angular-docs-cn/aio/content/guide/forms.md

@title Forms

@intro A form creates a cohesive, effective, and compelling data entry experience. An Angular form coordinates a set of data-bound user controls, tracks changes, validates input, and presents errors.

@description We’ve all used a form to log in, submit a help request, place an order, book a flight, schedule a meeting, and perform countless other data entry tasks. Forms are the mainstay of business applications.

Any seasoned web developer can slap together an HTML form with all the right tags. It's more challenging to create a cohesive data entry experience that guides the user efficiently and effectively through the workflow behind the form.

That takes design skills that are, to be frank, well out of scope for this guide.

It also takes framework support for two-way data binding, change tracking, validation, and error handling ... which we shall cover in this guide on Angular forms.

We will build a simple form from scratch, one step at a time. Along the way we'll learn how to:

• Build an Angular form with a component and template
• Use ngModel to create two-way data bindings for reading and writing input control values
• Track state changes and the validity of form controls
• Provide visual feedback using special CSS classes that track the state of the controls
• Display validation errors to users and enable/disable form controls
• Share information across HTML elements using template reference variables

Run the .

Template-driven forms

Many of us will build forms by writing templates in the Angular template syntax with the form-specific directives and techniques described in this guide.

That's not the only way to create a form but it's the way we'll cover in this guide.We can build almost any form we need with an Angular template — login forms, contact forms, pretty much any business form. We can lay out the controls creatively, bind them to data, specify validation rules and display validation errors, conditionally enable or disable specific controls, trigger built-in visual feedback, and much more.

It will be pretty easy because Angular handles many of the repetitive, boilerplate tasks we'd otherwise wrestle with ourselves.

We'll discuss and learn to build a template-driven form that looks like this:

Here at the Hero Employment Agency we use this form to maintain personal information about heroes. Every hero needs a job. It's our company mission to match the right hero with the right crisis!

Two of the three fields on this form are required. Required fields have a green bar on the left to make them easy to spot.

If we delete the hero name, the form displays a validation error in an attention-grabbing style:

Note that the submit button is disabled, and the "required" bar to the left of the input control changed from green to red.

We'll customize the colors and location of the "required" bar with standard CSS. We'll build this form in small steps:

1. Create the Hero model class.
2. Create the component that controls the form.
3. Create a template with the initial form layout.
4. Bind data properties to each form control using the ngModel two-way data binding syntax.
5. Add a name attribute to each form input control.
6. Add custom CSS to provide visual feedback.
7. Show and hide validation error messages.
8. Handle form submission with ngSubmit.
9. Disable the form’s submit button until the form is valid.

Setup

Follow the setup instructions for creating a new project named angular-forms.

Create the Hero model class

As users enter form data, we'll capture their changes and update an instance of a model. We can't lay out the form until we know what the model looks like.

A model can be as simple as a "property bag" that holds facts about a thing of application importance. That describes well our Hero class with its three required fields (id, name, power) and one optional field (alterEgo).

In the !{_appDir} directory, create the following file with the given content:

{@example 'forms/ts/src/app/hero.ts'}

It's an anemic model with few requirements and no behavior. Perfect for our demo.

The TypeScript compiler generates a public field for each public constructor parameter and assigns the parameter’s value to that field automatically when we create new heroes.

The alterEgo is optional, so the constructor lets us omit it; note the (?) in alterEgo?.

We can create a new hero like this:

Create a form component

An Angular form has two parts: an HTML-based template and a component class to handle data and user interactions programmatically. We begin with the class because it states, in brief, what the hero editor can do.

Create the following file with the given content: There’s nothing special about this component, nothing form-specific, nothing to distinguish it from any component we've written before.

Understanding this component requires only the Angular concepts covered in previous guides.

1. The code imports the Angular core library, and the Hero model we just created.
2. The @Component selector value of "hero-form" means we can drop this form in a parent template with a <hero-form> tag.
3. The moduleId: module.id property sets the base for module-relative loading of the templateUrl.
4. The templateUrl property points to a separate file for the template HTML.
5. We defined dummy data for model and powers, as befits a demo. Down the road, we can inject a data service to get and save real data or perhaps expose these properties as inputs and outputs for binding to a parent component. None of this concerns us now and these future changes won't affect our form.
6. We threw in a diagnostic property to return a JSON representation of our model. It'll help us see what we're doing during our development; we've left ourselves a cleanup note to discard it later.

Why the separate template file?

Why don't we write the template inline in the component file as we often do elsewhere?

