Task.Yield() / Task.Delay(0) hang

Code (CSharp):

1. using System.Threading.Tasks;
2.  
3. using UnityEngine;
4.  
5. public class Test : MonoBehaviour
6. {
7.     void Start()
8.     {
9.         TestAsync();
10.     }
11.  
12.     async void TestAsync()
13.     {
14.         int i = 0;
15.         while (true)
16.         {
17.             Debug.Log($"### TestAsync: {++i}");
18.             await Task.Delay(1); // ok
19.             // await Task.Delay(0); // hang!
20.             // await Task.Yield(); // hang!
21.         }
22.     }
23. }
24.  

While posting this thread, I do notice that there is another thread talking about a similar issue: https://forum.unity.com/threads/help-with-async-await.470214/

But regardless, the above code when attached to a scene will hang the whole Unity editor when using Task.Delay(0) and Task.Yield(), is this the expected behavior?

 

Did the process continuation move from Fixed Update ? I can't find any documentation on it. It'd be great to see that in the player loop here: https://docs.unity3d.com/Manual/ExecutionOrder.html

 

I stumbled across this and did some digging (Unity 2019 LTS). It seems it moved to Update (evaluated once per frame). It is called between Update() and LateUpdate() and after coroutine "yield null".

Here is how the updated lifecycle graph would look like:



One caveat: they may be processed after the "Internal animation update", I have not tested that.
Test was done with a loop like this:

Code (CSharp):

1. while (predicate() == false)
2. {
3.     await Task.Yield();
4. }

If you want to be extra sure it's done only once per frame you may use something like this:

Code (CSharp):

1.  
2. // Thanks to https://github.com/modesttree/Unity3dAsyncAwaitUtil
3. // License MIT: https://github.com/modesttree/Unity3dAsyncAwaitUtil/blob/master/License.md
4. using System;
5. using System.Threading;
6. using System.Threading.Tasks;
7. using UnityEngine;
8.  
9. public static class Awaiters
10. {
11.     readonly static WaitForUpdate _waitForUpdate = new WaitForUpdate();
12.  
13.     public static WaitForUpdate NextFrame
14.     {
15.         get { return _waitForUpdate; }
16.     }
17.  
18.     public static async Task Until(CancellationToken ct, Func<bool> predicate)
19.     {
20.         while(predicate() == false && ct.IsCancellationRequested == false)
21.         {
22.             await NextFrame;
23.         }
24.     }
25.  
26.     public static async Task While(CancellationToken ct, Func<bool> predicate)
27.     {
28.         while (predicate() == true && ct.IsCancellationRequested == false)
29.         {
30.             await NextFrame;
31.         }
32.     }
33. }

Would be nice to get some confirmation for this.

 

I am facing the new bug in Unity2021.3.0f1:
When I use await Task.Yield() in Editor, everything works fine, but after I build the game, it just 'freeze' on this line of code. Here's the test code:

Code (CSharp):

1.     void Start()
2.     {
3.         Test();
4.     }
5.     async void Test()
6.     {
7.         Debug.LogError("Test Begin");
8.         await Task.Yield();
9.         Debug.LogError("Test End");//Will not be executed!
10.     }

 

Are you using IL2CPP? If yes, then it doesn’t work.

 

Thanks for your reply, I am using the good old Mono.

 

For async/await use this: https://github.com/Cysharp/UniTask