MonoBehaviour lifecycle in Unity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output order of these MonoBehaviour methods?

Consider a Unity script attached to an active GameObject. Which sequence of method calls will Unity invoke first when the scene starts?

Unity

using UnityEngine;

public class TestLifecycle : MonoBehaviour
{
    void Awake() { Debug.Log("Awake"); }
    void Start() { Debug.Log("Start"); }
    void OnEnable() { Debug.Log("OnEnable"); }
}

A Start, Awake, OnEnable

B OnEnable, Awake, Start

C Awake, OnEnable, Start

D Awake, Start, OnEnable

💡 Hint

Remember that Awake is called before OnEnable, and Start is called after both.

Unity calls Awake first when the script instance is loaded, then OnEnable when the object becomes active, and finally Start before the first frame update.

❓ Predict Output

intermediate

What happens if you disable a GameObject during Update?

Given this script, what will be the output when the GameObject is active and Update disables it?

Unity

using UnityEngine;

public class DisableInUpdate : MonoBehaviour
{
    void Update()
    {
        Debug.Log("Update called");
        gameObject.SetActive(false);
    }

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

A Only OnDisable called

B Update called\nOnDisable called

C Only Update called

D OnDisable called\nUpdate called

💡 Hint

Think about the order of method calls within the same frame.

Update runs first and logs "Update called". Then disabling the GameObject triggers OnDisable immediately, logging "OnDisable called".

🔧 Debug

advanced

Why does Start not get called when re-enabling a previously active GameObject?

Look at this script attached to a GameObject that starts active (so Start has already been called), is then disabled, and re-enabled at runtime. Why does Start not print "Start called" again?

Unity

using UnityEngine;

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

// Elsewhere in code:
// gameObject.SetActive(false);
// gameObject.SetActive(true);

A Start is only called once when the script instance is first enabled, not every time the GameObject is enabled.

B Start is called only if Awake is called first, which is missing here.

C Start is called only if OnEnable is overridden in the script.

D Start is called only if the GameObject is enabled in the editor before play.

💡 Hint

Think about when Start is triggered in the lifecycle.

Start is called exactly once per MonoBehaviour instance, just before the first frame update the first time the script is enabled. Subsequent enables after disabling do not call Start again.

📝 Syntax

advanced

Which method signature is correct for OnDestroy in MonoBehaviour?

Choose the correct method signature for the OnDestroy callback in a MonoBehaviour script.

A void OnDestroy()

B void OnDestroy(bool isPermanent)

C public void OnDestroy()

D void OnDestroy(GameObject obj)

💡 Hint

Check the Unity documentation for OnDestroy method signature.

OnDestroy has no parameters and returns void. It is called when the MonoBehaviour will be destroyed.

🚀 Application

expert

How many times is FixedUpdate called per second if Time.fixedDeltaTime = 0.02?

In Unity, if Time.fixedDeltaTime is set to 0.02 seconds, how many times will FixedUpdate be called per second?

A 20 times

B Depends on frame rate

C 100 times

D 50 times

💡 Hint

Think about how fixedDeltaTime relates to FixedUpdate frequency.

FixedUpdate is called every fixedDeltaTime seconds. If fixedDeltaTime is 0.02, then 1 / 0.02 = 50 calls per second.

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