如何在不改变dropdown的模型值的情况下，从dropdown绑定的文本框中获取模型值

<select ng-model="selectedItem">
  <option value="1">选项1</option>
  <option value="2">选项2</option>
  <option value="3">选项3</option>
</select>

$scope.selectedItem = '1';
$scope.selectedItemText = '选项1';

<select ng-model="selectedItem" ng-change="updateSelectedItemText()">
  <option value="1">选项1</option>
  <option value="2">选项2</option>
  <option value="3">选项3</option>
</select>

$scope.updateSelectedItemText = function() {
  if ($scope.selectedItem === '1') {
    $scope.selectedItemText = '选项1';
  } else if ($scope.selectedItem === '2') {
    $scope.selectedItemText = '选项2';
  } else if ($scope.selectedItem === '3') {
    $scope.selectedItemText = '选项3';
  }
};