` element reveals a set of nested message `divs` but only if there are "name" errors 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 `maxlength`. The full template repeats this kind of layout for each data entry control on the form. {@a why-check} ~~~ {.l-sub-section} #### Why check _dirty_ and _touched_? The app shouldn't show errors for a new hero before the user has had a chance to edit the value. The checks for `dirty` and `touched` prevent premature display of errors. Learn about `dirty` and `touched` in the [Forms](guide/forms) guide. ~~~ The component class manages the hero model used in the data binding as well as other code to support the view. Use this template-driven validation technique when working with static forms with simple, standard validation rules. Here are the complete files for the first version of `HeroFormTemplateCompononent` in the template-driven approach:

{@a template2} ## Template-driven forms with validation messages in code While the layout is straightforward, there are obvious shortcomings with the way it's handling validation messages: * It takes a lot of HTML to represent all possible error conditions. This gets out of hand when there are many controls and many validation rules. * There's a lot of JavaScript logic in the HTML. * The messages are static strings, hard-coded into the template. It's easier to maintain _dynamic_ messages in the component class. In this example, you can move the logic and the messages into the component with a few changes to the template and component. Here's the hero name again, excerpted from the revised template (Template 2), next to the original version:

The `` element HTML is almost the same. There are noteworthy differences: - The hard-code error message `` are gone. - There's a new attribute, `forbiddenName`, that is actually a custom validation directive. It invalidates the control if the user enters "bob" in the name ``([try it](guide/form-validation#live-example)). See the [custom validation](guide/form-validation#custom-validation) section later in this cookbook for more information on custom validation directives. - The `#name` template variable is gone because the app no longer refers to the Angular control for this element. - Binding to the new `formErrors.name` property is sufficent to display all name validation error messages. {@a component-class} ### Component class The original component code for Template 1 stayed the same; however, Template 2 requires some changes in the component. This section covers the code necessary in Template 2's component class to acquire the Angular form control and compose error messages. The first step is to acquire the form control that Angular created from the template by querying for it. Look back at the top of the component template at the `#heroForm` template variable in the `

` element: The `heroForm` variable is a reference to the control model that Angular derived from the template. Tell Angular to inject that model into the component class's `currentForm` property using a `@ViewChild` query: Some observations: - Angular `@ViewChild` queries for a template variable when you pass it the name of that variable as a string (`'heroForm'` in this case). - The `heroForm` object changes several times during the life of the component, most notably when you add a new hero. Periodically inspecting it reveals these changes. - Angular calls the `ngAfterViewChecked` [lifecycle hook method](guide/lifecycle-hooks) when anything changes in the view. That's the right time to see if there's a new `heroForm` object. - When there _is_ a new `heroForm` model, `formChanged()` subscribes to its `valueChanges` _Observable_ property. The `onValueChanged` handler looks for validation errors after every keystroke. The `onValueChanged` handler interprets user data entry. The `data` object passed into the handler contains the current element values. The handler ignores them. Instead, it iterates over the fields of the component's `formErrors` object. The `formErrors` is a dictionary of the hero fields that have validation rules and their current error messages. Only two hero properties have validation rules, `name` and `power`. The messages are empty strings when the hero data are valid. For each field, the `onValueChanged` handler does the following: - Clears the prior error message, if any. - Acquires the field's corresponding Angular form control. - If such a control exists _and_ it's been changed ("dirty") _and_ it's invalid, the handler composes a consolidated error message for all of the control's errors. Next, the component needs some error messages of course—a set for each validated property with one message per validation rule: Now every time the user makes a change, the `onValueChanged` handler checks for validation errors and produces messages accordingly. {@a improvement} ### The benefits of messages in code Clearly the template got substantially smaller while the component code got substantially larger. It's not easy to see the benefit when there are just three fields and only two of them have validation rules. Consider what happens as the number of validated fields and rules increases. In general, HTML is harder to read and maintain than code. The initial template was already large and threatening to get rapidly worse with the addition of more validation message `

