title: Add property observer to global variable inside class in Swift
body: <p>I have a variable <code>globalVariable</code> declared at global scope that may change at any time. </p>
  
  <p>Different <code>ViewControllers</code> in my app need to react differently, when <code>globalVariable</code> changes. </p>
  
  <p>Thus it would be desirable to add a property observer in each <code>ViewController</code> that execute the needed code when <code>globalVariable</code> changes.</p>
  
  <p><em>I cannot seem to achieve it with <code>override</code> or <code>extension</code></em>. <em>What is the way to go here?</em></p>