title: what is the CFNotificationCenter callback function observer?
body: <p>I'm new to the CFNotificationCenter. I was reading the documentation at developer.apple and it says that in the CallbackFunction that observer is "an arbitrary value other than NULL that identifies the observer". Can the observer be an object so that I can access its methods inside the callback function?
  These are the parameters as shown in the documentation.</p>
  
  <pre><code>   void MyCallBack (
         CFNotificationCenterRef center,
         void *observer,
         CFStringRef name,
         const void *object,
         CFDictionaryRef userInfo
      );
  </code></pre>
  
  <p>Another question. Why is it allowed for an observer to be nul when adding an observer??</p>
  
  <pre><code>void CFNotificationCenterAddObserver (
     CFNotificationCenterRef center,
     const void *observer,
     CFNotificationCallback callBack,
     CFStringRef name,
     const void *object,
     CFNotificationSuspensionBehavior suspensionBehavior
  );
  </code></pre>
  
  <p>"The observer. In OS X v10.3 and later, this parameter may be NULL."</p>