如何在启用和禁用多个复选框时设置ng-model null

<input type="checkbox" ng-model="selectedItems" ng-change="toggleSelection('checkbox1')"> Checkbox 1
<input type="checkbox" ng-model="selectedItems" ng-change="toggleSelection('checkbox2')"> Checkbox 2
<input type="checkbox" ng-model="selectedItems" ng-change="toggleSelection('checkbox3')"> Checkbox 3

angular.module('myApp', [])
  .controller('myController', function($scope) {
    $scope.selectedItems = [];

    $scope.toggleSelection = function(item) {
      var index = $scope.selectedItems.indexOf(item);
      if (index > -1) {
        $scope.selectedItems.splice(index, 1);
      } else {
        $scope.selectedItems.push(item);
      }
    };
  });