

Not to crash when using a custom loop runner.

What actually happened

The engine crashed when using a custom

Additional information

Nothing really more to say, except the original goal was to have some kind of interop between Seed (a Rust frontend framework) and Bevy. My goal was to add some HTML UI that can drove the engine and vice versa.

[rparrett]

#1255 strikes again?

It looks like set_runner needs to be called earlier. (Before bevy_webgl2?)

[codec-abc]

It seems like changing order of plugin does fix the problem. About the order it seems it need to be set even before add_plugins(DefaultPlugins) otherwise it still crash.

[codec-abc]

I checked the solution given by @rparrett but I don't think it solves the problem. If I put log in the runner function it doesn't show up. Also the call to app.run() does not return control. I did not understand the code and logic behind custom runner yet but if it works exactly the same with or without it, this doesn't seem right.

[rparrett]

It looks like bevy_winit via DefaultPlugins sets its own runner that probably overwrites the one you are setting.

It's not clear to me why you wouldn't be able to override that one by calling set_runner after DefaultPlugins though. (But sure enough, that panics)

The bevy_winit runner does a bunch of seemingly useful stuff like handling input events, so overriding may or may not be the way to go anyhow depending on your requirements.

[codec-abc]

Actually what I want is to be able to run the main event loop of the engine myself so my Rust web app can control it. After trying a simpler program it seems to work by just retrieving the App from the AppBuilder and calling update on it. But it only works if I remove the default, rendering and input systems. At which point I have a engine that cannot do rendering nor process input which isn't really useful. So I believe there are 2 issues here:

1- The custom loop is not taken into account if it registered first.
2- Whether we use a custom loop or do the "running" of the main engine loop ourselves, the engine crash complaining about any resources.

Do you share this analysis and do you see a way to make progress from here?

[codec-abc]

Okay after digging some more and a lot of help from the Discord people, the reason why it is crashing is because if I use my own runner I overwrite the one set by winit which take care to manage the rendering context, the input is so on.

So basically, setting a custom loop runner work as intended but I cannot run a Bevy instance externally in a WASM context I need winit to set up a few things to get the 3D, input and so on to work. Unfortunately, I didn't found a way to let the winit plugin yield control to me so I can run the event loop myself.

[CleanCut]

(I came to this issue via #1729 ...)

So in essence, you can't actually use a custom runner unless you omit most of Bevy's functionality, because adding a custom runner replaces the custom winit runner that processes rendering and input (just minor stuff 😝).

What's a good way to solve this?

The only thing that comes to my mind is to find some way to add new logic, rather than replace it. Think: a chain of functions to be run in sequence by the runner (rather than replacing the runner). Would that be a viable approach? Are there better ways to solve this need?

[bjorn3]

When using winit, you have to let winit drive the event loop. On many platforms the event loop can't return without tearing it down conpletely or exiting the process. On macOS for example a new NSApplication is created with the given event callback and then it's run method is invoked. This method doesn't finish until the event loop exits completely: https://github.com/rust-windowing/winit/blob/86748fbc681ea070bc37e90a569c192555b402cd/src/platform_impl/macos/event_loop.rs#L176

[bjorn3]

The only thing that comes to my mind is to find some way to add new logic, rather than replace it. Think: a chain of functions to be run in sequence by the runner (rather than replacing the runner). Would that be a viable approach? Are there better ways to solve this need?

That could work I think.

[zicklag]

The only thing that comes to my mind is to find some way to add new logic, rather than replace it. Think: a chain of functions to be run in sequence by the runner (rather than replacing the runner). Would that be a viable approach? Are there better ways to solve this need?

Or maybe the runner could receive a closure of vector of closures/function pointers that it is responsible for running every frame? So your runner would be responsible for driving the winit event loop?

[CleanCut]

Hmm. Given that (at least on macOS) the native window function (NSApplication::run()) never returns between frames...and runs its own event loop, I don't see how we could possibly drive the event loop externally, by definition! If that's true, perhaps the custom runner functionality should be documented as "only works without a window" and this issue should be closed.

I think in my case I can reconsider my approach and set up a bevy system to callback into my external application every frame and sync any info it needs to sync, rather than attempting to drive Bevy from the external application.

[CleanCut]

I got my workaround concept functioning (external, internal), though I had to resort to a lazy_static!{} to pass logic to Bevy systems to execute.

If anyone's curious I'm working on a wrapper over Bevy to simplify the API down to as close to plain Rust concepts as possible (sacrificing performance and features along the way, but adding built-in assets and systems). It's intended to be a simplified game engine for folks just starting to learn Rust to use as a library for practice projects without the need to learn complicated game engine concepts like ECS, etc. It should be a more interesting approach to learning than the old typical "let's make a to do list" or "let's make an account record" project.

I'll stop hijacking the thread now. 😆

[politrons]

my requirement is to run the event loop but not in the main thread, since this main thread is already used by iOS. Amy idea how complex it would be to configure the wind event loop to run in another thread?