task 可以通过CancellationTokenSource.token 来控制线程是否终止。
同时 还可以注册一个委托，来回调。
| Register(Action) | Registers a delegate that will be called when this CancellationToken is canceled. |
| ThrowIfCancellationRequested | Throws a OperationCanceledException if this token has had cancellation requested. |