/// <summary>
/// Determines the <see cref="TimeSpan"/> in which the <paramref name="key"/> expires.
/// </summary>
/// <param name="cache">The cache to use.</param>
/// <param name="key">The key to check.</param>
/// <param name="scheduler"><see cref="IScheduler"/> to perform the check on.</param>
/// <returns>
/// An observable stream that returns the <see cref="TimeSpan"/> in which the <paramref name="key"/> expires.
/// </returns>
public static IObservable <TimeSpan> ExpiresIn <TKey, TValue>(
this IObservableCache <TKey, TValue> cache,
TKey key, IScheduler scheduler = null)
{
if (cache == null)
{
throw new ArgumentNullException(nameof(cache));
}
if (key == null)
{
throw new ArgumentNullException(nameof(key));
}
return(cache.ExpiresIn(Observable.Return(key), scheduler));
}