Scene Tick Order: Full Frame Pipeline — FixedUpdate, Update, PreRender, ProcessDeletes, and Stage Hooks

Scene Tick Order: The Full Frame Pipeline

Understanding the exact order of operations in a scene tick is critical for writing correct game logic.

Game Tick Order (Scene.GameTick)

1. UpdateTime( timeDelta ) — advances TimeNow and TimeDelta (scaled by TimeScale)
2. SyncContext.FrameStage.Update.Trigger() — async frame stage
3. Fixed Update loop (if ProjectSettings.Physics.UseFixedUpdate):

1. ProcessDeletes() — destroy queued objects
2. Signal( Stage.StartUpdate )
3. InternalUpdate():

1. Signal( Stage.UpdateBones )
2. PreRender() — OnPreRender() on all pre-render components (skipped if headless)
3. ProcessDeletes()
4. Signal( Stage.FinishUpdate )
5. SoundHandle.FlushCreatedSounds()
6. SyncContext.FrameStage.PreRender.Trigger()

Key Insights

• ProcessDeletes() is called multiple times per frame — after fixed update and after update. Objects destroyed with Destroy() are queued and removed at these points.
• OnStart() is called lazily from RunPendingStarts(), which runs at the start of both fixed update and regular update. A component enabled mid-frame will have OnStart called before its first OnUpdate.
• Signal( Stage.Interpolation ) runs before OnUpdate, so transform interpolation is already applied when your update code runs.
• OnPreRender() is skipped entirely on dedicated servers (Application.IsHeadless).
• The TimeScale property on Scene scales TimeDelta but not RealTime.
• Network updates (SceneNetworkUpdate()) run at the start of SharedTick, before fixed update.

GameObjectSystem Stage Hooks

public class MySystem : GameObjectSystem<MySystem>
{
    public MySystem( Scene scene ) : base( scene )
    {
        Listen( Stage.StartUpdate, 0, OnStartUpdate, "MySystem.OnStartUpdate" );
        Listen( Stage.PhysicsStep, 0, OnPhysicsStep, "MySystem.OnPhysicsStep" );
        Listen( Stage.FinishUpdate, 0, OnFinishUpdate, "MySystem.OnFinishUpdate" );
    }

    void OnStartUpdate() { /* runs before OnUpdate on all components */ }
    void OnPhysicsStep() { /* runs during fixed update, after component FixedUpdate */ }
    void OnFinishUpdate() { /* runs after all OnUpdate calls */ }
}

The order parameter controls priority within a stage (negative = before default, positive = after).