There is no “right” answer for all occasions. We like inline templates when they are short. Most form templates won't be short. TypeScript and JavaScript files generally aren't the best place to write (or read) large stretches of HTML and few editors are much help with files that have a mix of HTML and code. We also like short files with a clear and obvious purpose like this one.

Form templates tend to be quite large even when displaying a small number of fields so it's usually best to put the HTML template in a separate file. We'll write that template file in a moment. Before we do, we'll take a step back and revise the app.module.ts and app.component.ts to make use of the new HeroFormComponent.

Revise app.module.ts

app.module.ts defines the application's root module. In it we identify the external modules we'll use in our application and declare the components that belong to this module, such as our HeroFormComponent.

Because template-driven forms are in their own module, we need to add the FormsModule to the array of imports for our application module before we can use forms.

Replace the contents of the "QuickStart" version with the following:

{@example 'forms/ts/src/app/app.module.ts'}

There are three changes:

1. We import FormsModule and our new HeroFormComponent.

2. We add the FormsModule to the list of imports defined in the ngModule decorator. This gives our application access to all of the template-driven forms features, including ngModel.

3. We add the HeroFormComponent to the list of declarations defined in the ngModule decorator. This makes the HeroFormComponent component visible throughout this module.

If a component, directive, or pipe belongs to a module in the `imports` array, ​_DON'T_​ re-declare it in the `declarations` array.
If you wrote it and it should belong to this module, ​_DO_​ declare it in the `declarations` array.

Revise app.component.ts

AppComponent is the application's root component. It will host our new HeroFormComponent.

Replace the contents of the "QuickStart" version with the following:

{@example 'forms/ts/src/app/app.component.ts'}

There are only two changes. The template is simply the new element tag identified by the component's selector property. This will display the hero form when the application component is loaded. We've also dropped the name field from the class body.

Create an initial HTML form template

Create the new template file with the following contents:

{@example 'forms/ts/src/app/hero-form.component.html' region='start'}

That is plain old HTML 5. We're presenting two of the Hero fields, name and alterEgo, and opening them up for user input in input boxes.

The Name <input> control has the HTML5 required attribute; the Alter Ego <input> control does not because alterEgo is optional.

We've got a Submit button at the bottom with some classes on it for styling.

We are not using Angular yet. There are no bindings, no extra directives, just layout.

The container, form-group, form-control, and btn classes come from Twitter Bootstrap. Purely cosmetic. We're using Bootstrap to give the form a little style!

<header>
  Angular forms do not require a style library
</header>

Angular makes no use of the `container`, `form-group`, `form-control`, and `btn` classes or
the styles of any external library. Angular apps can use any CSS library, or none at all.

Let's add the stylesheet. Open index.html and add the following link to the <head>:

Add powers with *ngFor

Our hero must choose one super power from a fixed list of Agency-approved powers. We maintain that list internally (in HeroFormComponent).

We'll add a select to our form and bind the options to the powers list using ngFor, a technique seen previously in the Displaying Data guide.

Add the following HTML immediately below the Alter Ego group: This code repeats the <option> tag for each power in the list of powers. The pow template input variable is a different power in each iteration; we display its name using the interpolation syntax.

Two-way data binding with ngModel

Running the app right now would be disappointing.

We don't see hero data because we are not binding to the Hero yet. We know how to do that from earlier guides. Displaying Data taught us property binding. User Input showed us how to listen for DOM events with an event binding and how to update a component property with the displayed value.

Now we need to display, listen, and extract at the same time.

We could use the techniques we already know, but instead we'll introduce something new: the [(ngModel)] syntax, which makes binding the form to the model super easy.

Find the <input> tag for Name and update it like this:

We added a diagnostic interpolation after the input tag so we can see what we're doing. We left ourselves a note to throw it away when we're done. Focus on the binding syntax: [(ngModel)]="...".

If we run the app right now and started typing in the Name input box, adding and deleting characters, we'd see them appearing and disappearing from the interpolated text. At some point it might look like this.

The diagnostic is evidence that values really are flowing from the input box to the model and back again.

That's two-way data binding! For more information about [(ngModel)] and two-way data bindings, see the Template Syntax page. Notice that we also added a name attribute to our <input> tag and set it to "name" which makes sense for the hero's name. Any unique value will do, but using a descriptive name is helpful. Defining a name attribute is a requirement when using [(ngModel)] in combination with a form.

Internally Angular creates FormControl instances and registers them with an NgForm directive that Angular attached to the <form> tag. Each FormControl is registered under the name we assigned to the name attribute. We'll talk about NgForm later in this guide. Let's add similar [(ngModel)] bindings and name attributes to Alter Ego and Hero Power. We'll ditch the input box binding message and add a new binding (at the top) to the component's diagnostic property. Then we can confirm that two-way data binding works for the entire hero model.

