title: Can I override setValue: forKey:?
body: <p>I have an object that is likely to have NSNull set as a value on all of its properties, and I'm working with code that doesn't play nice with NSNull values.  Is it possible to override setValue: forKey: in my object like this?</p>
  
  <pre><code>-(void)setValue:(id)value forKey:(NSString *)key {
  
      if ([value isEqual:[NSNull null]]) {
          [super setValue:nil forKey:key];
      } else {
          [super setValue:value forKey:key];
      }
  }
  </code></pre>
  
  <p>The method doesn't get called when <code>object.property = [NSNull null]</code> gets set on it.  How can I make this the default behavior for my setters?</p>