angular-cn/aio/content/guide/form-validation.md

Form Validation

Improve overall data quality by validating user input for accuracy and completeness.

This page shows how to validate user input in the UI and display useful validation messages using both reactive and template-driven forms. It assumes some basic knowledge of the two forms modules.

If you're new to forms, start by reviewing the Forms and Reactive Forms guides.

Template-driven validation

To add validation to a template-driven form, you add the same validation attributes as you would with native HTML form validation. Angular uses directives to match these attributes with validator functions in the framework.

Every time the value of a form control changes, Angular runs validation and generates either a list of validation errors, which results in an INVALID status, or null, which results in a VALID status.

You can then inspect the control's state by exporting ngModel to a local template variable. The following example exports NgModel into a variable called name:

Note the following:

• The <input> element carries the HTML validation attributes: required and minlength. It also carries a custom validator directive, forbiddenName. For more information, see Custom validators section.

• #name="ngModel" exports NgModel into a local variable called name. NgModel mirrors many of the properties of its underlying FormControl instance, so you can use this in the template to check for control states such as valid and dirty. For a full list of control properties, see the AbstractControl API reference.

• The *ngIf on the <div> 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 <div> can present a custom message for one of the possible validation errors. There are messages for required, minlength, and forbiddenName.

Why check dirty and touched?

You may not want your application to display errors before the user has a chance to edit the form. The checks for dirty and touched prevent errors from showing until the user does one of two things: changes the value, turning the control dirty; or blurs the form control element, setting the control to touched.

Reactive form validation

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

There are two types of validator functions: sync validators and async validators.

• Sync validators: 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: 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.

Note: for performance reasons, Angular only runs async validators if all sync validators pass. Each must complete before errors are set.

Built-in validators

You can choose to write your own validator functions, 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 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. See below:

{@a reactive-component-class}

Note that:

• The name control sets up two built-in validators—Validators.required and Validators.minLength(4)—and one custom validator, forbiddenNameValidator. For more details see the Custom validators section in this guide.
• As these validators are all sync validators, you pass them in as the second argument.
• Support multiple validators by passing the functions in as an array.
• This example 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 shorthands for the template.

If you look at the template for the name input again, it is fairly similar to the template-driven example.

Key takeaways:

• The form no longer exports any directives, and instead uses the name getter defined in the component class.
• The required attribute is still present. While it's not necessary for validation purposes, you may want to keep it in your template for CSS styling or accessibility reasons.

Custom validators

Since the built-in validators won't always match the exact use case of your application, sometimes you'll want to create a custom validator.

Consider the forbiddenNameValidator function from previous examples in this guide. Here's what the definition of that function looks like:

The function is actually 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.

Adding to reactive forms

In reactive forms, custom validators are fairly simple to add. All you have to do is pass the function directly to the FormControl.

Adding to template-driven forms

In template-driven forms, you don't have direct access to the FormControl instance, so you can't pass the validator in like you can for reactive forms. Instead, you need to add a directive to the template.

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, a 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 simply add its selector, appForbiddenName, to any input element to activate it. For example:

You may have noticed 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

Like in AngularJS, 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

The hero form uses the .ng-valid and .ng-invalid classes to set the color of each form control's border.

Cross field validation

This section shows how to perform cross field validation. It assumes some basic knowledge of creating custom validators.

If you haven't created custom validators before, start by reviewing the custom validators section.

In the following section, we will make sure that our heroes do not reveal their true identities by filling out the Hero Form. We will do that by validating that the hero names and alter egos do not match.

Adding to reactive forms

The form has the following structure:

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, we should perform the validation in a common ancestor control: the FormGroup. That way, we can query the FormGroup for the child controls which will allow us to compare their values.

To add a validator to the FormGroup, pass the new validator in as the second argument on creation.

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.

First we retrieve the child controls by calling the FormGroup's get method. Then we simply compare the values of the name and alterEgo controls.

If the values do not match, the hero's identity remains secret, and we can safely return null. Otherwise, the hero's identity is revealed and we must mark the form as invalid by returning an error object.

Next, to provide better user experience, we show an appropriate error message when the form is invalid.

Note that we check if:

• the FormGroup has the cross validation error returned by the identityRevealed validator,
• the user is yet to interact with the form.

Adding to template driven forms

First we must create a directive that will wrap the validator function. We provide it as the validator using the NG_VALIDATORS token. If you are not sure why, or you do not fully understand the syntax, revisit the previous section.

Next, we have to add the directive to the html template. Since the validator must be registered at the highest level in the form, we put the directive on the form tag.

To provide better user experience, we show an appropriate error message when the form is invalid.

Note that we check if:

• the form has the cross validation error returned by the identityRevealed validator,
• the user is yet to interact with the form.

This completes the cross validation example. We managed to:

• validate the form based on the values of two sibling controls,
• show a descriptive error message after the user interacted with the form and the validation failed.

You can run the to see the complete reactive and template-driven example code.