/// <summary>
/// Downloads the specified syndication <paramref name="feeds"/> and merges them into an observable sequence.
/// </summary>
/// <param name="formatter">The object that reads each feed.</param>
/// <param name="client">The object used to make the web request.</param>
/// <param name="feeds">The <see cref="Uri"/> objects identifying the feeds to be downloaded.</param>
/// <remarks>
/// <alert type="tip">
/// To get the original <see cref="SyndicationFeed"/> for each <see cref="SyndicationItem"/>, use the <see cref="SyndicationItem.SourceFeed"/> property.
/// </alert>
/// </remarks>
/// <returns>An observable sequence of items from all of the feeds merged together.</returns>
public static IObservable<SyndicationItem> DownloadObservable(this SyndicationFeedFormatter formatter, WebClient client, IEnumerable<Uri> feeds)
{
Contract.Requires(formatter != null);
Contract.Requires(client != null);
Contract.Requires(feeds != null);
Contract.Ensures(Contract.Result<IObservable<SyndicationItem>>() != null);
return (from uri in feeds
select Observable.Defer(() => from stream in client.OpenReadObservable(uri)
select GetFeed(stream, formatter)))
.Concat()
.SelectMany(feed => feed.Items.Do(item =>
{
if (item.SourceFeed == null)
{
item.SourceFeed = feed;
}
}));
}
