title: How to validate Vuetify text field asynchronously?
body: <p><a href="https://vuetifyjs.com/en/components/text-fields" rel="noreferrer">Text fields</a> in Vuetify have <code>rules</code> props, which take an array of functions returning <code>true</code> or an error string. How to make them async, so that the validation could be made server-side using XHR?</p>
  
  <p>Something like:</p>
  
  <pre><code>&lt;v-text-field :rules="[v =&gt; { axios.get('/check?value=' + val).then(() =&gt; { return true }) }]"&gt;
  </code></pre>