title: When is PHPhotoLibraryChangeObserver photoLibraryDidChange: called?
body: <p>I am developing an iOS app with the Photos framework, and I am trying to get the change observer working.  I am registering the change observer with this call <code>[[PHPhotoLibrary sharedPhotoLibrary] registerChangeObserver:self];</code>.  After registering the observer, I switch to the Camera app and take a photo. The callback method <code>photoLibraryDidChange:</code> does not get called immediately after taking the photo, but only after I re-enter my app.</p>
  
  <p>Is that how it is supposed to work?  I could not find anything in the documentation that said when exactly the callback would be called.  Ideally I would get notified as soon as the picture was taken and put into the photo library, but that does not seem to be the case.  Does the user have to re-enter my app before I am notified of the change?</p>