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

async UniTask ListenAsync(UniTaskCompletionSource source, CancellationToken ct)
{
    // This await will be cancelled if ct is cancelled, but the source stays alive.
    await source.Task.AttachExternalCancellation(ct);

    // if we got here, the source completed and you can handle the signal
}

var cts = new CancellationTokenSource();
var listen = ListenAsync(mySource, cts.Token);

// later, if this listener should stop waiting:
cts.Cancel(); // only this waiter is cancelled; others keep waiting

class Signal
{
    UniTaskCompletionSource tcs = new UniTaskCompletionSource();

    public UniTask NextAsync(CancellationToken ct)
        => tcs.Task.AttachExternalCancellation(ct);

    public void Fire()
    {
        tcs.TrySetResult();               // complete current waiters
        tcs = new UniTaskCompletionSource(); // new round for the next event
    }
}