winitrequires control over the principal class. You must create the event loop before other parts of your application initialize NSApplication

[dezbracaty]

Description

Background

I built a program combine Qt/Qml and bevy , and I can run it normally on ubuntu system , but when run it on macos, it output a error, and I think this is a platform related problem. So I hope to know how to solve it.

project address

https://github.com/dezbracaty/BevyQml

Information

After running App::new() .add_plugins(DefaultPlugins) .add_systems(Startup, setup) .add_systems(Update, animate_cube) .run();
It crash and output :
2024-07-02T05:56:27.686954Z INFO bevy_diagnostic::system_information_diagnostics_plugin::internal: SystemInfo { os: "MacOS 15.0 ", kernel: "24.0.0", cpu: "Apple M1 Pro", core_count: "10", memory: "16.0 GiB" } thread '' panicked at /Users/allen/.cargo/registry/src/index.crates.io-6f17d22bba15001f/winit-0.30.3/src/platform_impl/macos/event_loop.rs:220:13: winitrequires control over the principal class. You must create the event loop before other parts of your application initialize NSApplication note: run withRUST_BACKTRACE=1 environment variable to display a backtrace fatal runtime error: failed to initiate panic, error 5 [1] 33997 abort /Users/allen/Projects/RustProjects/BevyQml/build/bevyQml/example_qml_minimal

macOS version

Macos 15 beta2
rustc 1.79.0 (129f3b996 2024-06-10)

Winit version

cargo tree -i winit
winit v0.30.3
├── accesskit_winit v0.21.1
│ └── bevy_winit v0.14.0-dev (https://github.com/bevyengine/bevy?branch=main#f607be87)
│ └── bevy_internal v0.14.0-dev (https://github.com/bevyengine/bevy?branch=main#f607be87)
│ └── bevy v0.14.0-dev (https://github.com/bevyengine/bevy?branch=main#f607be87)
│ └── qml_minimal v0.1.0 (/Users/allen/Projects/RustProjects/BevyQml/bevyQml/rust)
└── bevy_winit v0.14.0-dev (https://github.com/bevyengine/bevy?branch=main#f607be87) (*)

[PaulDance]

I've encountered this recently and found a solution for my case that involved using Winit through Slint and tray-icon.

Under macOS, the windowing event loop must be started from the main thread, as otherwise AppKit crashes the process. Furthermore, it must also be started before any other such constructs, as otherwise Winit crashes the process on a panic here:

winit/src/platform_impl/apple/appkit/event_loop.rs

Lines 212 to 223 in dfea49f

	let mtm = MainThreadMarker::new()
	.expect("on macOS, `EventLoop` must be created on the main thread!");
	let app: Retained<NSApplication> =
	unsafe { msg_send_id![WinitApplication::class(), sharedApplication] };
	if !app.is_kind_of::<WinitApplication>() {
	panic!(
	"`winit` requires control over the principal class. You must create the event \
	loop before other parts of your application initialize NSApplication"
	);
	}

where it checks that the returned class is the correct one instead of working on incorrect data, most probably due to AppKit global state.

I was able to fix my case by ensuring Winit's event loop was always started first and before tray-icon's. A similar solution should be possible for you: try to find out what is the other event loop that is running. Temporarily patching Winit locally in order to display context before the aforementioned panic should help for example.

If the other event loop is out of your control, it could be a Bevy-specific issue. I also see that you're using macOS 15 and maybe Winit does not support it yet as it's still in beta: have you tried it under 14.6?

[madsmtm]

Indeed, the issue is that something is started before EventLoop::new, which means we no longer have control over the NSApplication. I plan to fix this so that Winit doesn't need control over the NSApplication.