Unity

Unity Deep Engineering Guide

Unity is a component-based game engine built around GameObjects, MonoBehaviours, and a C# scripting layer. Modern Unity adds Data-Oriented Technology Stack (DOTS/E.C.S.) and two Scriptable Render Pipelines (URP/HDRP). This guide covers how a lead engineer thinks about Unity: architecture, C# performance, data-oriented design, rendering, physics, multiplayer, and profiling.

1. Engine Architecture

Unity Player/Editor
  Application      (UnityEngine)
    SceneManager   -> load/unload Scenes (additive)
      GameObject   (identity + transform)
        Component  (MonoBehaviour, Renderer, Collider, Animator...)
    PlayerLoop:    (fixed update order, engine-driven)
      ScriptableRunLoops -> custom systems injected
    Module systems: Physics (PhysX/Box2D), Audio (FMOD), UI (UGUI/UI Toolkit),
                    Rendering (URP/HDRP/Built-in), Input (new Input System)

1.1 The Player Loop & Update Order

Within one frame Unity calls systems in a fixed order:

void Update()   // per rendered frame
void FixedUpdate() // fixed timestep: physics, network ticks (default 0.02s)
void LateUpdate()  // camera follow, post-movement

Rules:

• Never move Rigidbody in Update(); move via FixedUpdate or use rb.velocity only in FixedUpdate. Changing transform in Update fights the physics solver.
• Camera interpolation in LateUpdate.
• Scene load is synchronous unless using Addressables/SceneManager.LoadSceneAsync.

1.2 Multiple Scenes (Additive Loading)

• Split levels into persistent + gameplay scenes; stream with LoadSceneAsync + AllowSceneActivation = false to keep the load from blocking.
• Use SceneManager.MergeScenes or DontDestroyOnLoad root object.

2. GameObject & Component Model

public class Health : MonoBehaviour {
    public int maxHp = 100;
    int current;

    void OnEnable()  { /* add self to manager lists */ }
    void OnDisable() { /* remove self */ }
}

2.1 GetComponent Performance

GetComponent<T>() per frame per object is a hot-path mistake:

• Cache component references in Awake() once.
• Prefer TryGetComponent.
• GetComponentsInChildren only at load or on structural change.
• Hot update loops use managers (single static list of registered components), not per-object Find.

2.2 FindObjectOfType / GameObject.Find

These are O(n) with allocations. They MUST not run on the per-frame path. For singletons, register in Awake, destroy in OnDestroy:

public static GameManager I;  // global accessor
void Awake() { I = this; }
void OnDestroy() { if (I == this) I = null; }

3. The DOTS / ECS Stack

Unity DOTS replaces MonoBehaviours with archetype-based ECS:

// Component - plain struct, no MonoBehaviour
public struct Velocity : IComponentData { public float3 Value; }

// System
[BurstCompile]
partial struct MoveSystem : ISystem {
    [BurstCompile]
    public void OnUpdate(ref SystemState state) {
        foreach (var (transform, v) in
                 SystemAPI.Query<RefRW<LocalTransform>, RefRO<Velocity>>())
        {
            transform.ValueRW.Position += v.ValueRO.Value * SystemAPI.Time.DeltaTime;
        }
    }
}

• Entities stored in archetypes (SoA per component) → cache-friendly.
• Burst + Jobs compiles to native SIMD-optimized code.
• SubScene baking: author GameObject scenes → baked entities at build.

When to use DOTS vs classic GameObjects

4. Rendering Pipelines

4.1 Choose the Pipeline

4.2 URP vs HDRP in practice

• URP: forward rendering, 2D lights, SRP batcher, simplified.
• HDRP: deferred, clustered lighting, volumetric fog, SSR, RT reflections (DXR).
• Both are ScriptableRenderPipeline; they run in the Rendering.Update player-loop phase.

4.3 Draw Call Reduction

1. Enable SRP Batcher (URP/HDRP property UseSRPBatcher) — batches dynamic shadows+lit objects with compatible materials.
2. GPU instancing: Graphics.DrawMeshInstanced for repeated meshes (grass, crowds, particles).
3. Static batching: mark static → combined at build.
4. Addressables + SDF 2D: not draw-call related, but stream textures to control bandwidth.
5. Minimize material variants (KeywordEnum, MaterialPropertyBlock for per-object color).
6. Bake lighting (Lightmapping) instead of dynamic.

4.4 Shader Lab & Shader Graph

• Shader Graph (node-based) is preferred for artists; expose exposed properties only.
• Hand-written shaders: write in HLSL; SRP uses HLSLPROGRAM blocks and #pragma target 3.0+.
• Watch out: Unity editor implicit #pragma multi_compile _ _ALPHATEST_ON; use #pragma shader_feature_local to avoid variants.

