RssChannelDom DownloadChannelDom(string url)
{
// look for disk cache first
RssChannelDom dom = TryLoadFromDisk(url);
if (dom != null)
{
return(dom);
}
// download the feed
byte[] feed = new WebClient().DownloadData(url);
// parse it as XML
XmlDocument doc = new XmlDocument();
doc.Load(new MemoryStream(feed));
// parse into DOM
dom = RssXmlHelper.ParseChannelXml(doc);
// set expiry
string ttlString = null;
dom.Channel.TryGetValue("ttl", out ttlString);
int ttlMinutes = GetTtlFromString(ttlString, _defaultTtlMinutes);
DateTime utcExpiry = DateTime.UtcNow.AddMinutes(ttlMinutes);
dom.SetExpiry(utcExpiry);
// save to disk
TrySaveToDisk(doc, url, utcExpiry);
return(dom);
}
RssChannelDom TryLoadFromDisk(string url)
{
if (_directoryOnDisk == null)
{
return(null); // no place to cache
}
// look for all files matching the prefix
// looking for the one matching url that is not expired
// removing expired (or invalid) ones
string pattern = GetTempFileNamePrefixFromUrl(url) + "_*.rss";
string[] files = Directory.GetFiles(_directoryOnDisk, pattern, SearchOption.TopDirectoryOnly);
foreach (string rssFilename in files)
{
XmlDocument rssDoc = null;
bool isRssFileValid = false;
DateTime utcExpiryFromRssFile = DateTime.MinValue;
string urlFromRssFile = null;
try {
rssDoc = new XmlDocument();
rssDoc.Load(rssFilename);
// look for special XML comment (before the root tag)'
// containing expiration and url
XmlComment comment = rssDoc.DocumentElement.PreviousSibling as XmlComment;
if (comment != null)
{
string c = comment.Value;
int i = c.IndexOf('@');
long expiry;
if (long.TryParse(c.Substring(0, i), out expiry))
{
utcExpiryFromRssFile = DateTime.FromBinary(expiry);
urlFromRssFile = c.Substring(i + 1);
isRssFileValid = true;
}
}
}
catch {
// error processing one file shouldn't stop processing other files
}
// remove invalid or expired file
if (!isRssFileValid || utcExpiryFromRssFile < DateTime.UtcNow)
{
try {
File.Delete(rssFilename);
}
catch {
}
// try next file
continue;
}
// match url
if (urlFromRssFile == url)
{
// found a good one - create DOM and set expiry (as found on disk)
RssChannelDom dom = RssXmlHelper.ParseChannelXml(rssDoc);
dom.SetExpiry(utcExpiryFromRssFile);
return(dom);
}
}
// not found
return(null);
}