title: anyone know when the method -photoLibraryDidChange called?
body: <p>i have test the method for many times, one thing for sure ,when the photo library changes ,the method was called. but after the change , the method would be called another twice( doesn't do anything ). do any one know why </p>
  
  <pre><code>2015-08-25 14:16:04.420 PhotoLibrary[25742:3293667] enter photoLibraryDidChange methods 
  2015-08-25 14:16:04.445 PhotoLibrary[25742:3283461] something inserted. 
  2015-08-25 14:16:17.199 PhotoLibrary[25742:3293667] enter photoLibraryDidChange methods 
  2015-08-25 14:16:17.522 PhotoLibrary[25742:3293668] enter photoLibraryDidChange methods 
  2015-08-25 14:17:04.762 PhotoLibrary[25742:3295134] enter photoLibraryDidChange methods 
  2015-08-25 14:17:04.796 PhotoLibrary[25742:3283461] something changed. 
  2015-08-25 14:17:18.056 PhotoLibrary[25742:3295135] enter photoLibraryDidChange methods 
  2015-08-25 14:17:18.366 PhotoLibrary[25742:3295137] enter photoLibraryDidChange methods 
  2015-08-25 14:18:22.915 PhotoLibrary[25742:3297134] enter photoLibraryDidChange methods 
  2015-08-25 14:18:22.932 PhotoLibrary[25742:3283461] something changed. 
  2015-08-25 14:18:34.275 PhotoLibrary[25742:3297133] enter photoLibraryDidChange methods 
  2015-08-25 14:18:34.602 PhotoLibrary[25742:3297130] enter photoLibraryDidChange methods
  </code></pre>