Hot patching systems with subsecond by mockersf · Pull Request #19309 · bevyengine/bevy

mockersf · 2025-05-20T10:50:01Z

Objective

Enable hot patching systems with subsecond
Fixes Support hotpatching systems #19296

Solution

First commit is the naive thin layer
Second commit only check the jump table when the code is hot patched instead of on every system execution
Depends on subsecond: Typed HotFn pointer DioxusLabs/dioxus#4153 for a nicer API, but could be done without
Everything in second commit is feature gated, it has no impact when the feature is not enabled

Testing

Check dependencies without the feature enabled: nothing dioxus in tree
Run the new example: text and color can be changed

github-actions · 2025-05-20T10:58:05Z

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Please review the instructions for writing release notes, then expand or revise the content in the release notes directory to showcase your changes.

examples/ecs/hotpatching_systems.rs

janhohenheim

Consider adding the documentation on how to setup your system (i.e. download CLI, change linker, etc.) from https://github.com/TheBevyFlock/bevy_simple_subsecond_system :)

jakkos-net · 2025-05-20T12:13:36Z

Just tried this on Linux with 2 of my existing projects. All I had to do was enable the feature and run with dx serve --hot-patch and all systems would hot-patch. Great stuff!

crates/bevy_app/src/hotpatch.rs

janhohenheim · 2025-05-20T14:48:19Z

Little update: https://github.com/TheBevyFlock/bevy_simple_subsecond_system now supports changing the signatures of systems at runtime. Would be nice if this PR did so too, but I'm also fine with leaving that for a follow-up :)

MrGVSV · 2025-05-20T17:51:14Z

The future is now

Cargo.toml

examples/ecs/hotpatching_systems.rs

crates/bevy_dev_tools/src/hotpatch.rs

bushrat011899

A couple notes more than anything else. Really awesome to see this working, this will be a massive selling point of 0.17

crates/bevy_ecs/src/system/exclusive_function_system.rs

crates/bevy_ecs/src/system/system.rs

crates/bevy_ecs/src/system/function_system.rs

crates/bevy_dev_tools/src/hotpatch.rs

crates/bevy_ecs/src/lib.rs

janhohenheim · 2025-05-21T03:41:11Z

Update again: I added an opt-in to rerun hot-patched systems to the prototype. This makes it way nicer to edit UIs live, as it means we can just rerun regular setup systems that are typically in OnEnter or Startup. Which breaks their run contracts, but I think that's fine as long as it's opt-in per function.
Not sure if we want to land that in the MVP, just pointing out that this possibility exists :)

crates/bevy_ecs/Cargo.toml

release-content/release-notes/hot_patching.md

Co-authored-by: JMS55 <47158642+JMS55@users.noreply.github.com>

Co-authored-by: Jan Hohenheim <jan@hohenheim.ch>

crates/bevy_dev_tools/src/hotpatch.rs

crates/bevy_ecs/src/lib.rs

cart

I think this is in a good spot, once my minor feedback is addressed. Exciting times!

alice-i-cecile added this to the 0.17 milestone May 20, 2025

alice-i-cecile added X-Needs-SME This type of work requires an SME to approve it. M-Release-Note Work that should be called out in the blog due to impact A-Dev-Tools Tools used to debug Bevy applications. labels May 20, 2025