Angular watch在同一指令中更改另一个实例

Question

我正在尝试使用angular指令实现经典的Country > State > City组合框。

我愿意通过自定义属性对name/depends-on链接它们，但我不知道是否有更好的表单。

我的HTML如下：

<div ng-app="app" ng-controller="Ctrl">
  <multiselect source-url="/countries.json" auto-init="true" name="Contries">
    <select multiple ng-model="$parent.selectedOptions" ng-change="$parent.handleChange()">
      <optgroup ng-repeat="(continent, countries) in $parent.optionList" label="{{continent}}">
        <option ng-repeat="country in countries" value="{{country.code}}">
          {{country.code}} - {{country.name}}
        </option>
      </optgroup>
    </select>
  </multiselect>
  <multiselect source-url="/states.json" depends-on="Countries" name="States">
    <select multiple ng-model="$parent.selectedOptions" ng-change="$parent.handleChange()">
      <optgroup ng-repeat="(continent, countries) in $parent.optionList" label="{{continent}}">
        <option ng-repeat="country in countries" value="{{country.code}}"> {{country.code}} - {{country.name}}
        </option>
      </optgroup>
    </select>
  </multiselect>
</div>

Javascript：

app.directive('multiselect', function () {
  return {
    restrict: 'E',
    transclude: true,
    scope: {
      sourceUrl : '@',
      autoInit : '@',
      dependsOn : '@'
    },
    controller: ['$scope', '$element', '$attrs', '$transclude', '$http', function ($scope, $element, $attrs, $transclude, $http) {

      $scope.handleChange = function handleChange() {
        console.log($scope.selectedOptions)
      }

      console.log($scope.dependsOn)

      function updateSource() {
        const config = {
          params : {}
        }

        if ($scope.dependsOn) {
          // how do I get dependsOnValue?????
          config.params[$scope.dependsOn.toLowerCase()] = $scope.dependsOnValue
        }

        $http.get($scope.sourceUrl, config)
          .then(function (response) {
            $scope.optionList = response.data
          })
      }

      if ($scope.autoInit) {
        updateSource()
      }
    }],
    template: '<ng-transclude></ng-transclude>'
  }
})

问题是：我如何关注Countries中的更改才能获得更新States的价值？

我试图让它尽可能重复使用。