linkReactive Forms

Reactive forms is an Angular technique for creating forms in a reactive style. This guide explains reactive forms as you follow the steps to build a "Hero Detail Editor" form.

Try the Reactive Forms live-example / download example.

You can also run the Reactive Forms Demo / download example version and choose one of the intermediate steps from the "demo picker" at the top.

linkIntroduction to Reactive Forms

Angular offers two form-building technologies: reactive forms and template-driven forms. The two technologies belong to the @angular/forms library and share a common set of form control classes.

But they diverge markedly in philosophy, programming style, and technique. They even have their own modules: the ReactiveFormsModule and the FormsModule.

linkReactive forms

Angular reactive forms facilitate a reactive style of programming that favors explicit management of the data flowing between a non-UI data model (typically retrieved from a server) and a UI-oriented form model that retains the states and values of the HTML controls on screen. Reactive forms offer the ease of using reactive patterns, testing, and validation.

With reactive forms, you create a tree of Angular form control objects in the component class and bind them to native form control elements in the component template, using techniques described in this guide.

You create and manipulate form control objects directly in the component class. As the component class has immediate access to both the data model and the form control structure, you can push data model values into the form controls and pull user-changed values back out. The component can observe changes in form control state and react to those changes.

One advantage of working with form control objects directly is that value and validity updates are always synchronous and under your control. You won't encounter the timing issues that sometimes plague a template-driven form and reactive forms can be easier to unit test.

In keeping with the reactive paradigm, the component preserves the immutability of the data model, treating it as a pure source of original values. Rather than update the data model directly, the component extracts user changes and forwards them to an external component or service, which does something with them (such as saving them) and returns a new data model to the component that reflects the updated model state.

Using reactive form directives does not require you to follow all reactive priniciples, but it does facilitate the reactive programming approach should you choose to use it.

linkTemplate-driven forms

Template-driven forms, introduced in the Template guide, take a completely different approach.

You place HTML form controls (such as <input> and <select>) in the component template and bind them to data model properties in the component, using directives like ngModel.

You don't create Angular form control objects. Angular directives create them for you, using the information in your data bindings. You don't push and pull data values. Angular handles that for you with ngModel. Angular updates the mutable data model with user changes as they happen.

For this reason, the ngModel directive is not part of the ReactiveFormsModule.

While this means less code in the component class, template-driven forms are asynchronous which may complicate development in more advanced scenarios.

linkAsync vs. sync

Reactive forms are synchronous while template-driven forms are asynchronous.

In reactive forms, you create the entire form control tree in code. You can immediately update a value or drill down through the descendants of the parent form because all controls are always available.

Template-driven forms delegate creation of their form controls to directives. To avoid "changed after checked" errors, these directives take more than one cycle to build the entire control tree. That means you must wait a tick before manipulating any of the controls from within the component class.

For example, if you inject the form control with a @ViewChild(NgForm) query and examine it in the ngAfterViewInit lifecycle hook, you'll discover that it has no children. You must wait a tick, using setTimeout, before you can extract a value from a control, test its validity, or set it to a new value.

The asynchrony of template-driven forms also complicates unit testing. You must wrap your test block in async() or fakeAsync() to avoid looking for values in the form that aren't there yet. With reactive forms, everything is available when you expect it to be.

linkChoosing reactive or template-driven forms

Reactive and template-driven forms are two different architectural paradigms, with their own strengths and weaknesses. Choose the approach that works best for you. You may decide to use both in the same application.

The rest of this page explores the reactive paradigm and concentrates exclusively on reactive forms techniques. For information on template-driven forms, see the Forms guide.

In the next section, you'll set up your project for the reactive form demo. Then you'll learn about the Angular form classes and how to use them in a reactive form.

linkSetup

Create a new project named angular-reactive-forms:

      
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      ng new angular-reactive-forms
    

linkCreate a data model

The focus of this guide is a reactive forms component that edits a hero. You'll need a hero class and some hero data.

