/// <summary>
/// This method attempts to returned a cached value, in the case of a
/// cache miss the fetchFunc will be used to provide a fresh value which
/// is then returned. In the case of a cache hit where the cache item is
/// considered stale (but not expired) as determined by <paramref name="refreshInterval"/>
/// the stale value will be produced first, followed by the fresh value
/// when the fetch func completes.
/// </summary>
/// <param name="blobCache">The cache to retrieve the object from.</param>
/// <param name="key">The key to look up the cache value with.</param>
/// <param name="fetchFunc">The fetch function.</param>
/// <param name="refreshInterval">
/// Cache objects with an age exceeding this value will be treated as stale
/// and the fetch function will be invoked to refresh it.
/// </param>
/// <param name="maxCacheDuration">
/// The maximum age of a cache object before the object is treated as
/// expired and unusable. Cache objects older than this will be treated
/// as a cache miss.
/// </param>
public static IObservable <byte[]> GetAndRefresh(
this IBlobCache blobCache,
string key,
Func <IObservable <byte[]> > fetchFunc,
TimeSpan refreshInterval,
TimeSpan maxCacheDuration)
{
return(Observable.Defer(() =>
{
var absoluteExpiration = blobCache.Scheduler.Now + maxCacheDuration;
try
{
return blobCache.GetAndFetchLatestBytes(
key,
fetchFunc,
createdAt => IsExpired(blobCache, createdAt, refreshInterval),
absoluteExpiration);
}
catch (Exception exc)
{
return Observable.Throw <byte[]>(exc);
}
}));
}