如何在angularjs中获取未选中的复选框值？

<div ng-app="myApp" ng-controller="myCtrl">
  <label ng-repeat="item in items">
    <input type="checkbox" ng-model="selectedItems[item]" ng-change="updateSelectedItems()"> {{ item }}
  </label>
</div>

angular.module('myApp', [])
  .controller('myCtrl', function($scope) {
    $scope.items = ['Item 1', 'Item 2', 'Item 3', 'Item 4'];
    $scope.selectedItems = {};

    $scope.updateSelectedItems = function() {
      $scope.unselectedItems = [];

      for (var item in $scope.items) {
        if (!$scope.selectedItems[item]) {
          $scope.unselectedItems.push(item);
        }
      }
    };
  });