Using the CLI, generate a new class named data-model:

      
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      ng generate class data-model
    

And copy the following into data-model.ts:

      
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      export class Hero {
  id = 0;
  name = '';
  addresses: Address[];
}

export class Address {
  street = '';
  city   = '';
  state  = '';
  zip    = '';
}

export const heroes: Hero[] = [
  {
    id: 1,
    name: 'Whirlwind',
    addresses: [
      {street: '123 Main',  city: 'Anywhere', state: 'CA',  zip: '94801'},
      {street: '456 Maple', city: 'Somewhere', state: 'VA', zip: '23226'},
    ]
  },
  {
    id: 2,
    name: 'Bombastic',
    addresses: [
      {street: '789 Elm',  city: 'Smallville', state: 'OH',  zip: '04501'},
    ]
  },
  {
    id: 3,
    name: 'Magneta',
    addresses: [ ]
  },
];

export const states = ['CA', 'MD', 'OH', 'VA'];
    

The file exports two classes and two constants. The Address and Hero classes define the application data model. The heroes and states constants supply the test data.

linkCreate a reactive forms component

Generate a new component named HeroDetail:

      
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      ng generate component HeroDetail
    

And import:

      
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      import { FormControl } from '@angular/forms';
    

Next, update the HeroDetailComponent class with a FormControl. FormControl is a directive that allows you to create and manage a FormControl instance directly.

      
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      export class HeroDetailComponent1 {
  name = new FormControl();
}
    

This creates a FormControl called name. It will be bound in the template to an HTML <input> element for the hero name.

A FormControl constructor accepts three, optional arguments: the initial data value, an array of validators, and an array of async validators.

linkCreate the template

Now update the component's template with the following markup.

      
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      <h2>Hero Detail</h2>
<h3><i>Just a FormControl</i></h3>
<label class="center-block">Name:
  <input class="form-control" [formControl]="name">
</label>
    

To let Angular know that this is the input that you want to associate to the name FormControl in the class, you need [formControl]="name" in the template on the <input>.

linkImport the ReactiveFormsModule

The HeroDetailComponent template uses the formControlName directive from the ReactiveFormsModule.

Do the following two things in app.module.ts:

1. Use a JavaScript import statement to access the ReactiveFormsModule.
2. Add ReactiveFormsModule to the AppModule's imports list.

      
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      import { NgModule }            from '@angular/core';
import { BrowserModule }       from '@angular/platform-browser';
import { ReactiveFormsModule } from '@angular/forms';  // <-- #1 import module

import { AppComponent }        from './app.component';
import { HeroDetailComponent } from './hero-detail/hero-detail.component';

@NgModule({
  declarations: [
    AppComponent,
    HeroDetailComponent,
  ],
  imports: [
    BrowserModule,
    ReactiveFormsModule // <-- #2 add to @NgModule imports
  ],
  bootstrap: [ AppComponent ]
})
export class AppModule { }
    

linkDisplay the HeroDetailComponent

Revise the AppComponent template so it displays the HeroDetailComponent.

      
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      <div class="container">
  <h1>Reactive Forms</h1>
  <app-hero-detail></app-hero-detail>
</div>
    

linkEssential form classes

This guide uses four fundamental classes to build a reactive form:

linkStyle the app

To use the bootstrap CSS classes that are in the template HTML of both the AppComponent and the HeroDetailComponent, add the bootstrap CSS stylesheet to the head of styles.css:

      
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      @import url('https://unpkg.com/bootstrap@3.3.7/dist/css/bootstrap.min.css');
    

Now that everything is wired up, serve the app with:

      
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      ng serve
    

The browser should display something like this:

linkAdd a FormGroup

Usually, if you have multiple FormControls, you register them within a parent FormGroup. To add a FormGroup, add it to the imports section of hero-detail.component.ts:

      
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      import { Component }              from '@angular/core';
import { FormControl, FormGroup } from '@angular/forms';
    

In the class, wrap the FormControl in a FormGroup called heroForm as follows:

      
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      export class HeroDetailComponent2 {
  heroForm = new FormGroup ({
    name: new FormControl()
  });
}
    

Now that you've made changes in the class, they need to be reflected in the template. Update hero-detail.component.html by replacing it with the following.

