title: When should we delete the cache data in an iPhone apps?
body: <p>We often cache image and data to improve our iPhone app's performance. But what strategy do you use to manage the cache data, such as delete or update it?</p>
  
  <p>I saved the images to TMP folder, but don't know when should I trigger to "checking out of date cache data and delete it": when the iPhone apps starts, or quits, or in idle time? </p>