title: How to set a value for my custom key (undefinedKey) in NSObject instance?
body: <p>I have subclassed NSObject. And I need to set an object (<code>NSMutableArray</code> instance) to an undefined key (<code>@"ItemsList"</code>) to that class's instance. How can I do that? For exmaple, I want to set,</p>
  
  <pre><code>[myObject setValue:myArray forUndefinedKey:@"ItemsList"];
  </code></pre>
  
  <p>What are the things I should do to make this work?</p>