如何根据其他下拉值设置dropdown的$dirty和$invalid状态

<select ng-model="selectedValue">
  <option value="option1">Option 1</option>
  <option value="option2">Option 2</option>
  <option value="option3">Option 3</option>
</select>

$scope.$watchGroup(['otherDropdown1', 'otherDropdown2'], function(newValues, oldValues) {
  var otherDropdown1Value = newValues[0];
  var otherDropdown2Value = newValues[1];

  if (otherDropdown1Value === 'someValue' && otherDropdown2Value === 'someOtherValue') {
    $scope.myForm.selectedValue.$dirty = true;
    $scope.myForm.selectedValue.$invalid = true;
  } else {
    $scope.myForm.selectedValue.$dirty = false;
    $scope.myForm.selectedValue.$invalid = false;
  }
});

<div ng-show="myForm.selectedValue.$dirty && myForm.selectedValue.$invalid">
  <p>Please select a valid option.</p>
</div>