Angular js指令在表单中未保存数据时显示浏览器后退按钮的警报

我是angularjs的新手。当表单有未保存的数据并且用户按下浏览器返回按钮时，是否有任何方法显示警报。任何关于如何实现这一目标的建议都将不胜感激。

更新：：

    angular.module('myApp.directives', []).directive('confirmOnExit', function() {
    return {
        link: function($scope, elem, attrs) {

            $scope.$on('$locationChangeStart', function(event, next, current) {
                if ($scope.myForm.$dirty) {
                    if(!confirm("Ahh the form is dirty, do u want to continue?")) {
                        event.preventDefault();
                    }
                }
            });
            window.onbeforeunload = function(){
                alert('me');
                if ($scope.myForm.$dirty) {
                    return "The Form is Dirty.";
                }
            }
        }
    };
});

更新的代码

    angular.module('myApp.directives', []).directive('confirmOnExit', function () {
    return {
        link: function ($scope, elem, attrs, $rootScope) {
            window.onbeforeunload = function () {
                if ($scope.myForm.$dirty) {
                    return " do you want to stay on the page?";
                }
            }
            $rootScope.$watch(function {
                return $location.path();
            },
            function (newValue, oldValue) {
                if (newValue != oldvalue) {
                    // here you can do your tasks
                } else {}
            },
            true);
            $scope.$on('$locationChangeStart', function (event, next, current) {
                if ($scope.myForm.$dirty) {
                    if (!confirm("The form is dirty, do you want to stay on the page?")) {
                        event.preventDefault();
                    }
                }
            });
        }
    };
});