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Blacksite/docs/adr/0004-editor-hot-reload.md
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2026-06-05 21:44:45 -04:00

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ADR 0004: Editor Gameplay Hot Reload

Status

Accepted

Context

Iterating on FPS controller feel (sim movement, player input, camera presentation) required restarting the entire editor after every Rust change. Full restarts are slow (GPU init, egui layout, scene reload) and break flow.

Competing approaches:

  1. Full editor restart — simple; too slow for gameplay iteration.
  2. Subprocess game binary — preserves editor shell; breaks in-process viewport render-to-texture (cross-process GPU sharing).
  3. In-process dylib hot reload — keeps one world and PIE model; swaps gameplay function bodies via hot-lib-reloader.

Decision

Add dev-only in-process hot reload for the full GamePlugin surface:

Layer Crate Hot reload?
Components, resources, TypeIds protocol, shared, settings, sim Static (never swapped)
Gameplay system bodies game_hot (cdylib + rlib) Yes
Plugin registration shell game, editor Static
Editor UI editor Static

Implementation:

  • crates/game_hot holds #[unsafe(no_mangle)] systems (sim, player, world, rendering bootstrap).
  • game registers systems once; with hot-reload feature, hot_module wraps dylib functions.
  • editor HotReloadPlugin (enabled via dev feature) polls LibReloadObserver, watches the dylib with notify, shows status in the bottom bar, and supports Ctrl+Shift+R manual refresh.
  • on_hot_reload re-applies idempotent bootstrap (camera FX tags, ambient light); spawns guard against duplicate player/world entities.

Release builds omit hot-reload; shipped game binary links game_hot statically.

Consequences

  • Gameplay iteration no longer requires editor restart for logic-only changes.
  • Component layout or schedule graph changes still require a full restart (documented in README).
  • Linux uses a single target/ directory; Windows may need separate bin/dylib target dirs to avoid DLL locking (follow-up).
  • dev feature on editor implies hot-reload (not optional in daily dev workflow).

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