title: Vue input required if others are non-empty
body: <p>I have a form collecting an address. A fully specified address is okay, a completely empty one is okay, too, but a partial address is not okay.</p>
  
  <p>I'm trying to express that this way:</p>
  
  <pre><code>&lt;v-text-field :required="reqIn(address)" v-model="address.street" label="Street"&gt;&lt;/v-text-field&gt;
  &lt;v-text-field :required="reqIn(address)" v-model="address.city" label="City"&gt;&lt;/v-text-field&gt;
  &lt;v-text-field :required="reqIn(address)" v-model="address.state" label="State"&gt;&lt;/v-text-field&gt;
  
  // methods
  reqIn (address) {
    // addresses may be all blank, or fully specified
    let totalLength = address.street.length + address.city.length + address.state.length + address.zip.length
    console.log(totalLength)
    return totalLength === 0
  }
  </code></pre>
  
  <p>I can see the method getting called, and I can see the total length changing as I add and delete chars to the inputs, but when <code>totalLength</code> reaches 0, I expect the fields to become styled as required (if they are empty).  But I don't see that happening.  Any idea what I'm doing wrong?</p>