/// <summary>
/// Works out how long to cache based on a default period and any expiry dates, relative to a start date.
/// </summary>
/// <param name="relativeToDate">The start date cache time spans are relative to, usually <see cref="DateTime.UtcNow"/>.</param>
/// <param name="defaultCachePeriod">The default cache period.</param>
/// <param name="contentExpiryDates">Expiry dates for any content, either part of a page or the whole page.</param>
/// <returns>An absolute time and relative timespan representing how long to cache the content for.</returns>
public CacheFreshness WorkOutCacheFreshness(DateTime relativeToDate, TimeSpan defaultCachePeriod, IList <DateTime> contentExpiryDates)
{
if (contentExpiryDates == null)
{
throw new ArgumentNullException("contentExpiryDates");
}
// Convert date to UTC so that it can be compared on an equal basis
relativeToDate = relativeToDate.ToUniversalTime();
// How long is this page fresh for?
var freshness = new CacheFreshness()
{
FreshFor = defaultCachePeriod,
FreshUntil = relativeToDate.Add(defaultCachePeriod)
};
for (var i = 0; i < contentExpiryDates.Count; i++)
{
// Convert date to UTC so that it can be compared on an equal basis
contentExpiryDates[i] = contentExpiryDates[i].ToUniversalTime();
// Check if some or all of the content expires sooner than the default cache period?
OverrideDueToContentExpiry(freshness, relativeToDate, contentExpiryDates[i]);
}
return(freshness);
}
private static void OverrideDueToContentExpiry(CacheFreshness freshness, DateTime relativeToDate, DateTime expiryDate)
{
if (expiryDate > relativeToDate && expiryDate < freshness.FreshUntil)
{
freshness.FreshFor = expiryDate.Subtract(relativeToDate);
freshness.FreshUntil = relativeToDate.Add(freshness.FreshFor);
}
}
