问向现有元素添加指令

EN

 app.directive('requiredIfDirty', function ($compile, $timeout) {
                        return {
                            restrict: "A",
                            require: // element must have ng-model attribute.
                            'ngModel',
                            link: // scope = the parent scope
                            // elem = the element the directive is on
                            // attr = a dictionary of attributes on the element
                            // ctrl = the controller for ngModel.
                            function (scope, elem, attr, ctrl) {
                                var unsubscribe = scope.$watch(attr.ngModel, function (oldValue, newValue) {
                                    if(angular.isUndefined(oldValue)) {
                                        return;
                                    }
                                    attr.$set("required", true);
                                    $timeout(function () {
                                        var newElement = $compile(elem)(scope);
                                        elem.replaceWith(newElement);
                                    }, 1);
                                    unsubscribe();
                                });
                            }
                        };
                    });