` elements. After moving the validation messaging to the component, the template grows more slowly and proportionally. Each field has approximately the same number of lines no matter its number of validation rules. The component also grows proportionally, at the rate of one line per validated field and one line per validation message. Both trends are manageable. Now that the messages are in code, you have more flexibility and can compose messages more efficiently. You can refactor the messages out of the component, perhaps to a service class that retrieves them from the server. In short, there are more opportunities to improve message handling now that text and logic have moved from template to code. {@a formmodule} ### _FormModule_ and template-driven forms Angular has two different forms modules—`FormsModule` and `ReactiveFormsModule`—that correspond with the two approaches to form development. Both modules come from the same `@angular/forms` library package. You've been reviewing the "Template-driven" approach which requires the `FormsModule`. Here's how you imported it in the `HeroFormTemplateModule`. ~~~ {.l-sub-section} This guide hasn't talked about the `SharedModule` or its `SubmittedComponent` which appears at the bottom of every form template in this cookbook. They're not germane to the validation story. Look at the [live example](guide/form-validation#live-example) if you're interested. ~~~ {@a reactive} ## Reactive forms with validation in code In the template-driven approach, you markup the template with form elements, validation attributes, and `ng...` directives from the Angular `FormsModule`. At runtime, Angular interprets the template and derives its _form control model_. **Reactive Forms** takes a different approach. You create the form control model in code. You write the template with form elements and `form...` directives from the Angular `ReactiveFormsModule`. At runtime, Angular binds the template elements to your control model based on your instructions. This approach requires a bit more effort. *You have to write the control model and manage it*. This allows you to do the following: * Add, change, and remove validation functions on the fly. * Manipulate the control model dynamically from within the component. * [Test](guide/form-validation#testing) validation and control logic with isolated unit tests. The following cookbook sample re-writes the hero form in _reactive forms_ style. {@a reactive-forms-module} ### Switch to the _ReactiveFormsModule_ The reactive forms classes and directives come from the Angular `ReactiveFormsModule`, not the `FormsModule`. The application module for the reactive forms feature in this sample looks like this: The reactive forms feature module and component are in the `src/app/reactive` folder. Focus on the `HeroFormReactiveComponent` there, starting with its template. {@a reactive-component-template} ### Component template Begin by changing the `` tag so that it binds the Angular `formGroup` directive in the template to the `heroForm` property in the component class. The `heroForm` is the control model that the component class builds and maintains. Next, modify the template HTML elements to match the _reactive forms_ style. Here is the "name" portion of the template again, revised for reactive forms and compared with the template-driven version:

Key changes are: - The validation attributes are gone (except `required`) because validating happens in code. - `required` remains, not for validation purposes (that's in the code), but rather for css styling and accessibility. ~~~ {.l-sub-section} A future version of reactive forms will add the `required` HTML validation attribute to the DOM element (and perhaps the `aria-required` attribute) when the control has the `required` validator function. Until then, apply the `required` attribute _and_ add the `Validator.required` function to the control model, as you'll see below. ~~~ - The `formControlName` replaces the `name` attribute; it serves the same purpose of correlating the input with the Angular form control. - The two-way `[(ngModel)]` binding is gone. The reactive approach does not use data binding to move data into and out of the form controls. That's all in code. ~~~ {.l-sub-section} The retreat from data binding is a principle of the reactive paradigm rather than a technical limitation. ~~~ {@a reactive-component-class} ### Component class The component class is now responsible for defining and managing the form control model. Angular no longer derives the control model from the template so you can no longer query for it. You can create the Angular form control model explicitly with the help of the `FormBuilder` class. Here's the section of code devoted to that process, paired with the template-driven code it replaces:

