angular.module('myApp', [])

​

.controller('myCtrl', function($scope) {

​

  $scope.foo = 'abc';

  // in my example, it's always going to be invalid

  $scope.fooValidity = false;

​

})

​

.directive('myDirective', function() {

  var directive = {

    scope: {

      isValid: '=myDirective'

    },

    require: 'ngModel',

    link: function($scope, $element, $attrs, $ngModel) {

      // "whatever" is the property which will be assinged on the ngModel $error object

      $ngModel.$validators.whatever = function(val) {

        // write your validation logic here

        // return true or false for valid / invalid

        return $scope.isValid;

      };

    }

  };

  return directive;

})

​

;