FadeTransition.cs

using System.Collections;
using System.Collections.Generic;
using System.Threading.Tasks;
using System.Runtime.CompilerServices;
using UnityEngine;

public class FadeTransition
{
    [SerializeField] CanvasGroup canvasGroup = null;

    public FadeTransition(CanvasGroup canvasGroup) {
        this.canvasGroup = canvasGroup;
        this.canvasGroup.alpha = 0;
    }

    public async Task StartFade(float time, float to, System.Action onComplete = null) {
        var tween = LeanTween.value(canvasGroup.gameObject, canvasGroup.alpha, to, time).setOnUpdate((v) => {
            canvasGroup.alpha = v;
        });
        await tween;
        onComplete?.Invoke();
    }
}
public static class LeanTweenExtension
{
    public static TaskAwaiter<bool> GetAwaiter(this LTDescr leanTween)
    {
        var tcs = new TaskCompletionSource<bool>();

        leanTween.setOnComplete(() => {
            tcs.SetResult(true);
        });

        return tcs.Task.GetAwaiter();
    }
}