- Inject `FormBuilder` in a constructor. - Call a `buildForm` method in the `ngOnInit` [lifecycle hook method](guide/lifecycle-hooks) because that's when you'll have the hero data. Call it again in the `addHero` method. ~~~ {.l-sub-section} A real app would retrieve the hero asynchronously from a data service, a task best performed in the `ngOnInit` hook. ~~~ - The `buildForm` method uses the `FormBuilder`, `fb`, to declare the form control model. Then it attaches the same `onValueChanged` handler (there's a one line difference) to the form's `valueChanges` event and calls it immediately to set error messages for the new control model. {@a formbuilder} #### _FormBuilder_ declaration The `FormBuilder` declaration object specifies the three controls of the sample's hero form. Each control spec is a control name with an array value. The first array element is the current value of the corresponding hero field. The optional second value is a validator function or an array of validator functions. Most of the validator functions are stock validators provided by Angular as static methods of the `Validators` class. Angular has stock validators that correspond to the standard HTML validation attributes. The `forbiddenNames` validator on the `"name"` control is a custom validator, discussed in a separate [section below](guide/form-validation#custom-validation). ~~~ {.l-sub-section} Learn more about `FormBuilder` in the [Introduction to FormBuilder](guide/reactive-forms) section of Reactive Forms guide. ~~~ {@a committing-changes} #### Committing hero value changes In two-way data binding, the user's changes flow automatically from the controls back to the data model properties. Reactive forms do not use data binding to update data model properties. The developer decides _when and how_ to update the data model from control values. This sample updates the model twice: 1. When the user submits the form. 1. When the user adds a new hero. The `onSubmit()` method simply replaces the `hero` object with the combined values of the form: ~~~ {.l-sub-section} This example is lucky in that the `heroForm.value` properties _just happen_ to correspond _exactly_ to the hero data object properties. ~~~ The `addHero()` method discards pending changes and creates a brand new `hero` model object. Then it calls `buildForm()` again which replaces the previous `heroForm` control model with a new one. The `` tag's `[formGroup]` binding refreshes the page with the new control model. Here's the complete reactive component file, compared to the two template-driven component files.

~~~ {.l-sub-section} Run the [live example](guide/form-validation#live-example) to see how the reactive form behaves, and to compare all of the files in this cookbook sample. ~~~ {@a custom-validation} ## Custom validation This cookbook sample has a custom `forbiddenNamevalidator()` function that's applied to both the template-driven and the reactive form controls. It's in the `src/app/shared` folder and declared in the `SharedModule`. Here's the `forbiddenNamevalidator()` function: 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"; the validator rejects 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}`). {@a custom-validation-directive} ### Custom validation directive In the reactive forms component, the `'name'` control's validator function list has a `forbiddenNameValidator` at the bottom. In the _template-driven_ example, the `` has the selector (`forbiddenName`) of a custom _attribute directive_, which rejects "bob". The corresponding `ForbiddenValidatorDirective` is a wrapper around the `forbiddenNameValidator`. Angular `forms` 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 validation directives. Here is the rest of the directive to help you get an idea of how it all comes together: ~~~ {.l-sub-section} If you are familiar with Angular validations, 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`. To see this in action, run the example and then type “bob” in the name of Hero Form 2. Notice that you get a validation error. Now change from `useExisting` to `useClass` and try again. This time, when you type “bob”, there's no "bob" error message. ~~~ ~~~ {.l-sub-section} For more information on attaching behavior to elements, see [Attribute Directives](guide/attribute-directives). ~~~ {@a testing} ## Testing Considerations You can write _isolated unit tests_ of validation and control logic in _Reactive Forms_. _Isolated unit tests_ probe the component class directly, independent of its interactions with its template, the DOM, other dependencies, or Angular itself. Such tests have minimal setup, are quick to write, and easy to maintain. They do not require the `Angular TestBed` or asynchronous testing practices. That's not possible with _template-driven_ forms. The template-driven approach relies on Angular to produce the control model and to derive validation rules from the HTML validation attributes. You must use the `Angular TestBed` to create component test instances, write asynchronous tests, and interact with the DOM. While not difficult, this takes more time, work and skill—factors that tend to diminish test code coverage and quality.