title: How to know if a file is "stored in the file-system caches"
body: <p>The docs for <code>NSDataReadingOptions</code> state:</p>
  
  <blockquote>
    <p>NSDataReadingUncached</p>
    
    <p>A hint indicating the file should not be stored in the file-system caches. For data being read once and discarded, this option can improve performance.</p>
  </blockquote>
  
  <p>That all makes sense. I'm curious if there's a way to know if a file already resided in the file-sysem caches.</p>
  
  <p>For example, if I need to perform a large amount of file reading, it might make sense to prioritise reading files which already reside in the cache.</p>