` element reveals a set of nested message `divs` but only if the `name` is invalid and the control is either `dirty` or `touched`. * Each nested `

` can present a custom message for one of the possible validation errors. There are messages for `required`, `minlength`, and `forbiddenName`. {@a dirty-or-touched}

To prevent the validator from displaying errors before the user has a chance to edit the form, you should check for either the `dirty` or `touched` states in a control. * When the user changes the value in the watched field, the control is marked as "dirty". * When the user blurs the form control element, the control is marked as "touched".

{@a reactive-form-validation} ## Validating input in reactive forms In a reactive form, the source of truth is the component class. Instead of adding validators through attributes in the template, you add validator functions directly to the form control model in the component class. Angular then calls these functions whenever the value of the control changes. ### Validator functions Validator functions can be either synchronous or asynchronous. * **Sync validators**: Synchronous functions that take a control instance and immediately return either a set of validation errors or `null`. You can pass these in as the second argument when you instantiate a `FormControl`. * **Async validators**: Asynchronous functions that take a control instance and return a Promise or Observable that later emits a set of validation errors or `null`. You can pass these in as the third argument when you instantiate a `FormControl`. For performance reasons, Angular only runs async validators if all sync validators pass. Each must complete before errors are set. ### Built-in validator functions You can choose to [write your own validator functions](#custom-validators), or you can use some of Angular's built-in validators. The same built-in validators that are available as attributes in template-driven forms, such as `required` and `minlength`, are all available to use as functions from the `Validators` class. For a full list of built-in validators, see the [Validators](api/forms/Validators) API reference. To update the hero form to be a reactive form, you can use some of the same built-in validators—this time, in function form, as in the following example. {@a reactive-component-class} In this example, the `name` control sets up two built-in validators—`Validators.required` and `Validators.minLength(4)`—and one custom validator, `forbiddenNameValidator`. (For more details see [custom validators](#custom-validators) below.) All of these validators are synchronous, so they are passed as the second argument. Notice that you can support multiple validators by passing the functions in as an array. This example also adds a few getter methods. In a reactive form, you can always access any form control through the `get` method on its parent group, but sometimes it's useful to define getters as shorthand for the template. If you look at the template for the `name` input again, it is fairly similar to the template-driven example. This form differs from the template-driven version in that it no longer exports any directives. Instead, it uses the `name` getter defined in the component class. Notice that the `required` attribute is still present in the template. Although it's not necessary for validation, it should be retained to for accessibility purposes. {@a custom-validators} ## Defining custom validators The built-in validators don't always match the exact use case of your application, so you sometimes need to create a custom validator. Consider the `forbiddenNameValidator` function from previous [reactive-form examples](#reactive-component-class). Here's what the definition of that function looks like. The function is a factory that takes a regular expression to detect a _specific_ forbidden name and returns a validator function. In this sample, the forbidden name is "bob", so the validator will reject any hero name containing "bob". Elsewhere it could reject "alice" or any name that the configuring regular expression matches. The `forbiddenNameValidator` factory returns the configured validator function. That function takes an Angular control object and returns _either_ null if the control value is valid _or_ a validation error object. The validation error object typically has a property whose name is the validation key, `'forbiddenName'`, and whose value is an arbitrary dictionary of values that you could insert into an error message, `{name}`. Custom async validators are similar to sync validators, but they must instead return a Promise or observable that later emits null or a validation error object. In the case of an observable, the observable must complete, at which point the form uses the last value emitted for validation. {@a adding-to-reactive-forms} ### Adding custom validators to reactive forms In reactive forms, add a custom validator by passing the function directly to the `FormControl`. {@a adding-to-template-driven-forms} ### Adding custom validators to template-driven forms In template-driven forms, add a directive to the template, where the directive wraps the validator function. For example, the corresponding `ForbiddenValidatorDirective` serves as a wrapper around the `forbiddenNameValidator`. Angular recognizes the directive's role in the validation process because the directive registers itself with the `NG_VALIDATORS` provider, as shown in the following example. `NG_VALIDATORS` is a predefined provider with an extensible collection of validators. The directive class then implements the `Validator` interface, so that it can easily integrate with Angular forms. Here is the rest of the directive to help you get an idea of how it all comes together. Once the `ForbiddenValidatorDirective` is ready, you can add its selector, `appForbiddenName`, to any input element to activate it. For example:

Notice that the custom validation directive is instantiated with `useExisting` rather than `useClass`. The registered validator must be _this instance_ of the `ForbiddenValidatorDirective`—the instance in the form with its `forbiddenName` property bound to “bob". If you were to replace `useExisting` with `useClass`, then you’d be registering a new class instance, one that doesn’t have a `forbiddenName`.

