title: Mocking KVO with OCMock
body: <p>I would like to test that Key-Value-Observation is working properly for a class of mine. It has one property, which depends on another. They are set up like so:</p>
  
  <pre><code>+ (NSSet *)keyPathsForValuesAffectingSecondProperty {
      return [NSSet setWithObjects:
              @"firstProperty",
              nil];
  }
  
  - (NSArray *)secondProperty {
      return [self.firstProperty array];
  }
  </code></pre>
  
  <p>I want to run a unit test to verify that when <code>firstProperty</code> changes, an object bound to <code>secondProperty</code> gets a notification. At first I thought I would be able to use <code>+[OCMockObject observerMock]</code>, but it looks like that can only be used with <code>NSNotificationCenter</code>. What would be the best way to test this?</p>