      
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      <h2>Hero Detail</h2>
<h3><i>FormControl in a FormGroup</i></h3>
<form [formGroup]="heroForm">
  <div class="form-group">
    <label class="center-block">Name:
      <input class="form-control" formControlName="name">
    </label>
  </div>
</form>
    

Notice that now the single <input> is in a <form> element.

formGroup is a reactive form directive that takes an existing FormGroup instance and associates it with an HTML element. In this case, it associates the FormGroup you saved as heroForm with the <form> element.

Because the class now has a FormGroup, you must update the template syntax for associating the <input> with the corresponding FormControl in the component class. Without a parent FormGroup, [formControl]="name" worked earlier because that directive can stand alone, that is, it works without being in a FormGroup. With a parent FormGroup, the name <input> needs the syntax formControlName=name in order to be associated with the correct FormControl in the class. This syntax tells Angular to look for the parent FormGroup, in this case heroForm, and then inside that group to look for a FormControl called name.

linkTaking a look at the form model

When the user enters data into an <input>, the value goes into the form model. To see the form model, add the following line after the closing <form> tag in the hero-detail.component.html:

      
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      <p>Form value: {{ heroForm.value | json }}</p>
    

The heroForm.value returns the form model. Piping it through the JsonPipe renders the model as JSON in the browser:

The initial name property value is the empty string. Type into the name <input> and watch the keystrokes appear in the JSON.

In real life apps, forms get big fast. FormBuilder makes form development and maintenance easier.

linkIntroduction to FormBuilder

The FormBuilder class helps reduce repetition and clutter by handling details of control creation for you.

To use FormBuilder, import it into hero-detail.component.ts. You can remove FormControl:

      
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      import { Component }              from '@angular/core';
import { FormBuilder, FormGroup } from '@angular/forms';
    

Use it to refactor the HeroDetailComponent into something that's easier to read and write, by following this plan:

• Explicitly declare the type of the heroForm property to be FormGroup; you'll initialize it later.
• Inject a FormBuilder into the constructor.
• Add a new method that uses the FormBuilder to define the heroForm; call it createForm().
• Call createForm() in the constructor.

The revised HeroDetailComponent looks like this:

      
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      export class HeroDetailComponent3 {
  heroForm: FormGroup; // <--- heroForm is of type FormGroup

  constructor(private fb: FormBuilder) { // <--- inject FormBuilder
    this.createForm();
  }

  createForm() {
    this.heroForm = this.fb.group({
      name: '', // <--- the FormControl called "name"
    });
  }
}
    

FormBuilder.group is a factory method that creates a FormGroup.   FormBuilder.group takes an object whose keys and values are FormControl names and their definitions. In this example, the name control is defined by its initial data value, an empty string.

Defining a group of controls in a single object makes your code more compact and readable because you don't have to write repeated new FormControl(...) statements.

linkValidators.required

Though this guide doesn't go deeply into validations, here is one example that demonstrates the simplicity of using Validators.required in reactive forms.

First, import the Validators symbol.

      
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      import { Component }                          from '@angular/core';
import { FormBuilder, FormGroup, Validators } from '@angular/forms';
    

To make the name FormControl required, replace the name property in the FormGroup with an array. The first item is the initial value for name; the second is the required validator, Validators.required.

      
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      this.heroForm = this.fb.group({
  name: ['', Validators.required ],
});
    

Update the diagnostic message at the bottom of the template to display the form's validity status.

