SubApp::extract cannot be called due to borrow checker

[anlumo]

Bevy version

0.14.0-rc.3

What you did

I'm trying to call extract/update on a single SubApp instead of all at once.

What went wrong

Here is the code:

fn run_sub_app(app: &mut App, label: impl AppLabel) {
    let world = app.world_mut();
    if let Some(sub_app) = app.get_sub_app_mut(label) {
        sub_app.extract(world);
        sub_app.update();
    }
}

• what were you expecting?

I'd like to run extract.

• what actually happened?

Compiler error in the if let line: cannot borrow app mutable more than once, first borrow happens in the line defining the world variable.

I don't see a way to use the extract function at all, since the world I have to pass is always part of the enclosing context, and thus the borrow checker will never allow that.

Additional information

SubApps::update works, because it can do a partial borrow of self, but since I don't have access to SubApps, I can't do the same from outside the bevy_app crate.

This is an inherent limitation of Rust's borrow checker, because it can't do partial borrows on references returned from function calls.

[alice-i-cecile]

Can you give a bit more context on why you're using this pattern? I'd like to remove sub-apps completely, and want a better sense of the advanced use cases.

[anlumo]

I have a UI system (not bevy's!) and want to display bevy views as UI views, and there might be multiples of them. There's a place in the app where I have to tell bevy to render that specific view to wgpu's queue right now (synchronously), which should be achievable by running sub_app.update();. This shouldn't render the other views, only the one I want right now (because there might be UI elements between two views).

It would be nice to have separate worlds for these views. I thought about using completely separate bevy Apps for this, but then I couldn't share entities when necessary. Also, I'd have to set up all systems for every App again and again.

[alice-i-cecile]

Okay, so pretty standard multi-world / entity partitioning stuff. Thanks!

As for this issue, please feel free to make a simple PR to expose an API to do so.

[Soulghost]

I have made a simple PR to expose an API called update_sub_app_by_label to resolve this issue, could you please verify if this is in line with our expectations?

[anlumo]

I have made a simple PR to expose an API called update_sub_app_by_label to resolve this issue, could you please verify if this is in line with our expectations?

Yes, thank you!