MonoBehaviour lifecycle in Unity - Step-by-Step Execution

Concept Flow - MonoBehaviour lifecycle

Game Start

↓

Awake() called

↓

OnEnable() called

↓

Start() called

↓

Update() called every frame

↓

FixedUpdate() called every physics step

↓

LateUpdate() called every frame after Update

↓

OnDisable() called

↓

OnDestroy() called

↓

Game End

This flow shows the order in which Unity calls MonoBehaviour lifecycle methods from game start to end.

Execution Sample

Unity

void Awake() { Debug.Log("Awake called"); }
void Start() { Debug.Log("Start called"); }
void Update() { Debug.Log("Update called"); }

This code logs messages when Awake, Start, and Update methods are called during the game lifecycle.

Execution Table

Step	Method Called	When Called	Action	Output
1	Awake()	When script instance is loaded	Initialize variables	Logs: Awake called
2	OnEnable()	When object becomes active	Prepare object	No output in sample
3	Start()	Before first frame update	Setup before gameplay	Logs: Start called
4	Update()	Every frame	Game logic update	Logs: Update called
5	FixedUpdate()	Every physics step	Physics calculations	No output in sample
6	LateUpdate()	Every frame after Update	Follow-up logic	No output in sample
7	OnDisable()	When object becomes inactive	Cleanup or pause	No output in sample
8	OnDestroy()	When object is destroyed	Final cleanup	No output in sample
9	End	Game ends or object destroyed	Stop lifecycle	No further calls

💡 Lifecycle ends when object is destroyed or game stops.

Variable Tracker

Variable	Start	After Awake	After OnEnable	After Start	After Update	After OnDisable	After OnDestroy
isActive	false	false	true	true	true	false	false
initialized	false	true	true	true	true	true	false

Key Moments - 3 Insights

Why is Awake called before Start?

Awake is called when the script instance loads to initialize variables early, while Start is called just before the first frame to set up gameplay. See execution_table rows 1 and 3.

Why does Update run many times but Awake only once?

Update runs every frame to keep game logic updated, but Awake runs only once to initialize. See execution_table rows 1 and 4.

What happens when the object is disabled?

OnDisable is called to pause or clean up before the object becomes inactive. See execution_table row 7 and variable_tracker for isActive changes.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the object considered active?

A Step 7 OnDisable()

B Step 2 OnEnable()

C Step 1 Awake()

D Step 8 OnDestroy()

💡 Hint

Check variable_tracker column After OnEnable where isActive changes to true.

According to the execution_table, which method runs every frame during gameplay?

A Start()

B Awake()

C Update()

D OnDisable()

💡 Hint

Look at Step 4 in execution_table where Update() is called every frame.

If the object is destroyed, which method is called last according to the lifecycle?

A OnDestroy()

B OnDisable()

C Start()

D Awake()

💡 Hint

See execution_table Step 8 for OnDestroy() called when object is destroyed.

Concept Snapshot

MonoBehaviour lifecycle methods run in this order:
Awake() -> OnEnable() -> Start() -> Update() (every frame) -> FixedUpdate() (physics) -> LateUpdate() -> OnDisable() -> OnDestroy()
Awake initializes early, Start runs before first frame, Update runs every frame.
OnDisable and OnDestroy handle cleanup.
This order controls script behavior during game play.

Full Transcript

The MonoBehaviour lifecycle in Unity starts with Awake, which initializes the script instance when loaded. Then OnEnable is called when the object becomes active. Start runs before the first frame update to prepare gameplay. Update runs every frame to handle game logic. FixedUpdate runs every physics step for physics calculations. LateUpdate runs after Update for follow-up logic. When the object is disabled, OnDisable is called to pause or clean up. Finally, OnDestroy is called when the object is destroyed for final cleanup. Variables like isActive track if the object is active, changing after OnEnable and OnDisable. This lifecycle ensures scripts run in the correct order during the game.

Practice

1. Which MonoBehaviour method is called exactly once when the script instance is being loaded, before any Start methods?

easy

A. FixedUpdate()

B. Update()

C. Awake()

D. OnDestroy()

Solution

1. Step 1: Understand MonoBehaviour initialization order

   Awake() is called once when the script instance is loaded, before Start().

2. Step 2: Differentiate from other lifecycle methods

   Update() and FixedUpdate() run repeatedly, and OnDestroy() runs when the object is destroyed.

