How to add custom validation to an AngularJS form?

Angular-UI's project includes a ui-validate directive, which will probably help you with this. It let's you specify a function to call to do the validation.

Have a look at the demo page: http://angular-ui.github.com/, search down to the Validate heading.

From the demo page:

<input ng-model="email" ui-validate='{blacklist : notBlackListed}'>
<span ng-show='form.email.$error.blacklist'>This e-mail is black-listed!</span>

then in your controller:

function ValidateCtrl($scope) {
  $scope.blackList = ['[email protected]','[email protected]'];
  $scope.notBlackListed = function(value) {
    return $scope.blackList.indexOf(value) === -1;
  };
}

You can use ng-required for your validation scenario ("if these 3 fields are filled in, then this field is required":

<div ng-app>
    <input type="text" ng-model="field1" placeholder="Field1">
    <input type="text" ng-model="field2" placeholder="Field2">
    <input type="text" ng-model="field3" placeholder="Field3">
    <input type="text" ng-model="dependentField" placeholder="Custom validation"
        ng-required="field1 && field2 && field3">
</div>

You can use Angular-Validator.

Example: using a function to validate a field

<input  type = "text"
    name = "firstName"
    ng-model = "person.firstName"
    validator = "myCustomValidationFunction(form.firstName)">

Then in your controller you would have something like

$scope.myCustomValidationFunction = function(firstName){ 
   if ( firstName === "John") {
       return true;
    }

You can also do something like this:

<input  type = "text"
        name = "firstName"
        ng-model = "person.firstName"
        validator = "'!(field1 && field2 && field3)'"
        invalid-message = "'This field is required'">

(where field1 field2, and field3 are scope variables. You might also want to check if the fields do not equal the empty string)

If the field does not pass the validator then the field will be marked as invalid and the user will not be able to submit the form.

For more use cases and examples see: https://github.com/turinggroup/angular-validator

Disclaimer: I am the author of Angular-Validator

Here's a cool way to do custom wildcard expression validations in a form (from: Advanced form validation with AngularJS and filters):

<form novalidate="">  
   <input type="text" id="name" name="name" ng-model="newPerson.name"
      ensure-expression="(persons | filter:{name: newPerson.name}:true).length !== 1">
   <!-- or in your case:-->
   <input type="text" id="fruitName" name="fruitName" ng-model="data.fruitName"
      ensure-expression="(blacklist | filter:{fruitName: data.fruitName}:true).length !== 1">
</form>

app.directive('ensureExpression', ['$http', '$parse', function($http, $parse) {
    return {
        require: 'ngModel',
        link: function(scope, ele, attrs, ngModelController) {
            scope.$watch(attrs.ngModel, function(value) {
                var booleanResult = $parse(attrs.ensureExpression)(scope);
                ngModelController.$setValidity('expression', booleanResult);
            });
        }
    };
}]);

jsFiddle demo (supports expression naming and multiple expressions)

It's similar to ui-validate, but you don't need a scope specific validation function (this works generically) and ofcourse you don't need ui.utils this way.

I recently created a directive to allow for expression-based invalidation of angular form inputs. Any valid angular expression can be used, and it supports custom validation keys using object notation. Tested with angular v1.3.8

        .directive('invalidIf', [function () {
        return {
            require: 'ngModel',
            link: function (scope, elm, attrs, ctrl) {

                var argsObject = scope.$eval(attrs.invalidIf);

                if (!angular.isObject(argsObject)) {
                    argsObject = { invalidIf: attrs.invalidIf };
                }

                for (var validationKey in argsObject) {
                    scope.$watch(argsObject[validationKey], function (newVal) {
                        ctrl.$setValidity(validationKey, !newVal);
                    });
                }
            }
        };
    }]);

You can use it like this:

<input ng-model="foo" invalid-if="{fooIsGreaterThanBar: 'foo > bar',
                                   fooEqualsSomeFuncResult: 'foo == someFuncResult()'}/>

Or by just passing in an expression (it will be given the default validationKey of "invalidIf")

<input ng-model="foo" invalid-if="foo > bar"/>

@synergetic I think @blesh suppose to put function validate as below

function validate(value) {
    var valid = blacklist.indexOf(value) === -1;
    ngModel.$setValidity('blacklist', valid);
    return valid ? value : undefined;
}

ngModel.$formatters.unshift(validate);
ngModel.$parsers.unshift(validate);

Update:

Improved and simplified version of previous directive (one instead of two) with same functionality:

.directive('myTestExpression', ['$parse', function ($parse) {
    return {
        restrict: 'A',
        require: 'ngModel',
        link: function (scope, element, attrs, ctrl) {
            var expr = attrs.myTestExpression;
            var watches = attrs.myTestExpressionWatch;

            ctrl.$validators.mytestexpression = function (modelValue, viewValue) {
                return expr == undefined || (angular.isString(expr) && expr.length < 1) || $parse(expr)(scope, { $model: modelValue, $view: viewValue }) === true;
            };

            if (angular.isString(watches)) {
                angular.forEach(watches.split(",").filter(function (n) { return !!n; }), function (n) {
                    scope.$watch(n, function () {
                        ctrl.$validate();
                    });
                });
            }
        }
    };
}])

Example usage:

<input ng-model="price1" 
       my-test-expression="$model > 0" 
       my-test-expression-watch="price2,someOtherWatchedPrice" />
<input ng-model="price2" 
       my-test-expression="$model > 10" 
       my-test-expression-watch="price1" 
       required />

Result: Mutually dependent test expressions where validators are executed on change of other's directive model and current model.

