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.