title: How to use 'onchange' in Yii2 MaskedInput widget
body: <p>I have an Yii2 app with advanced template. There is 2 fields on my form. 1st: MaskedInput and 2nd textInput with readonly attribute.
  So, I wanted to fill 2nd textInput automatically right after I have filled MaskedInput. For this I tried to use onchange. But I got following error: </p>
  
  <blockquote>
  <pre><code>Unknown Property – yii\base\UnknownPropertyException
  Setting unknown property: yii\widgets\MaskedInput::onchange
  </code></pre>
  </blockquote>
  
  <p>Here is my code:</p>
  
  <pre><code>&lt;?php $form = ActiveForm::begin(); ?&gt;
  &lt;div class="row"&gt;
      &lt;div class="col-xs-3"&gt;
          &lt;?= Html::label("Ser num")?&gt;
          &lt;?= MaskedInput::widget(['name'=&gt;'serNum',
              'mask'=&gt;'AA 9999999',
              'onchange'=&gt;'
                  $.post("index.php?r=act/actid&amp;serNum='.'"+$(this).val(),function(data){
                  $("select#ser-sernum").html(data);
          });
              '
          ])?&gt;
      &lt;/div&gt;
      &lt;div class="col-xs-3"&gt;
          &lt;?= $form-&gt;field($model, 'sernum')-&gt;textInput(['readonly'=&gt;true]) ?&gt;
      &lt;/div&gt;
      &lt;div class="col-xs-6"&gt;
          &lt;b id="actstatus"&gt;&lt;/b&gt;
      &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>