title: How to extend default implementation (with an extension) adding a didSet on a property?
body: <p>How can I implement <code>didSet</code> on MyProtocol <code>var</code> through an extension?</p>
  
  <p>I need to run some specific code after it has been set.</p>
  
  <p>I tried this, but I get this error:</p>
  
  <blockquote>
    <p>Extensions may not contain stored properties</p>
  </blockquote>
  
  <pre><code>protocol MyProtocol {
      var contact: MyContact? { get set }
  }
  
  extension MyProtocol {
      var contact: MyContact? {
          didSet {
              // some code
          }
      }
  }
  </code></pre>