After revision, the core of our form should look like this:

• Each input element has an id property that is used by the label element's for attribute to match the label to its input control.
• Each input element has a name property that is required by Angular forms to register the control with the form. If we run the app now and changed every hero model property, the form might display like this:

The diagnostic near the top of the form confirms that all of our changes are reflected in the model.

Delete the {{diagnostic}} binding at the top as it has served its purpose.

Track control state and validity with ngModel

A form isn't just about data binding. We'd also like to know the state of the controls in our form.

Using ngModel in a form gives us more than just a two way data binding. It also tells us if the user touched the control, if the value changed, or if the value became invalid.

The NgModel directive doesn't just track state; it updates the control with special Angular CSS classes that reflect the state. We can leverage those class names to change the appearance of the control.

<th>
  State
</th>


<th>
  Class if true
</th>


<th>
  Class if false
</th>

<td>
  Control has been visited
</td>


<td>
  <code>ng-touched</code>
</td>


<td>
  <code>ng-untouched</code>
</td>

<td>
  Control's value has changed
</td>


<td>
  <code>ng-dirty</code>
</td>


<td>
  <code>ng-pristine</code>
</td>

<td>
  Control's value is valid
</td>


<td>
  <code>ng-valid</code>
</td>


<td>
  <code>ng-invalid</code>
</td>

Let's temporarily add a template reference variable named spy to the Name <input> tag and use it to display the input's CSS classes. Now run the app, and look at the Name input box. Follow the next four steps precisely:

1. Look but don't touch.
2. Click inside the name box, then click outside it.
3. Add slashes to the end of the name.
4. Erase the name.

The actions and effects are as follows:

We should see the following transitions and class names:

The ng-valid/ng-invalid pair is the most interesting to us, because we want to send a strong visual signal when the values are invalid. We also want to mark required fields. To create such visual feedback, let's add definitions for the ng-* CSS classes.

Delete the #spy template reference variable and the TODO as they have served their purpose.

Add custom CSS for visual feedback

We can mark required fields and invalid data at the same time with a colored bar on the left of the input box:

We achieve this effect by adding these class definitions to a new forms.css file that we add to our project as a sibling to index.html:

{@example 'forms/ts/src/forms.css'}

Update the <head> of index.html to include this style sheet:

Show and hide validation error messages

We can do better. The Name input box is required and clearing it turns the bar red. That says something is wrong but we don't know what is wrong or what to do about it. We can leverage the control's state to reveal a helpful message.

Here's the way it should look when the user deletes the name:

To achieve this effect we extend the <input> tag with

1. a template reference variable
2. the "is required" message in a nearby <div> which we'll display only if the control is invalid.

Here's an example of adding an error message to the name input box: We need a template reference variable to access the input box's Angular control from within the template. Here we created a variable called name and gave it the value "ngModel".

Why "ngModel"? A directive's exportAs property tells Angular how to link the reference variable to the directive. We set name to ngModel because the ngModel directive's exportAs property happens to be "ngModel". We control visibility of the name error message by binding properties of the name control to the message <div> element's hidden property. In this example, we hide the message when the control is valid or pristine; pristine means the user hasn't changed the value since it was displayed in this form.

This user experience is the developer's choice. Some folks want to see the message at all times. If we ignore the pristine state, we would hide the message only when the value is valid. If we arrive in this component with a new (blank) hero or an invalid hero, we'll see the error message immediately, before we've done anything.

Some folks find that behavior disconcerting. They only want to see the message when the user makes an invalid change. Hiding the message while the control is "pristine" achieves that goal. We'll see the significance of this choice when we add a new hero to the form.

The hero Alter Ego is optional so we can leave that be.

Hero Power selection is required. We can add the same kind of error handling to the <select> if we want, but it's not imperative because the selection box already constrains the power to valid values.

We'd like to add a new hero in this form. We place a "New Hero" button at the bottom of the form and bind its click event to a newHero component method.

{@example 'forms/ts/src/app/hero-form.component.html' region='new-hero-button-no-reset'}

{@example 'forms/ts/src/app/hero-form.component.ts' region='new-hero'}

Run the application again, click the New Hero button, and the form clears. The required bars to the left of the input box are red, indicating invalid name and power properties. That's understandable as these are required fields. The error messages are hidden because the form is pristine; we haven't changed anything yet.

Enter a name and click New Hero again. The app displays a Name is required error message! We don't want error messages when we create a new (empty) hero. Why are we getting one now?

