Allow Bevy to start from non-main threads on supported platforms

[torsteingrindvik]

Objective

Allow Bevy apps to run without requiring to start from the main thread.
This allows other projects and applications to do things like spawning a normal or scoped
thread and run Bevy applications there.

The current behaviour if you try this is a panic.

Solution

Allow this by default on platforms winit supports this behaviour on (x11, Wayland, Windows).

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Changelog

Added

• Added the ability to start Bevy apps outside of the main thread on x11, Wayland, Windows

[mockersf]

not just on macOS, x11 is enabled by default in Bevy on all platforms

[torsteingrindvik]

Thanks, new comment is hopefully more correct

[hymm]

can you add this for windows too?

[torsteingrindvik]

can you add this for windows too?

Added.

Missed it because for me, https://docs.rs/winit/latest/winit/?search=any_thread does not show Windows 🤔
But I see it in the source.

[maniwani]

Yeah, this looks good. Just, users need to make sure any non-send resources get initialized in the same thread the app will live in (i.e. construct the app inside the new thread, don't add plugins in the main thread and then move it there).

[mockersf]

I don't have any of the platforms impacted, but it should work as long as winit works 👍

I think it should be configurable by the user

[torsteingrindvik]

I don't have any of the platforms impacted, but it should work as long as winit works 👍

I think it should be configurable by the user

I initially thought to make it configurable, but then I realized it only enables more flexiblity for the users of the affected platforms.

Since as far as I can tell it has no downsides, I see no point in not allowing any thread to start Bevy on platforms which support it.

Happy to make it configurable if you want to, in that case could you let me know which way to go:

1. Feature flag
2. A resource
3. A field on the winit plugin
4. Something else?

[JMS55]

Imo this should be off by default, because a user may not realize that it works only on the platform they're developing on, and then run into issues later when running on a different platform.

I'm in favor of a field on the winit plugin.

[torsteingrindvik]

Imo this should be off by default, because a user may not realize that it works only on the platform they're developing on, and then run into issues later when running on a different platform.

I'm in favor of a field on the winit plugin.

Good point, changed to this behavior now.

[torsteingrindvik]

@james7132 there was some issues with docs, so changed the doc link slightly but kept it to local symbols.