如何淡出父母的$mdDialog？

angular.module('myApp').controller('ParentController', function($scope, $mdDialog) {
  $scope.openDialog = function() {
    $mdDialog.show({
      templateUrl: 'dialog-template.html',
      controller: 'DialogController'
    });
  };

  $scope.closeDialog = function() {
    $mdDialog.hide();
  };
});

对话框控制器：
angular.module('myApp').controller('DialogController', function($scope, $mdDialog) {
  $scope.closeDialog = function() {
    $mdDialog.hide();
  };
});

对话框模板（dialog-template.html）：
<md-dialog>
  <md-dialog-content>
    <p>This is a dialog.</p>
  </md-dialog-content>
  <md-dialog-actions>
    <md-button ng-click="closeDialog()">Close</md-button>
  </md-dialog-actions>
</md-dialog>