title: watch for a property in vue component
body: <p>I have an async operation in parent component and need to notify child, that this operation was completed. Could I watch in child for a property change. Somewhat like:</p>
  
  <pre><code>new Vue({
     props:['parentReady'],
     watch:{
       parentReady(val){
         alert('parentReady prop was changed');
       }
    }
  })
  </code></pre>