UnityWebRequestAwaiter.cs

public static class UnityWebRequestExtensions
{
    public static UnityWebRequestAwaiter GetAwaiter(this UnityWebRequestAsyncOperation asyncOperation)
    {
        return new UnityWebRequestAwaiter(asyncOperation);
    }
}

public class UnityWebRequestAwaiter : INotifyCompletion
{
    private Action _continuation;
    private bool _isPendingContinuation;
    private bool _isDone;
    
    public UnityWebRequestAwaiter(UnityWebRequestAsyncOperation asyncOperation)
    {
        asyncOperation.completed += OnRequestCompleted;
    }

    public bool IsCompleted => _isDone;

    public void GetResult() { }

    public void OnCompleted(Action continuation)
    {
        if (_isPendingContinuation)
        {
            CallNonNullContinuation(continuation);
        }
        else
        {
            _continuation = continuation;
        }
    }

    private void OnRequestCompleted(AsyncOperation asyncOp)
    {
        if (_continuation == null)
        {
            _isPendingContinuation = true;
        }
        else
        {
            CallNonNullContinuation(_continuation);
        }
    }

    private void CallNonNullContinuation(Action continuation)
    {
        _isDone = true;
        continuation();
    }
}