title: How to watch child properties changes from parent component
body: <p>I am using a date picker component library and i want to watch when a property of that component changes.</p>
  
  <p>I have tried this: </p>
  
  <pre><code>  watch: {
      '$refs.picker.popupVisible': {
        handler (new_value) {
          console.log('executed')
        },
        deep: true
      }
    }
  </code></pre>
  
  <p>Also this:</p>
  
  <pre><code>  computed: {
      picker () {
        console.log(this.$refs.picker.popupVisible)
        return this.$refs.picker.popupVisible
      }
    }
  </code></pre>
  
  <p>I know that the solution will be a vue.js hack because this is not the right way.If i had access to child component i would emit en event to parent but unfortunately i don't have.</p>