title: set hidden form field value with selected value
body: <p>currently I'm trying to get the selected value form a <code>dxSelectBox</code> and assign it to a hidden form field. I have tried it in the markup with template reference and I have tried it in the controller file (access the <code>DxiItemComponent</code> and get its value.</p>
  <p>Actual, I have this markup:</p>
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;dxi-item #selectBox dataField="managerId" [label]="{text: 'Felettes'}"
                editorType="dxSelectBox"
                [editorOptions]="{dataSource: managers,
                                  valueExpr: 'id',
                                  displayExpr: 'name',
                                  placeholder: 'Kérlek válassz',
                                  searchEnabled: true,
                                  searchExpr: 'name',
                                  searchMode: 'contains',
                                  openOnFieldClick: true,
                                  deferRendering: false,
                                  noDataText: 'Nincs megjelenítendő adat',
                                  onValueChanged: managerNameChanged}"&gt;
        &lt;dxi-validation-rule type="required" message="A mező kitöltése kötelező"&gt;&lt;/dxi-validation-rule&gt;
      &lt;/dxi-item&gt;
      &lt;dxi-item dataField="manager.name" value="selectBox.value.name"&gt;&lt;/dxi-item&gt;</code></pre>
  </div>
  </div>
  </p>
  <p>And the <code>managerNameChanged</code> looks like this:</p>
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>  managerNameChanged() {
      alert(this.selectBox._value);
    }</code></pre>
  </div>
  </div>
  </p>
  <p>But I get undefined for the <code>selectBox</code>. I declare it like this:</p>
  <pre><code>  @ViewChild(&quot;selectBox&quot;) selectBox: DxiItemComponent;
  </code></pre>
  <p>So, the question is, how do I get the value for the <code>dxSeletBox</code> editor?</p>