title: Property change subscription with Aurelia
body: <p>I have a property on my viewmodel which I want to listen to and trigger events based on its value, like this:</p>
  
  <pre><code>class viewModel {
    constructor() {
      this.value = '0';
      let val = 2;
      subscribe(this.value, callbackForValue);
      subscribe(val, callbackForVal);
    }
  }
  </code></pre>
  
  <p>Is this a feature of Aurelia? If so, how would I go about setting up such a subscription?</p>