Coroutine is blocking main thread on save image

Question

I have a coroutine that takes a screenshot and saves to the file system. But when the coroutine runs the game freezes momentarily and the event onFinishedScreenShot fires prematurely.

I thought that Coroutines were "on a background thread".

Is there a way to adjust my code save a screenshot without blocking the main thread and have the event onFinishedScreenShot fire AFTER the write file is finished? Or is there a better way of handling saving screenshots for games?

MySingleton.cs

public event Action<string> onFinishedScreenShot;

public IEnumerator TakeScreenshot(string filename) {

    Texture2D areaImage = new Texture2D(Screen.width, Screen.height, TextureFormat.RGB24, false);

    yield return new WaitForEndOfFrame();

    areaImage.ReadPixels(new Rect(0, 0, Screen.width, Screen.height), 0, 0, false);
    areaImage.Apply();

    byte[] bytes = areaImage.EncodeToPNG();
    File.WriteAllBytes(filename, bytes);

    if (onFinishedScreenShot != null) {
        onFinishedScreenShot(filename);
    }
}

AnotherClass.cs

private void Start() {
    LoadTexture(someFileName);
}


public void SomeMethodInAnotherClass() {
        StartCoroutine(MySingleton.TakeScreenshot(someFileName));
}


public void LoadTexture(string filename) {

    if (!filename.Equals("NOT SET") ) {

        byte[] bytes = File.ReadAllBytes(GetSaveFilename(filename));

        Texture2D texture = new Texture2D(500, 500, TextureFormat.DXT5, false);
        texture.filterMode = FilterMode.Trilinear;
        texture.LoadImage(bytes);

        RectTransform rt = locationSnapShot.rectTransform;

        Sprite sprite = Sprite.Create(texture, new Rect(0, 0, rt.rect.width, rt.rect.height), new Vector2(0.5f, 0.0f), 16.0f);
        locationSnapShot.sprite = sprite;

        Debug.Log("Removing event listener.");
        MySingleton.onFinishedScreenShot -= LoadTexture;
    } else {
        Debug.Log("No screen shot. Listening for a change.");
        MySingleton.onFinishedScreenShot += LoadTexture;
    }
}

score 3 · Accepted Answer · answered Nov 09 '17 at 16:29

Unity coroutines and "threading" are two entirely different features. Coroutines are a way of managing when your code executes but have nothing to do with threads. Think of the two like this:

Coroutines: "Do X, wait for Y to be done, then do Z."

Threads: "Do X and Y concurrently, then do Z."

If you want to open up your own threads, C# has the ability to do that via System.Threading but you'll need to be careful; almost 100% of Unity APIs (anything coming from the UnityEngine namespace) will assert when they aren't run on the main thread. You'll need to get your raw byte data, open a new thread, then write the file:

...
byte[] bytes = areaImage.EncodeToPNG();

// Start a new thread here
File.WriteAllBytes(filename, bytes);

if (onFinishedScreenShot != null) {
    onFinishedScreenShot(filename);
}

For your last question about the event firing: File.WriteAllBytes is a synchronous call so your event is absolutely being fired after the file is being written (unless you fire that event from anywhere else not listed in the question).

Once again you have helped me understand! Thank you! @Foggzie — user-44651, Nov 09 '17 at 19:19

score 0 · Answer 2 · answered Nov 09 '17 at 15:44

0

Maybe it's because you have a circular reference in your script.

MySingleton.onFinishedScreenShot += LoadTexture;

You are registering the handler in the handler function. Try doing this in the Start()-function. This could fix the issue.

answered Nov 09 '17 at 15:44

Doern

7
1

score 0 · Answer 3 · answered Nov 09 '17 at 18:59

This has nothing to do with coroutine. Call the File.WriteAllBytes function in another Thread with ThreadPool.QueueUserWorkItem. First, get UnityThread class from this post which makes it easier to call a function on the main Thread from another Thread. Below is what your code is supposed to look like.

public event Action<string> onFinishedScreenShot;

//Data to pass to the Thread function
public class SaveData
{
    public string fileName;
    public byte[] imgByteArray;
}

void Awake()
{
    //Enable the Thread callback API
    UnityThread.initUnityThread();
}

public IEnumerator TakeScreenshot(string filename)
{

    Texture2D areaImage = new Texture2D(Screen.width, Screen.height, TextureFormat.RGB24, false);

    yield return new WaitForEndOfFrame();

    areaImage.ReadPixels(new Rect(0, 0, Screen.width, Screen.height), 0, 0, false);
    areaImage.Apply();

    SaveData saveData = new SaveData();
    saveData.fileName = filename;
    saveData.imgByteArray = areaImage.EncodeToPNG();

    //Save on a new Thread
    WriteAllBytes(saveData);
}

void WriteAllBytes(SaveData saveData)
{
    ThreadPool.QueueUserWorkItem(new WaitCallback(saveFile));
}

private void saveFile(object state)
{
    SaveData saveData = (SaveData)state;
    File.WriteAllBytes(saveData.fileName, saveData.imgByteArray);

    //Invoke the event on the MainThread
    UnityThread.executeInUpdate(() =>
    {
        if (onFinishedScreenShot != null)
        {
            onFinishedScreenShot(filename);
        }
    });
}

I'm currently having the same issue and it actually doesn't come from File IO but instead from the line `areaImage.EncodeToPNG` which might take a while. Unfortunately it uses UnityEngine and as [Foggzie](https://stackoverflow.com/a/47206968/7111561) says there is no way to make this async if Unity doesn't add this — derHugo, Nov 08 '18 at 12:31
@derHugo There is a way to do that with a native C++ plugin in another Thread. — Programmer, Nov 08 '18 at 14:14

Coroutine is blocking main thread on save image

3 Answers3