5. Physics

• Unity uses PhysX (3D) and Box2D (2D).
• Fixed timestep at 0.02s (50Hz); fast small objects tunnel → use Physics.Burst/Physics.Solver or a dedicated CCD sensor.
• Sleeping: overlapping static triggers should disable colliders (isTrigger = true) to avoid touches every frame.
• Layers & matrix matter: Physics.IgnoreLayerCollision(playerLayer, enemyLayer, true) reduces contact generation.
• Physics.SyncTransforms() forces syncing; avoid calling per-frame.

6. Animation & Timing

• Animator state machine with transitions; avoid heavy tree layers per object.
• Time.deltaTime may be 0 on pause; use Time.unscaledDeltaTime for UI.
• Animation events are editor-time; use OnAnimationEvent or AnimationEvent binding.
• For 100s of animating characters prefer playables or AnimationUpdateMode fixed with simple lerps.

7. UI (UGUI / UI Toolkit / IMGUI)

• UGUI (Canvas): fine for HUD; keep panels/Canvas to 1-2, enable Vertex Buffer reuse, disable raycast on static.
• UI Toolkit (new): UXML/USS, better for menus and editor tooling; runtime since 2023.
• IMGUI: editor windows and debugging only, not runtime UI.
• Rule: no layout rebuild per frame; cache RectTransforms, minify GraphicRaycaster blockers.

8. Asset Pipeline

8.1 Addressables

Addressables = load by key, async, memory-managed:

Addressables.LoadAssetAsync<GameObject>("enemies/wolf").Completed += h => {
    var go = Instantiate(h.Result);
    go.AddComponent<DeferredUnload>();
};
Addressables.Release(handle);

• Use AssetReference fields instead of hardcoded strings.
• Set addressables remote catalog for updates.
• Remember: Instantiate(referencedObject) body must be released too.

8.2 Streaming & Background Loading

• Texture streaming (Texture2D.streamingEnabled) or Addressables load in OnTriggerEnter.
• Audio: AudioClip.Create streaming for ambience.
• Never Resources.Load in Update; use Resources.LoadAll once.

9. Multiplayer (Netcode for GameObjects / Netcode Transport / UnityTransport)

• Authoritative: server owns state; clients send inputs; interpolate snapshot positions.
• [ServerRpc] from client to server; [ClientRpc] from server to clients.
• Sim tick = fixed timestep; physics simulation server-side only; clients render interpolated states.
• Use NetworkVariable<T> — beware of per-frame NetworkVariable writes (delta encode, avoid 60/s floats).
• Prediction: PredictedPlayer pattern with client-side NetworkTransform interpolation.

10. Performance Rules (Lead Level)

1. Profile first: Unity Profiler + Burst metrics; GPU profiler (RenderDoc, GPUView).
2. ExecuteInEditMode scripts only where needed; keep editor-only work in #if UNITY_EDITOR.
3. Avoid transform.position in a loop (accessing Transform is cheap-ish, but allocations from Find/GetComponentsInChildren are not).
4. Garbage: minimize per-frame allocations; cache arrays; use NativeArray/NativeList in jobs.
5. Broke update: use FixedUpdate only for physics; game logic in Update.
6. Root motion vs scripted movement: profile Animator cost.
7. Hidden cost: Camera.main cached; avoid in Update.

10.1 Frame Budget Table Template

Total must fit in 16.6ms (or 1000/refresh).

11. Project Structure Convention

Assets/
  Scripts/         # namespaces by system
  Scenes/
  Prefabs/
  Addressables/    # addressable groups
  Materials/
  Textures/
  Audio/
  _BuildTools/     # editor scripts, CI
  .gitignore: Library/ Temp/ Logs/ UserSettings/

Use .asmdef (assembly definition) per folder to speed up compile times and control references.

12. Anti-Patterns

13. When to NOT Use Unity

• Native console/PC-only huge world with full control → custom engine + Vulkan (see skills/game/game-development/vulkan/SKILL.md).
• Web "tiny" canvas game → Engines like Phaser or vanilla canvas might be lighter.
• Deep customization of renderer required beyond URP/HDRP budget → engine-level control required.
• For ECS patterns independent of engine, see skills/game/game-engine/ecs-pattern/SKILL.md.

14. References

• skills/game/game-development/unity-csharp/SKILL.md — Unity C# scripting and DOTS internals
• skills/game/game-engine/ecs-pattern/references/unity-dots-in-practice.md — DOTS in real projects
• skills/game/game-engine/patterns/references/game-loop-and-timestep.md — update timing patterns
• skills/game/multiplayer-netcode/SKILL.md — netcode patterns & prediction
• skills/game/game-development/physics-engine/SKILL.md — physics internals used by engines