Engine Frame Loop Order: FrameStart, Scene Tick, and FrameEnd Sequence

Engine Frame Loop Order

The s&box engine frame loop runs in a specific order each frame. Understanding this is important for knowing when your code runs relative to rendering, physics, and networking.

Per-Frame Order (EngineLoop.RunFrame)

1. RealTime.Update — updates real-time clock
2. Time.Update — updates scene time
3. FrameStart():

- Steam callbacks - Performance stats - Input pump (g_pInputService.Pump()) - File watch callbacks - Texture animation tick - VR frame start - Resource cache pruning - Async task queue drain - IMenuDll.Tick() → IGameInstanceDll.Tick() → IMenuDll.LateTick() → IToolsDll.Tick() - Voice, text rendering - API events and stats - UI simulation (style resolution) - Sound handle tick - Input router frame - Party room tick

1. SourceEngineFrame() — native engine frame (rendering submission)
2. FrameEnd():

- Streamer tick - Async task queue drain - VR frame end - Interop string free - Project recompile tick - Frame-end disposables drain - Render target end-of-frame

Scene Tick Order (Scene.SharedTick)

Within IGameInstanceDll.Tick(), the scene ticks in this order:

1. ISceneStage.Start() events
2. Network update (SceneNetworkUpdate)
3. Fixed update loop (physics, OnFixedUpdate)
4. ProcessDeletes
5. Signal(StartUpdate)
6. InternalUpdate → OnUpdate on all components
7. Signal(UpdateBones)
8. PreRender → OnPreRender on all components
9. ProcessDeletes
10. Signal(FinishUpdate)
11. Sound flush
12. ISceneStage.End() events

Frame Rate Clamping

The engine sleeps to enforce RenderSettings.MaxFrameRate. When the app is inactive, RenderSettings.MaxFrameRateInactive is used (if lower). Headless servers are capped at 60 FPS. Benchmark mode has no cap.