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Lifecycle of GameObject

Minh Khoa

Minh Khoa

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1. Basics: The basic lifecycle of a GameObject + MonoBehaviour

When a GameObject is created in the Scene and attached with a script MonoBehaviourit goes through a lifecycle sequence:

1.1 Creation and activation

  • Awake()
    • Called only once when the script is loaded (before the scene even starts).
    • Used for: initializing variables, setting up references (for example: GetComponent, loading config).
    • Even if GameObject it is disabled, Awake it is still called.
  • OnEnable()
    • Called whenever GameObject or the component is enabled.
    • If you disable and then enable it again, OnEnable() it will run again.
    • Used for: registering events, resetting temporary state.

1.2 Lifecycle at runtime

  • Start()
    • Called before the first Update framebut after Awake and OnEnable.
    • Used for: executing logic that requires fully initialized data.
    • If GameObject it is disabled on the first frame then Start it will wait until it is enabled.
  • Update()
    • Called every frame, depending FPS.
    • Used for: gameplay logic (input checking, AIanimation state).
  • LateUpdate()
    • Called after all Update() in that frame.
    • Used for: handling “follow-up” (camera follow, syncing the transform after the character has moved).
  • FixedUpdate()
    • Called at a fixed frequency (default 0.02s, i.e. 50 times/second).
    • Used for: physics (Rigidbody, Force, Collider).
    • If you want stable physics in your game → code that moves the rigidbody should be placed here.

1.3 End of lifecycle

  • OnDisable()
    • Called when GameObject or the component is disabled.
    • Used for: unregistering events, clearing state, avoiding memory leaks.
  • OnDestroy()
    • Called when GameObject is Destroyed or the scene is unloaded.
    • Used for: releasing resources, saving state.

2. Advanced execution order (Script Execution Order)

Unity has a specific order when running scripts in a frame:

  1. FixedUpdate (0–n times depending on Time.fixedDeltaTime).
  2. Update (called for all scripts).
  3. LateUpdate.
  4. Rendering (OnPreRender, OnRenderObject, OnPostRender, OnRenderImage).
  5. Gizmos (OnDrawGizmos for the Editor).
  6. Coroutines (IEnumerator with yield).

⚡ Important: Unity allows adjusting script execution order in Project Settings > Script Execution Order.

3. Other special events in the lifecycle

In addition to the sequence above, there are many other callbacks:

  • OnApplicationPause(bool pause) → called when the app is sent to the background (mobile).
  • OnApplicationFocus(bool focus) → called when focus is lost/regained.
  • OnApplicationQuit() → called when the app closes (editor playmode stop also calls it).
  • OnValidate() → called in Editor whenever serialized values change (Inspector).
  • Reset() → called when you attach the script for the first time or choose Reset from the component menu.

4. Coroutine Lifecycle

Coroutine (IEnumerator) has its own lifecycle:

  • StartCoroutine() → runs in parallel with Update.
  • yield return null → waits 1 frame.
  • yield return new WaitForSeconds(t) → waits t real seconds.
  • Coroutine is stopped if GameObject it is disabled or StopCoroutine/StopAllCoroutines called.

5. Lifecycle in the Editor

If you make tools or Editor scripts, there is also:

  • [ExecuteInEditMode] → script runs even in the editor, without needing Play.
  • OnDrawGizmos() → draw gizmos in the Scene view.
  • OnValidate() → update when editing the Inspector.

6. Advanced lifecycle management

  • Dependency Injection (Zenject/Extenject): inject dependency in Awake instead of Start.
  • Pooling: reuse GameObject → avoid calling Awake/Start too many (use OnEnable/OnDisable instead of Destroy).
  • Async (UniTask/Task): replace Coroutine for a clearer flow.
  • Separation of Concern:
    • Awake: assign reference.
    • OnEnable: register event.
    • Start: gameplay init logic.
    • OnDisable: unregister event.
    • OnDestroy: cleanup.

7. Lifecycle illustration GameObject

Awake()
↓
OnEnable()
↓
Start()
↓
(Update → LateUpdate → FixedUpdate) [Loop đến khi disable/destroy]
↓
OnDisable()
↓
OnDestroy()

✅ In summary:

  • Awake: initialize variables (even if disabled).
  • OnEnable: register event, reset.
  • Start: initialize gameplay logic.
  • Update: every frame.
  • FixedUpdate: physics.
  • LateUpdate: camera/cleanup : after Update.
  • OnDisable: clear event.
  • OnDestroy: final cleanup.

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