set_runner doesn't work like it did in 0.11 after updating to 0.12

[ilyvion]

Bevy version

0.12

What you did

I have a Bevy plugin crate for using Bevy (mainly the ECS, although I also make use of the Bevy App to orchestrate the whole thing) with another game engine.

In order to support this, I'm using a custom runner, which up until 0.12, would always run before anything else, and so I could do some very early initialization safely there.

bevy/crates/bevy_app/src/app.rs

Lines 312 to 318 in 32a5c7d

	if app.plugins_state() == PluginsState::Ready {
	// If we're already ready, we finish up now and advance one frame.
	// This prevents black frames during the launch transition on iOS.
	app.finish();
	app.cleanup();
	app.update();
	}

👆 Thanks to these lines added between 0.11 and 0.12, I'm now getting panics, because systems get to run (thanks to the call to app.update()) before my custom runner gets to do its required initializations.

What went wrong

This change was very unexpected to me. I thought the runner would be in charge of everything, including running systems at all, but now this privilege has been taken from me, and it breaks everything.

Is there a way to make it so I can turn this new code off in later Bevy releases? I don't care about iOS support and want to be in total control over calls to app.update().

Additional information

If you're interested, this is my custom runner:

https://github.com/alexschrod/bevy_doryen/blob/140d7eb7a84a1ef9b8f064918d83f39e1f4d9508/src/lib.rs#L348-L381

And the reason it's important that only I get to call app.update() is that this needs to happen before and after each such call:

https://github.com/alexschrod/bevy_doryen/blob/140d7eb7a84a1ef9b8f064918d83f39e1f4d9508/src/lib.rs#L258-L260

[tim-blackbird]

You can avoid calling app.set_runner() and app.run() entirely and just manually call your existing runner function.
You may want to call app.finish() and app.cleanup() once before that in case any of your plugins make use of those.

[ilyvion]

I don't know what you mean by "I can avoid" those. Users of my plugin are expected to do

App::new()
    .add_plugins(DoryenPlugin)
    // [and everything else they want to use]
    .run();

like they would any other Bevy project. That's how I want the user experience to be; it's just Bevy coupled with a different engine. And as I pointed out, this has been working just fine for a long time (since at least 0.4 and through to 0.11) and this change realy feels to me like it has "undermined" what runners should have the privilege of being in control over in this stack.

[tim-blackbird]

Sorry about that. I see how it's used now.

It does feel like this change should have happened inside the runner, not outside of it.
I'll take a look at it.

[tim-blackbird]

I've opened #10389 to revert the changes made to App::run(). I'm a bit disappointed that it was unnecessarily changed like that.

Now I'm fairly confident #10389 will be merged, but I'm unsure if it will make it into a 0.12 patch release :(

[ickk]

This also breaks me