Inspecting the element in the browser tools reveals that the name input box is no longer pristine. The form remembers that we entered a name before clicking New Hero. Replacing the hero object did not restore the pristine state of the form controls.

We have to clear all of the flags imperatively which we can do by calling the form's reset() method after calling the newHero() method.

{@example 'forms/ts/src/app/hero-form.component.html' region='new-hero-button-form-reset'}

Now clicking "New Hero" both resets the form and its control flags.

Submit the form with ngSubmit

The user should be able to submit this form after filling it in. The Submit button at the bottom of the form does nothing on its own, but it will trigger a form submit because of its type (type="submit").

A "form submit" is useless at the moment. To make it useful, bind the form's ngSubmit event property to the hero form component's onSubmit() method: We slipped in something extra there at the end! We defined a template reference variable, #heroForm, and initialized it with the value "ngForm".

The variable heroForm is now a reference to the NgForm directive that governs the form as a whole.

The NgForm directive

What NgForm directive? We didn't add an NgForm directive!

Angular did. Angular creates and attaches an NgForm directive to the <form> tag automatically.

The NgForm directive supplements the form element with additional features. It holds the controls we created for the elements with an ngModel directive and name attribute, and monitors their properties including their validity. It also has its own valid property which is true only if every contained control is valid. We'll bind the form's overall validity via the heroForm variable to the button's disabled property using an event binding. Here's the code: If we run the application now, we find that the button is enabled — although it doesn't do anything useful yet.

Now if we delete the Name, we violate the "required" rule, which is duly noted in the error message. The Submit button is also disabled.

Not impressed? Think about it for a moment. What would we have to do to wire the button's enable/disabled state to the form's validity without Angular's help?

For us, it was as simple as:

1. Define a template reference variable on the (enhanced) form element.
2. Refer to that variable in a button many lines away.

Toggle two form regions (extra credit)

Submitting the form isn't terribly dramatic at the moment.

An unsurprising observation for a demo. To be honest, jazzing it up won't teach us anything new about forms. But this is an opportunity to exercise some of our newly won binding skills. If you aren't interested, go ahead and skip to this guide's conclusion. Let's do something more strikingly visual. Let's hide the data entry area and display something else.

Start by wrapping the form in a <div> and bind its hidden property to the HeroFormComponent.submitted property. The main form is visible from the start because the submitted property is false until we submit the form, as this fragment from the HeroFormComponent shows: When we click the Submit button, the submitted flag becomes true and the form disappears as planned.

Now the app needs to show something else while the form is in the submitted state. Add the following HTML below the <div> wrapper we just wrote: There's our hero again, displayed read-only with interpolation bindings. This <div> appears only while the component is in the submitted state.

The HTML includes an Edit button whose click event is bound to an expression that clears the submitted flag.

When we click the Edit button, this block disappears and the editable form reappears.

That's as much drama as we can muster for now.

Conclusion

The Angular form discussed in this guide takes advantage of the following framework features to provide support for data modification, validation, and more:

• An Angular HTML form template.
• A form component class with a @Component decorator.
• Handling form submission by binding to the NgForm.ngSubmit event property.
• Template reference variables such as #heroForm and #name.
• [(ngModel)] syntax for two-way data binding.
• The use of name attributes for validation and form element change tracking.
• The reference variable’s valid property on input controls to check if a control is valid and show/hide error messages.
• Controlling the submit button's enabled state by binding to NgForm validity.
• Custom CSS classes that provide visual feedback to users about invalid controls.

Our final project folder structure should look like this:

angular-forms src app app.component.ts

    <aio-file>
      app.module.ts
    </aio-file>


    <aio-file>
      hero.ts
    </aio-file>


    <aio-file>
      hero-form.component.html
    </aio-file>


    <aio-file>
      hero-form.component.ts
    </aio-file>


  </aio-folder>


  <aio-file>
    main.ts
  </aio-file>


  <aio-file>
    tsconfig.json
  </aio-file>


  <aio-file>
    index.html
  </aio-file>


</aio-folder>


<aio-file>
  node_modules ...
</aio-file>


<aio-file>
  package.json
</aio-file>

Here’s the code for the final version of the application:

{@example 'forms/ts/src/app/hero-form.component.ts' region='final'} {@example 'forms/ts/src/app/hero-form.component.html' region='final'} {@example 'forms/ts/src/app/hero.ts'} {@example 'forms/ts/src/app/app.module.ts'} {@example 'forms/ts/src/app/app.component.ts'} {@example 'forms/ts/src/main.ts'} {@example 'forms/ts/src/index.html'} {@example 'forms/ts/src/forms.css'}