MonoBehaviour lifecycle in Unity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the Awake() method in a MonoBehaviour?

Awake() is called when the script instance is being loaded. It is used to initialize variables or states before the game starts. It runs even if the script component is disabled.

beginner

When is the Start() method called in the MonoBehaviour lifecycle?

Start() is called before the first frame update, but only if the script component is enabled. It is used to set up things that depend on other objects being initialized.

intermediate

What is the difference between Update() and FixedUpdate()?

Update() runs once per frame and is used for regular frame-based logic like input checking. FixedUpdate() runs at fixed time intervals and is used for physics calculations.

intermediate

What does the OnEnable() method do in the MonoBehaviour lifecycle?

OnEnable() is called every time the script component is enabled or the GameObject it is attached to becomes active. It is useful for subscribing to events or resetting states.

intermediate

Explain the role of OnDestroy() in MonoBehaviour.

OnDestroy() is called when the MonoBehaviour will be destroyed. It is used to clean up resources, unsubscribe from events, or save data before the object is removed.

Which MonoBehaviour method is called only once when the script instance is loaded?

A Update()

B Start()

C FixedUpdate()

D Awake()

Awake() is called once when the script instance is loaded, before any other method.

Which method is best for handling physics updates in Unity?

A FixedUpdate()

B Update()

C LateUpdate()

D OnEnable()

FixedUpdate() runs at fixed intervals and is designed for physics calculations.

When is Start() called in the MonoBehaviour lifecycle?

A Before the script instance is loaded

B Before the first frame update if the script is enabled

C Every frame

D When the object is destroyed

Start() runs once before the first frame update, but only if the script is enabled.

What happens when OnEnable() is called?

A The script component or GameObject becomes active

B The frame updates

C The object is destroyed

D The script instance is loaded

OnEnable() is called when the script or GameObject becomes active.

Which method should you use to clean up resources before an object is removed?

A Awake()

B Start()

C OnDestroy()

D Update()

OnDestroy() is called before the object is destroyed and is used for cleanup.

Describe the order of MonoBehaviour lifecycle methods from object creation to destruction.

Think about what happens when the object is created, enabled, updated, disabled, and destroyed.

Explain when and why you would use FixedUpdate() instead of Update().

Consider timing and physics calculations.

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