3. Final Answer:

   Awake() -> Option C

4. Quick Check:

   Awake runs once at load = Awake() [OK]

Hint: Awake runs once at load, before Start [OK]

Common Mistakes:

• Confusing Awake with Start
• Thinking Update runs once
• Mixing OnDestroy timing

2. Which of the following is the correct syntax to declare the Update method in a MonoBehaviour script?

easy

A. void Update() {}

B. public void Update() {}

C. private Update() {}

D. Update void() {}

Solution

1. Step 1: Recall method declaration rules in C# for Unity

   MonoBehaviour lifecycle methods like Update are usually declared as void Update() without access modifiers.

2. Step 2: Check each option's syntax

   void Update() {} is correct syntax. public void Update() {} is valid C# but Unity recommends no access modifier. private Update() {} is missing return type. Update void() {} has wrong order.

3. Final Answer:

   void Update() {} -> Option A

4. Quick Check:

   Correct method syntax = void Update() {} [OK]

Hint: Update method is always void Update() {} [OK]

Common Mistakes:

• Using wrong order of return type and name
• Omitting void return type
• Adding private keyword incorrectly

3. What will be the output of the following code attached to a GameObject?

using UnityEngine;

public class TestLifecycle : MonoBehaviour {
    void Awake() {
        Debug.Log("Awake called");
    }
    void Start() {
        Debug.Log("Start called");
    }
    void Update() {
        Debug.Log("Update called");
        enabled = false;
    }
}

Assuming the game runs for one frame, what messages appear in the Console?

medium

A. ["Awake called", "Start called", "Update called"]

B. ["Awake called", "Start called"]

C. ["Start called", "Update called"]

D. ["Awake called", "Update called"]

Solution

1. Step 1: Understand method call order on first frame

   On first frame, Awake() runs once at load, then Start() runs before first Update, then Update() runs.

2. Step 2: Analyze Update disabling script

   The Update() method disables the script after logging, so it runs once this frame only.

3. Final Answer:

   ["Awake called", "Start called", "Update called"] -> Option A

4. Quick Check:

   Awake, Start, Update run once first frame [OK]

Hint: Awake and Start run before first Update [OK]

Common Mistakes:

• Thinking Update disables before logging
• Missing Awake call
• Assuming Update runs multiple times first frame

4. Identify the error in this MonoBehaviour script:

using UnityEngine;

public class Example : MonoBehaviour {
    void Start() {
        Debug.Log("Start called");
    }

    void Update() {
        Debug.Log("Update called");
    }

    void OnDestroy() {
        Debug.Log("Destroying object");
    }

    void awake() {
        Debug.Log("Awake called");
    }
}

medium

A. Start method must be static

B. OnDestroy method must return a bool

C. Update method cannot have Debug.Log calls

D. Method awake() should be Awake() with capital A

Solution

1. Step 1: Check method names for correct casing

   Unity lifecycle methods are case-sensitive. awake() is incorrect; it must be Awake().

2. Step 2: Verify other method signatures

   OnDestroy() returns void, Update() can have Debug.Log, and Start() is instance method, not static.

3. Final Answer:

   Method awake() should be Awake() with capital A -> Option D

4. Quick Check:

   Method names are case-sensitive = Awake() [OK]

Hint: Unity method names must match exact case [OK]

Common Mistakes:

• Ignoring case sensitivity
• Thinking OnDestroy returns bool
• Assuming Start must be static

5. You want to ensure a GameObject cleans up resources before it is destroyed. Which MonoBehaviour method should you override, and what is the best practice to use it?

hard

A. Override Start() and release resources there.

B. Override OnDestroy() and release resources or unsubscribe from events there.

C. Override Update() and check if object is destroyed to release resources.

D. Override Awake() and release resources there.

Solution

1. Step 1: Identify the cleanup lifecycle method

   OnDestroy() is called just before the object is destroyed, ideal for cleanup.

2. Step 2: Understand best practice for resource management

   Release resources and unsubscribe from events inside OnDestroy() to avoid memory leaks or errors.

3. Final Answer:

   Override OnDestroy() and release resources or unsubscribe from events there. -> Option B

4. Quick Check:

   Cleanup happens in OnDestroy() [OK]

Hint: Use OnDestroy to clean up before object removal [OK]

Common Mistakes:

• Trying to clean up in Start or Awake
• Checking destruction inside Update
• Not unsubscribing from events