AsyncOperationExtensions.cs

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

public static class AsyncOperationExtensions
{
    public static AsyncTools.Awaiter GetAwaiter(this AsyncOperation asyncOp) => new AsyncOperationAwaiter(asyncOp);

    private class AsyncOperationAwaiter : AsyncTools.Awaiter
    {
        private readonly AsyncOperation asyncOp;

        public AsyncOperationAwaiter(AsyncOperation asyncOp)
        {
            this.asyncOp = asyncOp;
        }

        public override bool IsCompleted => asyncOp.isDone;

        public override void OnCompleted(Action action)
        {
            Task.Factory.StartNew(async () =>
                                        {
                                            while (asyncOp.isDone == false)
                                            {
                                                await 0;
                                            }
                                            action();
                                        },
                CancellationToken.None,
                TaskCreationOptions.None,
                UnityScheduler.UpdateScheduler);
        }
    }
}