DontDestroyOnLoad usage in Unity - Mini Project: Build & Apply

Using DontDestroyOnLoad in Unity

📖 Scenario: You are making a simple Unity game where you want a music player object to keep playing music even when you change scenes. Normally, when you load a new scene, all objects are destroyed and recreated. But you want the music to continue without restarting.

🎯 Goal: Build a Unity script that uses DontDestroyOnLoad to keep a GameObject alive across scene changes.

📋 What You'll Learn

Create a C# script called MusicPlayer

In MusicPlayer, use DontDestroyOnLoad(gameObject) to keep the object alive

Add a check to prevent duplicates if the object already exists

Print a message to the console when the object is kept alive

💡 Why This Matters

🌍 Real World

Games often need objects like music players or managers to persist across scenes without restarting or duplicating.

💼 Career

Understanding how to manage persistent objects is important for game developers working with Unity to create smooth player experiences.

1

Create the MusicPlayer script with Awake method

Create a C# script called MusicPlayer with an Awake() method that prints "MusicPlayer Awake" to the console.

Unity

using UnityEngine;

public class MusicPlayer : MonoBehaviour
{
    // Your code here
}

Hint

The Awake() method runs when the object is created. Use Debug.Log to print messages.

2

Add DontDestroyOnLoad to keep the object alive

Inside the Awake() method, add DontDestroyOnLoad(gameObject); to keep the GameObject alive when loading new scenes.

Unity

using UnityEngine;

public class MusicPlayer : MonoBehaviour
{
    void Awake()
    {
        Debug.Log("MusicPlayer Awake");
        // Your code here
    }
}

Hint

Use DontDestroyOnLoad(gameObject); inside Awake() to keep the object alive across scenes.

3

Prevent duplicate MusicPlayer objects

Modify the Awake() method to check if another MusicPlayer already exists. If yes, destroy this duplicate object using Destroy(gameObject);. Use FindObjectsOfType<MusicPlayer>() to find existing objects.

Unity

using UnityEngine;

public class MusicPlayer : MonoBehaviour
{
    void Awake()
    {
        Debug.Log("MusicPlayer Awake");
        // Your code here
    }
}

Hint

Use FindObjectsOfType<MusicPlayer>() to find all MusicPlayer objects. If more than one exists, destroy the new one.

4

Print confirmation when object is kept alive

Add a Debug.Log message inside Awake() after DontDestroyOnLoad(gameObject); that prints "MusicPlayer will not be destroyed on load".

Unity

using UnityEngine;

public class MusicPlayer : MonoBehaviour
{
    void Awake()
    {
        MusicPlayer[] players = FindObjectsOfType();
        if (players.Length > 1)
        {
            Destroy(gameObject);
            return;
        }
        Debug.Log("MusicPlayer Awake");
        DontDestroyOnLoad(gameObject);
        // Your code here
    }
}

Hint

Print a message after calling DontDestroyOnLoad to confirm the object stays alive.

Practice

1. What is the main purpose of using DontDestroyOnLoad in Unity?

easy

A. To pause the game during scene loading

B. To destroy a GameObject immediately

C. To load a new scene asynchronously

D. To keep a GameObject alive when switching scenes

Solution

1. Step 1: Understand what DontDestroyOnLoad does

   This method prevents a GameObject from being destroyed when a new scene loads.

2. Step 2: Identify the purpose in scene management

   It keeps important objects like music players or managers alive across scenes.

3. Final Answer:

   To keep a GameObject alive when switching scenes -> Option D

4. Quick Check:

   DontDestroyOnLoad keeps objects alive = A [OK]

Hint: Remember: DontDestroyOnLoad means object stays between scenes [OK]

Common Mistakes:

• Thinking it destroys objects instead of preserving them
• Confusing it with scene loading functions
• Assuming it pauses the game

2. Which of the following is the correct way to use DontDestroyOnLoad in a Unity script?

easy

A. DontDestroyOnLoad(this);

B. DontDestroyOnLoad(Scene);

C. DontDestroyOnLoad(gameObject);

D. DontDestroyOnLoad(SceneManager);

Solution

1. Step 1: Identify the correct parameter for DontDestroyOnLoad

   The method requires a GameObject to keep alive, usually referenced as gameObject in scripts.

2. Step 2: Check each option

   DontDestroyOnLoad(gameObject); uses gameObject, which is correct. DontDestroyOnLoad(this); uses this (the script), which is incorrect. Options B and C use unrelated types.

3. Final Answer:

   DontDestroyOnLoad(gameObject); -> Option C

4. Quick Check:

   Use gameObject with DontDestroyOnLoad = D [OK]

Hint: Use gameObject, not this or Scene, with DontDestroyOnLoad [OK]

Common Mistakes:

• Passing 'this' instead of 'gameObject'
• Trying to pass Scene or SceneManager objects
• Calling DontDestroyOnLoad without parameters

3. Consider this Unity C# script attached to a GameObject:

void Awake() {
    DontDestroyOnLoad(gameObject);
}

void Start() {
    Debug.Log("Object alive");
}

What will happen when the scene changes?

medium

A. The GameObject will persist and "Object alive" will print once

B. The GameObject will be destroyed and "Object alive" will not print again

C. The GameObject will persist and "Object alive" will print every scene load

D. The GameObject will be duplicated on each scene load

Solution

1. Step 1: Understand when Awake and Start run

   Awake runs once when the object is created. Start runs once after Awake. Both run only once per object lifetime.

2. Step 2: Effect of DontDestroyOnLoad on scene change

   The GameObject is kept alive across scenes, so Awake and Start do not run again on scene load.

3. Final Answer:

   The GameObject will persist and "Object alive" will print once -> Option A

4. Quick Check:

   DontDestroyOnLoad keeps object alive, Awake/Start run once = A [OK]

Hint: Awake/Start run once; object stays alive with DontDestroyOnLoad [OK]

Common Mistakes:

• Thinking Start runs on every scene load
• Assuming object duplicates on scene change
• Believing object is destroyed on scene change

4. You wrote this code in a Unity script:

void Start() {
    DontDestroyOnLoad(this);
}

What is the problem and how to fix it?

medium

A. Error: DontDestroyOnLoad must be called in Awake, not Start

B. Error: 'this' is not a GameObject; fix by using DontDestroyOnLoad(gameObject);

C. No problem; code works fine as is

D. Error: DontDestroyOnLoad requires a Scene parameter

Solution

1. Step 1: Identify the parameter type for DontDestroyOnLoad

   DontDestroyOnLoad requires a GameObject, but 'this' refers to the script instance, not the GameObject.

2. Step 2: Correct the parameter

   Use gameObject to pass the GameObject the script is attached to.

3. Final Answer:

   Error: 'this' is not a GameObject; fix by using DontDestroyOnLoad(gameObject); -> Option B

4. Quick Check:

   DontDestroyOnLoad needs gameObject, not this = C [OK]

Hint: Pass gameObject, not this, to DontDestroyOnLoad [OK]

Common Mistakes:

• Passing 'this' instead of 'gameObject'
• Thinking DontDestroyOnLoad must be in Awake only
• Passing wrong types like Scene or SceneManager

5. You want to keep a music player GameObject alive across scenes but avoid duplicates if the scene reloads. Which code snippet correctly implements this using DontDestroyOnLoad?

hard

A. void Awake() {
if (FindObjectsOfType<MusicPlayer>().Length > 1) {
Destroy(gameObject);
} else {
DontDestroyOnLoad(gameObject);
}
}

B. void Awake() {
DontDestroyOnLoad(gameObject);
}

C. void Start() {
Destroy(gameObject);
DontDestroyOnLoad(gameObject);
}

D. void Awake() {
Destroy(gameObject);
}

Solution

1. Step 1: Understand the problem of duplicates

   Using DontDestroyOnLoad alone keeps the object alive, but reloading the scene creates duplicates.

2. Step 2: Use a check to destroy duplicates

   void Awake() { if (FindObjectsOfType<MusicPlayer>().Length > 1) { Destroy(gameObject); } else { DontDestroyOnLoad(gameObject); } } checks if more than one MusicPlayer exists and destroys the extra one, preventing duplicates.

3. Step 3: Verify other options

   void Awake() { DontDestroyOnLoad(gameObject); } keeps duplicates. void Start() { Destroy(gameObject); DontDestroyOnLoad(gameObject); } destroys then calls DontDestroyOnLoad (wrong order). void Awake() { Destroy(gameObject); } destroys always.

4. Final Answer:

   void Awake() { if (FindObjectsOfType<MusicPlayer>().Length > 1) { Destroy(gameObject); } else { DontDestroyOnLoad(gameObject); } } -> Option A

5. Quick Check:

   Check duplicates before DontDestroyOnLoad to avoid duplicates = B [OK]

Hint: Check existing objects before DontDestroyOnLoad to prevent duplicates [OK]

Common Mistakes:

• Not checking for duplicates causing multiple music players
• Destroying object before calling DontDestroyOnLoad
• Calling DontDestroyOnLoad without duplicate check