Loading screens with coroutines in Unity - Step-by-Step Execution

Concept Flow - Loading screens with coroutines

Start Loading Screen

↓

Start Coroutine

↓

Begin Async Load

↓

Check Load Progress

↓

Update UI

↓

Hide Loading Screen

The loading screen starts, a coroutine runs to load the scene asynchronously, updating progress until complete, then hides the loading screen.

Execution Sample

Unity

IEnumerator LoadSceneAsync()
{
  loadingScreen.SetActive(true);
  AsyncOperation op = SceneManager.LoadSceneAsync("GameScene");
  while (!op.isDone)
  {
    progressBar.fillAmount = op.progress;
    yield return null;
  }
  loadingScreen.SetActive(false);
}

This coroutine shows a loading screen, loads a scene in the background, updates a progress bar, and hides the screen when done.

Execution Table

Step	Action	AsyncOperation.isDone	AsyncOperation.progress	UI Update	Yield
1	Activate loading screen	False	0.0	Show loading screen	Yield null
2	Start loading scene async	False	0.1	Update progress bar to 10%	Yield null
3	Check loading progress	False	0.4	Update progress bar to 40%	Yield null
4	Check loading progress	False	0.7	Update progress bar to 70%	Yield null
5	Check loading progress	False	0.9	Update progress bar to 90%	Yield null
6	Check loading progress	True	1.0	Update progress bar to 100%	Exit loop
7	Deactivate loading screen	True	1.0	Hide loading screen	End coroutine

💡 AsyncOperation.isDone becomes True, loading completes, coroutine ends.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	Final
AsyncOperation.isDone	False	False	False	False	False	True	True
AsyncOperation.progress	0.0	0.1	0.4	0.7	0.9	1.0	1.0
loadingScreen.activeSelf	False	True	True	True	True	True	False
progressBar.fillAmount	0.0	0.1	0.4	0.7	0.9	1.0	1.0

Key Moments - 3 Insights

Why does the coroutine use 'yield return null' inside the while loop?

Because 'yield return null' pauses the coroutine until the next frame, allowing the loading progress to update smoothly each frame as shown in steps 2 to 5.

Why is the loading screen active before starting the async load?

Activating the loading screen first (step 1) ensures the user sees the loading UI immediately while the scene loads in the background, as seen in the execution_table.

What causes the while loop to exit?

The loop exits when AsyncOperation.isDone becomes True (step 6), meaning the scene has finished loading.

Visual Quiz - 3 Questions

Look at the execution table, what is AsyncOperation.progress at step 4?

A 0.9

B 0.4

C 0.7

D 1.0

💡 Hint

Check the 'AsyncOperation.progress' column at step 4 in the execution_table.

At which step does the loading screen get hidden?

A Step 6

B Step 7

C Step 5

D Step 1

💡 Hint

Look at the 'UI Update' column to find when the loading screen is hidden.

If 'yield return null' was removed from the loop, what would happen?

A The coroutine would complete instantly without waiting.

B The progress bar updates smoothly each frame.

C The loading screen would never activate.

D The coroutine would freeze and never update progress.

💡 Hint

Refer to the 'key_moments' explanation about 'yield return null' and the execution_table loop steps.

Concept Snapshot

Loading screens with coroutines in Unity:
- Activate loading UI
- Start async scene load with SceneManager.LoadSceneAsync
- Use a coroutine with while(!op.isDone) loop
- Update progress bar each frame with op.progress
- Use 'yield return null' to wait a frame
- Hide loading UI when loading completes

Full Transcript

This example shows how to use a coroutine in Unity to display a loading screen while a scene loads asynchronously. The coroutine activates the loading screen, starts loading the scene in the background, and repeatedly updates a progress bar based on the loading progress. It uses 'yield return null' inside a while loop to wait for the next frame, allowing the UI to update smoothly. When the loading finishes, the coroutine hides the loading screen and ends. The execution table traces each step, showing the AsyncOperation's progress and completion status, UI updates, and coroutine yields. Key moments clarify why the coroutine yields each frame and when the loading screen is shown and hidden. The visual quiz tests understanding of progress values, UI changes, and coroutine behavior.

Practice

1. What is the main purpose of using coroutines for loading screens in Unity?

easy

A. To run loading tasks without freezing the game UI

B. To speed up the loading process by using multiple threads

C. To automatically optimize game assets during loading

D. To replace the Update() method for all game logic

Solution

1. Step 1: Understand coroutine behavior

   Coroutines allow tasks to run over multiple frames without blocking the main thread.

2. Step 2: Relate to loading screens

   Using coroutines for loading lets the UI remain responsive and update progress smoothly.

3. Final Answer:

   To run loading tasks without freezing the game UI -> Option A

4. Quick Check:

   Coroutines keep UI responsive during loading = A [OK]

Hint: Coroutines prevent freezing by spreading work over frames [OK]

Common Mistakes:

• Thinking coroutines run on separate threads
• Believing coroutines speed up loading automatically
• Confusing coroutines with Update() method

2. Which of the following is the correct way to start a coroutine named LoadSceneAsync in Unity?

easy

A. StartCoroutine(LoadSceneAsync());

B. StartCoroutine(LoadSceneAsync);

C. StartCoroutine("LoadSceneAsync");

D. StartCoroutine.LoadSceneAsync();

Solution

1. Step 1: Recall coroutine syntax

   Coroutines are started by calling StartCoroutine with the IEnumerator method invoked, like StartCoroutine(MethodName()).

