public void Subscribe(IObservable <ITweetItem> items, Action <ITweetItem> optionalActionOnSubscribe = null)
{
optionalActionOnSubscribe = optionalActionOnSubscribe ?? (_ => { });
IsBusy = true;
_subscription.DisposeIfNotNull();
_subscription = items
.SubscribeOnThreadPool()
.ObserveOnDispatcher()
.Do(_ => IsBusy = false)
.Do(x => optionalActionOnSubscribe(x))
.Subscribe(x => Tweets.Append(x), () => IsBusy = false);
((IActivate)this).Activate();
}
