How can I safely unsubscribe from an Action in one line?

I wish this helpful for you No — although you could try to set a cookie. If one isn't in the request then either it is a new subscriber or someone who uses a system that doesn't support cookies (which I suspect would be quite common). No — although if the client supported cookies you could look to see when a unique id stopped requesting the feed. Hourly is fairly typical. Generally speaking, you don't. The typical use case for RSS is to inform people when there is new content, not to distribute the entire archive. You could use the format to distribute an archive, but that would typically be used for a one shot initialization of a system which would then subscribe to the subscription feed to get updates.

like below fixes the issue I want to subscribe on an IObservable and unsubscribe (dipose) the subscription right after receiving the first element of type T, i.e. I only want to call the action on the very first element I get after subscribing.

var source = new Subject();

source
  .Take(1)
  .Subscribe(Console.WriteLine);

source.OnNext(5);
source.OnNext(6);
source.OnError(new Exception("oops!"));
source.OnNext(7);
source.OnNext(8);

// Output is "5". Subscription is auto-disposed. Error is ignored.

I wish did fix the issue. It was an issue with beta 2 of Rails 5. I've tried it in the latest version - beta 4 and it is now triggered as expected.
Here's the github issue for reference: https://github.com/rails/rails/pull/23715

this will help This is what Observable.create is for. You can create your own observable with specified attach/detach handlers, and can even wrap existing observables with just 2 additional lines of code.

const obs = Rx.Observable.create(observer => {
  console.log('attach');
  // If you want to wrap another observable, call this:
  // const subs = other.subscribe(observer);
  return () => {
    // subs.unsubscribe();
    console.log('detach');
  };
});

console.log('subscribe subscription1');
const subscribtion1 = obs.subscribe(() => {});
console.log('subscribe subscription2');
const subscribtion2 = obs.subscribe(() => {});
setTimeout(() => {
  console.log('subscribtion1.dispose()');
  subscribtion1.unsubscribe();
}, 500);
setTimeout(() => {
  console.log('subscribtion2.dispose()');
  subscribtion2.unsubscribe();
}, 1000);

<script src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/5.0.0-rc.4/Rx.js"></script>

will help you
When you call complete on the subject, any subscribers will be automatically unsubscribed.

complete() {
  if (this.closed) {
    throw new ObjectUnsubscribedError();
  }
  this.isStopped = true;
  const { observers } = this;
  const len = observers.length;
  const copy = observers.slice();
  for (let i = 0; i < len; i++) {
    copy[i].complete();
  }
  this.observers.length = 0;
}