## Control status CSS classes Angular automatically mirrors many control properties onto the form control element as CSS classes. You can use these classes to style form control elements according to the state of the form. The following classes are currently supported. * `.ng-valid` * `.ng-invalid` * `.ng-pending` * `.ng-pristine` * `.ng-dirty` * `.ng-untouched` * `.ng-touched` In the following example, the hero form uses the `.ng-valid` and `.ng-invalid` classes to set the color of each form control's border. ## Cross-field validation A cross-field validator is a [custom validator](#custom-validators "Read about custom validators") that compares the values of different fields in a form and accepts or rejects them in combination. For example, you might have a form that offers mutually incompatible options, so that if the user can choose A or B, but not both. Some field values might also depend on others; a user might be allowed to choose B only if A is also chosen. The following cross validation examples show how to do the following: * Validate reactive or template-based form input based on the values of two sibling controls, * Show a descriptive error message after the user interacted with the form and the validation failed. The examples use cross-validation to ensure that heroes do not reveal their true identities by filling out the Hero Form. The validators do this by checking that the hero names and alter egos do not match. ### Adding cross-validation to reactive forms The form has the following structure: ```javascript const heroForm = new FormGroup({ 'name': new FormControl(), 'alterEgo': new FormControl(), 'power': new FormControl() }); ``` Notice that the `name` and `alterEgo` are sibling controls. To evaluate both controls in a single custom validator, you must perform the validation in a common ancestor control: the `FormGroup`. You query the `FormGroup` for its child controls so that you can compare their values. To add a validator to the `FormGroup`, pass the new validator in as the second argument on creation. ```javascript const heroForm = new FormGroup({ 'name': new FormControl(), 'alterEgo': new FormControl(), 'power': new FormControl() }, { validators: identityRevealedValidator }); ``` The validator code is as follows. The `identity` validator implements the `ValidatorFn` interface. It takes an Angular control object as an argument and returns either null if the form is valid, or `ValidationErrors` otherwise. The validator retrieves the child controls by calling the `FormGroup`'s [get](api/forms/AbstractControl#get) method, then compares the values of the `name` and `alterEgo` controls. If the values do not match, the hero's identity remains secret, both are valid, and the validator returns null. If they do match, the hero's identity is revealed and the validator must mark the form as invalid by returning an error object. To provide better user experience, the template shows an appropriate error message when the form is invalid. This `*ngIf` displays the error if the `FormGroup` has the cross validation error returned by the `identityRevealed` validator, but only if the user has finished [interacting with the form](#dirty-or-touched). ### Adding cross-validation to template-driven forms For a template-driven form, you must create a directive to wrap the validator function. You provide that directive as the validator using the [`NG_VALIDATORS` token](#adding-to-template-driven-forms "Read about providing validators"), as shown in the following example. You must add the new directive to the HTML template. Because the validator must be registered at the highest level in the form, the following template puts the directive on the `form` tag. To provide better user experience, we show an appropriate error message when the form is invalid. This is the same in both template-driven and reactive forms. ## Creating asynchronous validators Asynchronous validators implement the `AsyncValidatorFn` and `AsyncValidator` interfaces. These are very similar to their synchronous counterparts, with the following differences. * The `validate()` functions must return a Promise or an observable, * The observable returned must be finite, meaning it must complete at some point. To convert an infinite observable into a finite one, pipe the observable through a filtering operator such as `first`, `last`, `take`, or `takeUntil`. Asynchronous validation happens after the synchronous validation, and is performed only if the synchronous validation is successful. This check allows forms to avoid potentially expensive async validation processes (such as an HTTP request) if the more basic validation methods have already found invalid input. After asynchronous validation begins, the form control enters a `pending` state. You can inspect the control's `pending` property and use it to give visual feedback about the ongoing validation operation. A common UI pattern is to show a spinner while the async validation is being performed. The following example shows how to achieve this in a template-driven form. ```html ``` ### Implementing a custom async validator In the following example, an async validator ensures that heroes pick an alter ego that is not already taken. New heroes are constantly enlisting and old heroes are leaving the service, so the list of available alter egos cannot be retrieved ahead of time. To validate the potential alter ego entry, the validator must initiate an asynchronous operation to consult a central database of all currently enlisted heroes. The following code create the validator class, `UniqueAlterEgoValidator`, which implements the `AsyncValidator` interface. The constructor injects the `HeroesService`, which defines the following interface. ```typescript interface HeroesService { isAlterEgoTaken: (alterEgo: string) => Observable; } ``` In a real world application, the `HeroesService` would be responsible for making an HTTP request to the hero database to check if the alter ego is available. From the validator's point of view, the actual implementation of the service is not important, so the example can just code against the `HeroesService` interface. As the validation begins, the `UniqueAlterEgoValidator` delegates to the `HeroesService` `isAlterEgoTaken()` method with the current control value. At this point the control is marked as `pending` and remains in this state until the observable chain returned from the `validate()` method completes. The `isAlterEgoTaken()` method dispatches an HTTP request that checks if the alter ego is available, and returns `Observable` as the result. The `validate()` method pipes the response through the `map` operator and transforms it into a validation result. The method then, like any validator, returns `null` if the form is valid, and `ValidationErrors` if it is not. This validator handles any potential errors with the `catchError` operator. In this case, the validator treats the `isAlterEgoTaken()` error as a successful validation, because failure to make a validation request does not necessarily mean that the alter ego is invalid. You could handle the error differently and return the `ValidationError` object instead. After some time passes, the observable chain completes and the asynchronous validation is done. The `pending` flag is set to `false`, and the form validity is updated. ### Optimizing performance of async validators By default, all validators run after every form value change. With synchronous validators, this does not normally have a noticeable impact on application performance. Async validators, however, commonly perform some kind of HTTP request to validate the control. Dispatching an HTTP request after every keystroke could put a strain on the backend API, and should be avoided if possible. You can delay updating the form validity by changing the `updateOn` property from `change` (default) to `submit` or `blur`. With template-driven forms, set the property in the template. ```html ``` With reactive forms, set the property in the `FormControl` instance. ```typescript new FormControl('', {updateOn: 'blur'}); ```