Scene transitions in Unity - Step-by-Step Execution

Concept Flow - Scene transitions

Start Scene

↓

User triggers transition

↓

Load new scene async

↓

Wait for scene to load

↓

Activate new scene

↓

Unload old scene

↓

New scene active

This flow shows how Unity loads a new scene asynchronously, waits for it to finish, activates it, and unloads the old scene.

Execution Sample

Unity

using UnityEngine;
using UnityEngine.SceneManagement;
using System.Collections;

public class SceneSwitcher : MonoBehaviour {
  public void SwitchScene(string sceneName) {
    StartCoroutine(LoadScene(sceneName));
  }

  private IEnumerator LoadScene(string sceneName) {
    AsyncOperation asyncLoad = SceneManager.LoadSceneAsync(sceneName);
    while (!asyncLoad.isDone) {
      yield return null;
    }
  }
}

This code switches scenes by loading the new scene asynchronously and waiting until it finishes loading.

Execution Table

Step	Action	AsyncOperation.isDone	Yield	Result
1	Call SwitchScene("Level2")	false	Start coroutine	Coroutine started
2	Start LoadScene coroutine	false	Check isDone	Not done, yield return null
3	Wait frame	false	Check isDone	Still loading, yield return null
4	Wait frame	true	Check isDone	Loading done, exit loop
5	Coroutine ends	true	No yield	Scene loaded and active

💡 AsyncOperation.isDone becomes true, ending the loading loop and activating the new scene.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
asyncLoad.isDone	false	false	false	true	true

Key Moments - 2 Insights

Why do we use a coroutine and yield return null inside the loading loop?

Because loading a scene takes time, the coroutine waits each frame until asyncLoad.isDone is true, as shown in execution_table rows 2-4.

What happens if we try to activate the new scene before loading finishes?

The scene won't be ready and may cause errors or a blank screen. The code waits for asyncLoad.isDone to be true before continuing (see execution_table row 4).

Visual Quiz - 3 Questions

Look at the execution table, at which step does asyncLoad.isDone become true?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Check the 'AsyncOperation.isDone' column in execution_table rows.

According to variable_tracker, what is the value of asyncLoad.isDone after step 3?

A true

B null

C false

D undefined

💡 Hint

Look at the 'After Step 3' column for asyncLoad.isDone in variable_tracker.

If we remove 'yield return null' inside the while loop, what happens?

A The scene loads instantly

B The program freezes because the loop never yields

C The scene never loads

D The coroutine ends immediately

💡 Hint

Refer to the execution_table rows 2-4 where yielding allows waiting without freezing.

Concept Snapshot

Unity scene transitions use asynchronous loading.
Use SceneManager.LoadSceneAsync to load scenes without freezing.
Wait in a coroutine until loading completes (asyncLoad.isDone).
Activate the new scene after loading finishes.
Unload old scenes if needed to free memory.

Full Transcript

This visual execution shows how Unity handles scene transitions by loading a new scene asynchronously. The process starts when the user triggers a scene switch. The code calls LoadSceneAsync, which returns an AsyncOperation. A coroutine waits each frame, checking if asyncLoad.isDone is true. While loading is in progress, the coroutine yields control to avoid freezing the game. Once loading finishes, the coroutine ends, and the new scene becomes active. This method ensures smooth transitions without freezing or delays. Variables like asyncLoad.isDone change from false to true during loading, as tracked in the variable tracker. Key moments include understanding why yielding is necessary and what happens if you try to activate a scene too early. The quizzes test understanding of when loading completes and the importance of yielding.