Implement exclusive app hooks

[radiantgurl]

This PR depends on another PR #21239

Objective

• Modifying / accessing SubApps at runtime
• Opening a way for dynamic plugin integration
• Mod loading

Solution

• Exclusive app access hooks

Testing

Ran cargo check
Tested to ensure no runtime errors.

---

Showcase

cargo run --example exclusive_hook

[ChristopherBiscardi]

Code seems fine generally. The std::mem::take and loop of the hashmap happens every update regardless of if there are hooks defined or not.

I'm not aware of why this is x-contentious though, so this is just a review of the code as implemented, not a review of the exclusive hook feature.

I'd love to see an example for this in examples/

[alice-i-cecile]

I've marked this as Contentious because I don't immediately understand the use case, and am not convinced that this is the best way to accomplish these goals.

Better documentation may be the right fix, but this PR also needs end to end tests verifying that this works as intended.

[radiantgurl]

Any meetings or places to discuss it?

I'm currently trying to find out why im getting completely unrelated errors to this code even though i've already tested it.

This code aims to allow better modding support, maybe we can discuss with core team to implement a DynamicPlugin system to better utilize this and allow for dynamic plugin building?

My vision of bevy is a game engine with lots of redundancy and modularity, while allowing to dynamically modify the game engine at runtime without a restart being required, unlike winit's event handler. This would allow us to have a lot more modding support, maybe even make it built into bevy directly.

[radiantgurl]

Also i have no idea how to fix the CI issues considering its not related to my changes.

[james7132]

I'm currently trying to find out why im getting completely unrelated errors to this code even though i've already tested it.

CI was broken with the release of Rust 1.90. Just updated your branch to incorporate the fix..

My vision of bevy is a game engine with lots of redundancy and modularity, while allowing to dynamically modify the game engine at runtime without a restart being required, unlike winit's event handler. This would allow us to have a lot more modding support, maybe even make it built into bevy directly.

Is this not possible with our existing schedule APIs? Schedules already live in the World, and can be mutated at runtime from within an exclusive system. The only thing it's missing is the ability to change out the top level app runner while it is currently running. At the same time, I don't think this is particularly ergonomic right now, butthis does seem like an easy path to crashes/violated logical invariants, and thus shouldn't be explicitly encouraged unless there is a very compelling use case for it.

[radiantgurl]

The only thing it's missing is the ability to change out the top level app runner while it is currently running.

Not only that, it also doesnt allow you to access other subapps which is a shame. This not only allows access to other subapps but also allows modifying and creating new ones, giving further modding possibilities.

this does seem like an easy path to crashes/violated logical invariants

That issue was resolved by making app hooks unaccessible during app runtime, resulting in no violation. What is being done fully respects rust's borrow semantics, with no extra unsafe usage, meaning there should be no issues caused by this.

The only thing i have to add is to turn the hook hashmap into an option and make it panic when you attempt to access it during runtime.

[radiantgurl]

Added a new example cargo run --example exclusive_hook