angularjs – 带隔离范围和ng模型的指令

问题：
当在指令中更新模型时，调用方的值未更新。

HTML：

<test-ng-model ng-model="model" name="myel"></test-ng-model>

指示：

app.directive(
    'testNgModel',[
    '$timeout','$log',function ($timeout,$log) {

    function link($scope,$element,attrs,ctrl) {
        var counter1 = 0,counter2 = 0;

        ctrl.$render = function () {
            $element.find('.result').text(JSON.stringify(ctrl.$viewValue))
        }

        $element.find('.one').click(function () {
            if ($scope.$$phase) return;
            $scope.$apply(function () {
                var form = angular.isObject(ctrl.$viewValue) ? ctrl.$viewValue : {};
                form.counter1 = ++counter1;
                ctrl.$setViewValue(form);
            });
        });
        $element.find('.two').click(function () {
            if ($scope.$$phase) return;
            $scope.$apply(function () {
                var form = angular.isObject(ctrl.$viewValue) ? ctrl.$viewValue : {};
                form.counter2 = ++counter2;
                ctrl.$setViewValue(form);
            });
        });

        $scope.$watch(attrs.ngModel,function (current,old) {
            ctrl.$render()
        },true)
    }

    return {
        require: 'ngModel',restrict: 'E',link: link,//if isolated scope is not set it is working fine
        scope: true,template: '<div><input type="button" class="one" value="One"/><input type="button" class="two" value="Two"/><span class="result"></span></div>',replace: true
    };

}]);

演示：Fiddle

如果隔离示波器没有设置，它工作正常：fiddle