      
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      <p>Form value: {{ heroForm.value | json }}</p>
<p>Form status: {{ heroForm.status | json }}</p>
    

The browser displays the following:

Validators.required is working. The status is INVALID because the <input> has no value. Type into the <input> to see the status change from INVALID to VALID.

In a real app, you'd replace the diagnosic message with a user-friendly experience.

Using Validators.required is optional for the rest of the guide. It remains in each of the following examples with the same configuration.

For more on validating Angular forms, see the Form Validation guide.

linkMore FormControls

This section adds additional FormControls for the address, a super power, and a sidekick.

Additionally, the address has a state property. The user will select a state with a <select> and you'll populate the <option> elements with states. So import states from data-model.ts.

      
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      import { Component }                          from '@angular/core';
import { FormBuilder, FormGroup, Validators } from '@angular/forms';

import { states } from '../data-model';
    

Declare the states property and add some address FormControls to the heroForm as follows.

      
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      export class HeroDetailComponent4 {
  heroForm: FormGroup;
  states = states;

  constructor(private fb: FormBuilder) {
    this.createForm();
  }

  createForm() {
    this.heroForm = this.fb.group({
      name: ['', Validators.required ],
      street: '',
      city: '',
      state: '',
      zip: '',
      power: '',
      sidekick: ''
    });
  }
}
    

Then add corresponding markup in hero-detail.component.html as follows.

      
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      <h2>Hero Detail</h2>
<h3><i>A FormGroup with multiple FormControls</i></h3>
<form [formGroup]="heroForm">
  <div class="form-group">
    <label class="center-block">Name:
      <input class="form-control" formControlName="name">
    </label>
  </div>
  <div class="form-group">
    <label class="center-block">Street:
      <input class="form-control" formControlName="street">
    </label>
  </div>
  <div class="form-group">
    <label class="center-block">City:
      <input class="form-control" formControlName="city">
    </label>
  </div>
  <div class="form-group">
    <label class="center-block">State:
      <select class="form-control" formControlName="state">
          <option *ngFor="let state of states" [value]="state">{{state}}</option>
      </select>
    </label>
  </div>
  <div class="form-group">
    <label class="center-block">Zip Code:
      <input class="form-control" formControlName="zip">
    </label>
  </div>
  <div class="form-group radio">
    <h4>Super power:</h4>
    <label class="center-block"><input type="radio" formControlName="power" value="flight">Flight</label>
    <label class="center-block"><input type="radio" formControlName="power" value="x-ray vision">X-ray vision</label>
    <label class="center-block"><input type="radio" formControlName="power" value="strength">Strength</label>
  </div>
  <div class="checkbox">
    <label class="center-block">
      <input type="checkbox" formControlName="sidekick">I have a sidekick.
    </label>
  </div>
</form>


<p>Form value: {{ heroForm.value | json }}</p>
    

The revised template includes more text <input> elements, a <select> for the state, radio buttons for the power, and a <checkbox> for the sidekick.

You must bind the value property of the <option> with [value]="state". If you do not bind the value, the select shows the first option from the data model.

The component class defines control properties without regard for their representation in the template. You define the state, power, and sidekick controls the same way you defined the name control. You tie these controls to the template HTML elements in the same way, specifying the FormControl name with the formControlName directive.

See the API reference for more information about radio buttons, selects, and checkboxes.

linkNested FormGroups

To manage the size of the form more effectively, you can group some of the related FormControls into a nested FormGroup. For example, the street, city, state, and zip are ideal properties for an address FormGroup. Nesting groups and controls in this way allows you to mirror the hierarchical structure of the data model and helps track validation and state for related sets of controls.

You used the FormBuilder to create one FormGroup in this component called heroForm. Let that be the parent FormGroup. Use FormBuilder again to create a child FormGroup that encapsulates the address controls; assign the result to a new address property of the parent FormGroup.