2. Step 2: Check options

   StartCoroutine(LoadSceneAsync()); correctly calls StartCoroutine with LoadSceneAsync() including parentheses.

3. Final Answer:

   StartCoroutine(LoadSceneAsync()); -> Option A

4. Quick Check:

   StartCoroutine needs method call with parentheses = A [OK]

Hint: StartCoroutine requires parentheses to call the IEnumerator method [OK]

Common Mistakes:

• Omitting parentheses when calling the coroutine method
• Using string names instead of method calls
• Trying to call coroutine like a normal method

3. Given this coroutine code snippet, what will be printed to the console?

IEnumerator LoadScene()
{
    AsyncOperation asyncLoad = SceneManager.LoadSceneAsync("GameScene");
    while (!asyncLoad.isDone)
    {
        Debug.Log($"Loading progress: {asyncLoad.progress * 100}%");
        yield return null;
    }
    Debug.Log("Loading complete!");
}

medium

A. Loading progress: 0% ... Loading progress: 100% ... Loading complete!

B. Loading progress: 0% ... Loading progress: 50% ... Loading complete!

C. Loading progress: 0% ... Loading progress: 90% ... Loading complete!

D. Only "Loading complete!" is printed

Solution

1. Step 1: Understand AsyncOperation progress

   AsyncOperation.progress goes from 0 to 0.9 during loading; it reaches 0.9 before isDone becomes true.

2. Step 2: Analyze the loop output

   The loop prints progress from 0% up to 90%, then exits when isDone is true, printing "Loading complete!".

3. Final Answer:

   Loading progress: 0% ... Loading progress: 90% ... Loading complete! -> Option C

4. Quick Check:

   AsyncOperation.progress maxes at 0.9 before done = C [OK]

Hint: AsyncOperation.progress maxes at 0.9 before completion [OK]

Common Mistakes:

• Assuming progress reaches 100% before isDone
• Expecting progress to jump directly to 100%
• Ignoring that isDone triggers after 90% progress

4. Identify the error in this coroutine code for a loading screen:

IEnumerator LoadLevel()
{
    AsyncOperation asyncLoad = SceneManager.LoadSceneAsync("Level1");
    while (asyncLoad.isDone == false)
    {
        progressBar.fillAmount = asyncLoad.progress;
        yield return new WaitForSeconds(1);
    }
}

medium

A. AsyncOperation.progress should be multiplied by 100

B. The while loop condition should be asyncLoad.isDone == true

C. The coroutine never yields control to the main thread

D. Using WaitForSeconds delays UI updates unnecessarily

Solution

1. Step 1: Check yield statement impact

   Using yield return new WaitForSeconds(1) pauses the coroutine for 1 second each loop, causing slow UI updates.

2. Step 2: Suggest better yield usage

   Yielding null (yield return null) updates every frame, making progress bar smoother and more responsive.

3. Final Answer:

   Using WaitForSeconds delays UI updates unnecessarily -> Option D

4. Quick Check:

   WaitForSeconds slows progress updates = B [OK]

Hint: Use yield return null for smooth frame-by-frame updates [OK]

Common Mistakes:

• Confusing progress value scale
• Changing loop condition incorrectly
• Not yielding at all causing freeze

5. You want to create a loading screen that shows a progress bar and a "Loading..." text that updates smoothly while loading a scene asynchronously. Which approach correctly combines coroutines and UI updates?

hard

A. Load the scene synchronously and update UI after loading finishes

B. Start a coroutine that updates progressBar.fillAmount with asyncLoad.progress every frame and updates loadingText with percentage until asyncLoad.isDone is true

C. Start a coroutine that waits 5 seconds then sets progressBar.fillAmount to 1 and loadingText to "Done"

D. Use Update() method to check asyncLoad.progress and update UI, no coroutine needed

Solution

1. Step 1: Understand asynchronous loading with UI updates

   Using a coroutine to check asyncLoad.progress each frame allows smooth progress bar and text updates during loading.

2. Step 2: Evaluate options for UI responsiveness

   Start a coroutine that updates progressBar.fillAmount with asyncLoad.progress every frame and updates loadingText with percentage until asyncLoad.isDone is true correctly updates UI every frame until loading completes; other options either block UI or do not update smoothly.

3. Final Answer:

   Start a coroutine that updates progressBar.fillAmount with asyncLoad.progress every frame and updates loadingText with percentage until asyncLoad.isDone is true -> Option B

4. Quick Check:

   Coroutine + frame updates = smooth loading UI = D [OK]

Hint: Update UI inside coroutine each frame until loading completes [OK]

Common Mistakes:

• Using synchronous loading blocking UI
• Not updating UI every frame
• Waiting fixed time instead of checking progress