Bevy Simple Subsecond System

Hotpatch your Bevy systems, allowing you to change their code while the app is running and directly seeing the results! This is a intermediate solution you can use until Bevy has implement this feature upstream.

⚠️ Should work on Windows somehow, but I haven't yet figured out how! Let me know if you made it!

https://github.com/user-attachments/assets/a44e446b-b2bb-4e10-81c3-3f20cccadea0

First Time Installation

First, we'll install cargo-binstall. It's not strictly required, but it will make the setup much quicker. Click your OS below on instructions for how to do this

Set-ExecutionPolicy Unrestricted -Scope Process; iex (iwr "https://raw.githubusercontent.com/cargo-bins/cargo-binstall/main/install-from-binstall-release.ps1").Content

brew install cargo-binstall

or, if you don't use brew, same as on Linux.

curl -L --proto '=https' --tlsv1.2 -sSf https://raw.githubusercontent.com/cargo-bins/cargo-binstall/main/install-from-binstall-release.sh | bash

cargo install cargo-binstall

Now, we need to install the Dioxus CLI of the newest alpha build:

cargo binstall dioxus-cli@0.7.0-alpha.0

Then make sure you're not using LD as your linker. Click your OS below on instructions for how to do this

In case you already configured a linker, setting rustflags = ["-C", "link-arg=-fuse-ld=/path/to/your/linker"] is surprisingly not enough!

Sorry friend, I didn't test this. All I know is that you can install rust-lld.exe by running

cargo binstall cargo-binutils
rustup component add llvm-tools-preview

You're in luck! The default linker on macOS is already something other than LD. You don't have to change a thing :)

Download clang and mold for your distribution, e.g.

sudo apt-get install clang mold

Then, replace your system ld with a symlink to mold. The most brutal way to do this is:

cd /usr/bin
sudo mv ld ld-real
sudo ln -s mold ld

On NixOS you can do this in a shell by replacing:

pkgs.mkShell {
    # ..
}

with:

pkgs.mkShell.override {
    stdenv = pkgs.stdenvAdapters.useMoldLinker pkgs.clangStdenv;
} {
    # ..
}

Usage

Add the crate to your dependencies:

cargo add bevy_simple_subsecond_system

Then add the plugin to your app:

use bevy::prelude::*;
use bevy_simple_subsecond_system::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_plugins(SimpleSubsecondPlugin::default())
        // rest of the setup
        .run();
}

Now you can annotate your systems with #[hot] to enable hotpatching for them:

use bevy_simple_subsecond_system::prelude::*;

#[hot]
fn greet() {
    info!("Hello from a hotpatched system! Try changing this string while the app is running!")
}

Note that greet is a regular Bevy system, so use whatever parameters you'd like.

After adding the system to your app, run it with

dx serve --hot-patch

Now try changing the string and saving the file while the app is running. If all goes well, it should print your new string!

use bevy::prelude::*;
use bevy_simple_subsecond_system::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_plugins(SimpleSubsecondPlugin::default())
        .add_systems(Update, greet)
        .run();
}

#[hot]
fn greet() {
    info!("Hello from a hotpatched system! Try changing this string while the app is running!")
}

Examples

Run the examples with

dx serve --hot-patch --example name_of_the_example

e.g.

dx serve --hot-patch --example patch_on_update

Known Limitations

Cannot combine mold as your Rust linker with a global target dir
Using this breaks dynamic linking
A change in the definition of structs that appear in hot-patched systems at runtime will result in your query failing to match, as that new type does not exist in World yet.
- Practically speaking, this means you should not change the definition of Resources and Components of your system at runtime
All hotpatched systems run as exclusive systems, meaning they won't run in parallel
Only the topmost binary is hotpatched, meaning your app is not allowed to have a lib.rs or a workspace setup.
Attaching a debugger is problaby not going to work. Let me know if you try!
I did not test all possible ways in which systems can be used. Does piping work? Does bevy_mod_debugdump still work? Probably. Let me know!
Return values like -> bevy::prelude::Result are currently unsupported. That is not a technical limitation, but I couldn't make the macro happy.

Compatibility