Looks like UniTask.Yield(PlayerLoopTiming.Update) is not resuming at Update sometimes

[rage2050a]

Hello,

I'm using Unity 2023.2.11f1 and UniTask 2.5.4.
Here's what I found (Japanese). https://qiita.com/NekoBeko2024/items/51e57efc60436568dd73
The description below is same as what in the URL above.

Summary

To be short, it looks to me that

await UniTask.Yield(PlayerLoopTiming.Update, cancellationToken: cancel);

is not resuming the execution flow at Update sometimes and the behaviour is not repeatable.

Procedure

That is,

I started from a Standard (URP) scene, created an empty GameObject, added MyUniTaskTest component as below..

using Cysharp.Threading.Tasks;
using System.Threading;
using UnityEngine;

public class MyUniTaskTest : MonoBehaviour
{
    string _phase;
    bool _done;

    void OnEnable()
    {
        _done = false;
    }

    void FixedUpdate()
    {
        _phase = "FixedUpdate_Begin";
        _phase = "FixedUpdate_End";
    }

    void Update()
    {
        _phase = "Update_Begin";
        if (!_done)
        {
            _done = true;
            MyTaskAsync(destroyCancellationToken).Forget();
        }
        _phase = "Update_End";        
    }

    void LateUpdate()
    {
        _phase = "LateUpdate_Begin";
        _phase = "LateUpdate_End";        
    }

    async UniTaskVoid MyTaskAsync(CancellationToken cancel)
    {
        print($"MyTaskAsync");
        for (int i = 0; i<30; i++)
        {
            print($"{i}: {Time.frameCount} {_phase}");
            await UniTask.Yield(PlayerLoopTiming.Update, cancellationToken: cancel);
        }
    }
}

Run the scene in Play mode.

Results

What's printed in the Console was as follows.

i	1st run	2nd run	3rd run
0	Update_Begin	Update_Begin	Update_Begin
1	FixedUpdate_End	FixedUpdate_End	FixedUpdate_End
2	FixedUpdate_End	FixedUpdate_End	FixedUpdate_End
3	FixedUpdate_End	FixedUpdate_End	FixedUpdate_End
4	FixedUpdate_End	LateUpdate_End	LateUpdate_End
5	FixedUpdate_End	LateUpdate_End	LateUpdate_End
6	LateUpdate_End	LateUpdate_End	FixedUpdate_End
7	FixedUpdate_End	FixedUpdate_End	LateUpdate_End
8	LateUpdate_End	LateUpdate_End	LateUpdate_End
9	LateUpdate_End	FixedUpdate_End	LateUpdate_End

Expected

If the task is resumed at the timing of Update, which is after FixedUpdate and before LateUpdate, the output would be either FixedUpdate_End, Update_Begin, or Update_End.

Why is LateUpdate_End printed sometimes?

Correct me if I am using the API wrongly or I I misunderstand something.

[timcassell]

FixedUpdate is not guaranteed to run every frame. PlayerLoopTiming.Update runs before your MonoBehaviour's Update in the frame. Therefore, the only things it could ever be are Update_Begin FixedUpdate_End or LateUpdate_End, because FixedUpdate_End or LateUpdate_End were the last things to be updated on the previous frame.

[rage2050a]

PlayerLoopTiming.Update runs before your MonoBehaviour's Update in the frame.

Aha, then it makes sense. Thank you!

Because the test project runs with Time / Fixed Timestamp = 0.02, Quality / VSync Count = Every V Blank, and my display has 120Hz frequency, FixedUpdate is called less frequently than Update.

Suppose at the n-th frame ended. _phase = "LateUpdate_End";
Then starts (n+1)-th frame without FixedUpdate. UniTask runs before MyUniTaskTest.Update. So I see LateUpdate_End.