title: Reactive Cocoa diferences between KVC and instance object
body: <p>I use this code to subscribe events with my mutable array:</p>
  
  <pre><code>[RACObserve(self, marray) subscribeNext:^....
  </code></pre>
  
  <p>I am new in RAC and I don't understand why for example if I use:</p>
  
  <pre><code>NSMutableArray *keyValueCodingArray = [self mutableArrayValueForKey:@"marray"];
  
  [keyValueCodingArray addObject:@"string"];
  </code></pre>
  
  <p>everything works ok and if I add something to my <code>keyValueCodingArray</code> which is my <code>marray</code> it reacts on events and if I remove some object from keyValueCodingArray it also react.</p>
  
  <p>But if use the instance directly the RACObserve block wont work. I mean seems like it does not observe any changes with array:</p>
  
  <pre><code>[self.marray removeObjectAtIndex:0];
  </code></pre>
  
  <p>What is the differences?</p>