public async Task <FeedSubscription> Subscribe(string feedUrl)
{
if (feedUrl == null)
{
throw new ArgumentNullException(nameof(feedUrl));
}
if (!await RssFeed.IsValid(feedUrl))
{
throw new FeedException("Invalid feed url or feed content is not supported");
}
var feed = await RssFeed.CreateFromUrl(feedUrl);
var subscription = feed.SubscribeTo();
await feedRepository.AddSubscription(subscription);
return(subscription);
}