Introduce mutable version of ChangedRes<>

Currently if I want to a) execute a system only if a certain resource has been modified and b) to modify that resource in that system, I can do something like this:

pub fn my_system(
    _: ChangedRes<MyRes>,
    mut my_resource: ResMut<MyRes>,
) {
//...
}

It would be nice to be able to use just one parameter instead, like this:

pub fn my_system(
    mut my_resource: ChangedResMut<MyRes>,
) {
//...
}

wait that first code snippet works... Isn't that totally UB by providing a &MyRes and an &mut MyRes??

use bevy::prelude::*;

fn main() {
    App::build()
        .add_default_plugins()
        .add_startup_system(startup.system())
        .add_system(ub.system())
        .run();
}

#[derive(Debug)]
pub struct MyRes(f32);

pub fn startup(mut commands: Commands) {
    commands.insert_resource(MyRes(0.));
}

pub fn ub(res_1: Res<MyRes>, mut res_2: ResMut<MyRes>) {
    dbg!(&*res_1);
    res_2.0 += 2.;
    dbg!(&*res_1);
}

Welp this prints out an ever increasing number so I'm gonna go ahead and say this is UB unless I have a fundamental misunderstanding of how resources work x]

@EllenNyan I am not a big expert in smart pointers, but isn't the logic in your example the same as in the following?

use std::cell::RefCell;
use std::rc::Rc;

fn main() {
    let res_1 = Rc::new(RefCell::new(0.));
    let res_2 = Rc::clone(&res_1);
    
    println!("res_1 before mutation = {:?}", *res_1);
    println!("res_2 before mutation = {:?}", *res_2);

    *res_2.borrow_mut() += 2.;

    println!("res_1 after mutation = {:?}", *res_1);
    println!("res_2 after mutation = {:?}", *res_2);
}

At least I would intuitively expect resources to work somewhat like this.

Resources in bevy aren't using Rc<RefCell<T>>, Res<T> internally just contains an &'a T, ResMut<T> stores a *mut T internally.

For what it's worth, for each systems have the same issue.

pub fn ub_2(mut left: Mut<usize>, right: &usize) {
    dbg!(right);
    *left += 2;
    dbg!(right);
}

this will print out a constantly ascending number :)

FWIW, this doesn't panic and does go infinite:

use bevy::prelude::*;

fn main() {
    App::build()
        .add_default_plugins()
        .add_startup_system(startup.system())
        .add_system(ub.system())
        .run();
}

pub struct MyRes(Vec<usize>);

pub fn startup(mut commands: Commands) {
    commands.insert_resource(MyRes(vec![0]));
}

pub fn ub(res_1: Res<MyRes>, mut res_2: ResMut<MyRes>) {
    dbg!("start of the system");
    for i in 0..res_1.0.len() {
        dbg!(res_1.0[i]);
        res_2.0.push(i);
    }
}

This should be solved by #1313.

@TheRawMeatball

Double-checking: @TheRawMeatball is this resolved now?

Yes, it is. Closing.

Update petgraph requirement from 0.6 to 0.7 (#18224)

Navigation Menu

Repository navigation

Introduce mutable version of ChangedRes<> #598

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Introduce mutable version of ChangedRes<> #598

Description

Activity

BoxyUwU commented on Sep 29, 2020

BoxyUwU commented on Sep 29, 2020

ropewalker commented on Sep 29, 2020

BoxyUwU commented on Sep 29, 2020

ropewalker commented on Sep 29, 2020

alice-i-cecile commented on Feb 17, 2021

alice-i-cecile commented on Mar 7, 2021

TheRawMeatball commented on Mar 7, 2021

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions