DontDestroyOnLoad usage in Unity

Introduction

DontDestroyOnLoad keeps an object alive when you change scenes in a game. This helps keep important things like player data or music playing without restarting.

You want background music to keep playing when moving between game levels.

You need to keep player settings or scores without resetting after loading a new scene.

You have a game manager object that controls game state and should not be destroyed.

You want to keep a network connection alive across different scenes.

Syntax

Unity

DontDestroyOnLoad(gameObject);

Call this inside a script attached to the GameObject you want to keep.

Usually called in the Awake() or Start() method.

Examples

Keeps the current GameObject alive when loading new scenes.

Unity

void Awake() {
    DontDestroyOnLoad(gameObject);
}

Another way to keep the GameObject alive, called in Start instead of Awake.

Unity

void Start() {
    DontDestroyOnLoad(this.gameObject);
}

Sample Program

This script keeps the MusicPlayer object alive when you change scenes. It prints a message when it starts.

Unity

using UnityEngine;

public class MusicPlayer : MonoBehaviour {
    void Awake() {
        DontDestroyOnLoad(gameObject);
    }

    void Start() {
        Debug.Log("Music player started and will not be destroyed on load.");
    }
}

Important Notes

If you call DontDestroyOnLoad on multiple copies of the same object, you may get duplicates after scene loads.

Use a singleton pattern to avoid duplicates when using DontDestroyOnLoad.

Summary

DontDestroyOnLoad keeps objects alive across scene changes.

Call it in Awake or Start on the GameObject you want to keep.

Useful for music, game managers, and persistent data.

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