Test expression has local $model variable which you should use to compare it to other variables.

Previously:

I've made an attempt to improve @Plantface code by adding extra directive. This extra directive very useful if our expression needs to be executed when changes are made in more than one ngModel variables.

.directive('ensureExpression', ['$parse', function($parse) {
    return {
        restrict: 'A',
        require: 'ngModel',
        controller: function () { },
        scope: true,
        link: function (scope, element, attrs, ngModelCtrl) {
            scope.validate = function () {
                var booleanResult = $parse(attrs.ensureExpression)(scope);
                ngModelCtrl.$setValidity('expression', booleanResult);
            };

            scope.$watch(attrs.ngModel, function(value) {
                scope.validate();
            });
        }
    };
}])

.directive('ensureWatch', ['$parse', function ($parse) {
    return {
        restrict: 'A',
        require: 'ensureExpression',
        link: function (scope, element, attrs, ctrl) {
            angular.forEach(attrs.ensureWatch.split(",").filter(function (n) { return !!n; }), function (n) {
                scope.$watch(n, function () {
                    scope.validate();
                });
            });
        }
    };
}])

Example how to use it to make cross validated fields:

<input name="price1"
       ng-model="price1" 
       ensure-expression="price1 > price2" 
       ensure-watch="price2" />
<input name="price2" 
       ng-model="price2" 
       ensure-expression="price2 > price3" 
       ensure-watch="price3" />
<input name="price3" 
       ng-model="price3" 
       ensure-expression="price3 > price1 && price3 > price2" 
       ensure-watch="price1,price2" />

ensure-expression is executed to validate model when ng-model or any of ensure-watch variables is changed.

In AngularJS the best place to define Custom Validation is Cutsom directive. AngularJS provide a ngMessages module.

ngMessages is a directive that is designed to show and hide messages based on the state of a key/value object that it listens on. The directive itself complements error message reporting with the ngModel $error object (which stores a key/value state of validation errors).

For custom form validation One should use ngMessages Modules with custom directive.Here i have a simple validation which will check if number length is less then 6 display an error on screen

 <form name="myform" novalidate>
                <table>
                    <tr>
                        <td><input name='test' type='text' required  ng-model='test' custom-validation></td>
                        <td ng-messages="myform.test.$error"><span ng-message="invalidshrt">Too Short</span></td>
                    </tr>
                </table>
            </form>

Here is how to create custom validation directive

angular.module('myApp',['ngMessages']);
        angular.module('myApp',['ngMessages']).directive('customValidation',function(){
            return{
            restrict:'A',
            require: 'ngModel',
            link:function (scope, element, attr, ctrl) {// 4th argument contain model information 

            function validationError(value) // you can use any function and parameter name 
                {
                 if (value.length > 6) // if model length is greater then 6 it is valide state
                 {
                 ctrl.$setValidity('invalidshrt',true);
                 }
                 else
                 {
                 ctrl.$setValidity('invalidshrt',false) //if less then 6 is invalide
                 }

                 return value; //return to display  error 
                }
                ctrl.$parsers.push(validationError); //parsers change how view values will be saved in the model
            }
            };
        });

$setValidity is inbuilt function to set model state to valid/invalid

I extended @Ben Lesh's answer with an ability to specify whether the validation is case sensitive or not (default)

use:

<input type="text" name="fruitName" ng-model="data.fruitName" blacklist="Coconuts,Bananas,Pears" caseSensitive="true" required/>

code:

angular.module('crm.directives', []).
directive('blacklist', [
    function () {
        return {
            restrict: 'A',
            require: 'ngModel',
            scope: {
                'blacklist': '=',
            },
            link: function ($scope, $elem, $attrs, modelCtrl) {

                var check = function (value) {
                    if (!$attrs.casesensitive) {
                        value = (value && value.toUpperCase) ? value.toUpperCase() : value;

                        $scope.blacklist = _.map($scope.blacklist, function (item) {
                            return (item.toUpperCase) ? item.toUpperCase() : item
                        })
                    }

                    return !_.isArray($scope.blacklist) || $scope.blacklist.indexOf(value) === -1;
                }

                //For DOM -> model validation
                modelCtrl.$parsers.unshift(function (value) {
                    var valid = check(value);
                    modelCtrl.$setValidity('blacklist', valid);

                    return value;
                });
                //For model -> DOM validation
                modelCtrl.$formatters.unshift(function (value) {
                    modelCtrl.$setValidity('blacklist', check(value));
                    return value;
                });
            }
        };
    }
]);

Some great examples and libs presented in this thread, but they didn't quite have what I was looking for. My approach: angular-validity -- a promise based validation lib for asynchronous validation, with optional Bootstrap styling baked-in.

An angular-validity solution for the OP's use case might look something like this:

<input  type="text" name="field4" ng-model="field4"
        validity="eval"
        validity-eval="!(field1 && field2 && field3 && !field4)"
        validity-message-eval="This field is required">

Here's a Fiddle, if you want to take it for a spin. The lib is available on GitHub, has detailed documentation, and plenty of live demos.