title: Cache Expiration Implementation using NSCache
body: <p>I am using NSCache to implement caching in my app.  I want to add expiration to it so it will obtain new data after some time.  What are the options and what's the best approach?</p>
  
  <p>Should I look at the timestamp when the cache is accessed and invalidate it then? Should the cache automatically invalidate itself by using a fixed interval timer?</p>