      
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      export class HeroDetailComponent5 {
  heroForm: FormGroup;
  states = states;

  constructor(private fb: FormBuilder) {
    this.createForm();
  }

  createForm() {
    this.heroForm = this.fb.group({ // <-- the parent FormGroup
      name: ['', Validators.required ],
      address: this.fb.group({ // <-- the child FormGroup
        street: '',
        city: '',
        state: '',
        zip: ''
      }),
      power: '',
      sidekick: ''
    });
  }
}
    

When you change the structure of the form controls in the component class, you must make corresponding adjustments to the component template.

In hero-detail.component.html, wrap the address-related FormControls in a <div>. Add a formGroupName directive to the div and bind it to "address". That's the property of the address child FormGroup within the parent FormGroup called heroForm. Leave the <div> with the name <input>.

To make this change visually obvious, add an <h4> header near the top with the text, Secret Lair. The new address HTML looks like this:

      
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      <div formGroupName="address" class="well well-lg">
  <h4>Secret Lair</h4>
  <div class="form-group">
    <label class="center-block">Street:
      <input class="form-control" formControlName="street">
    </label>
  </div>
  <div class="form-group">
    <label class="center-block">City:
      <input class="form-control" formControlName="city">
    </label>
  </div>
  <div class="form-group">
    <label class="center-block">State:
      <select class="form-control" formControlName="state">
        <option *ngFor="let state of states" [value]="state">{{state}}</option>
      </select>
    </label>
  </div>
  <div class="form-group">
    <label class="center-block">Zip Code:
      <input class="form-control" formControlName="zip">
    </label>
  </div>
</div>
    

After these changes, the JSON output in the browser shows the revised form model with the nested address FormGroup:

This shows that the template and the form model are talking to one another.

linkInspect FormControl Properties

You can inspect an individual FormControl within a form by extracting it with the get() method. You can do this within the component class or display it on the page by adding the following to the template, immediately after the {{form.value | json}} interpolation as follows:

      
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      <p>Name value: {{ heroForm.get('name').value }}</p>
    

To get the state of a FormControl that’s inside a FormGroup, use dot notation to traverse to the control.

      
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      <p>Street value: {{ heroForm.get('address.street').value}}</p>
    

You can use this technique to display any property of a FormControl such as one of the following:

Read about other FormControl properties in the AbstractControl API reference.

One common reason for inspecting FormControl properties is to make sure the user entered valid values. Read more about validating Angular forms in the Form Validation guide.

linkThe data model and the form model

At the moment, the form is displaying empty values. The HeroDetailComponent should display values of a hero, possibly a hero retrieved from a remote server.

In this app, the HeroDetailComponent gets its hero from a parent HeroListComponent.

The hero from the server is the data model. The FormControl structure is the form model.

The component must copy the hero values in the data model into the form model. There are two important implications:

1. The developer must understand how the properties of the data model map to the properties of the form model.

2. User changes flow from the DOM elements to the form model, not to the data model.

The form controls never update the data model.

The form and data model structures don't need to match exactly. You often present a subset of the data model on a particular screen. But it makes things easier if the shape of the form model is close to the shape of the data model.

In this HeroDetailComponent, the two models are quite close.

Here are the definitions of Hero and Address in data-model.ts:

      
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      export class Hero {
  id = 0;
  name = '';
  addresses: Address[];
}

export class Address {
  street = '';
  city   = '';
  state  = '';
  zip    = '';
}
    

Here, again, is the component's FormGroup definition.

      
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      this.heroForm = this.fb.group({
  name: ['', Validators.required ],
  address: this.fb.group({
    street: '',
    city: '',
    state: '',
    zip: ''
  }),
  power: '',
  sidekick: ''
});
    

There are two significant differences between these models:

1. The Hero has an id. The form model does not because you generally don't show primary keys to users.

2. The Hero has an array of addresses. This form model presents only one address, which is covered in the section on FormArray below.

Keeping the two models close in shape facilitates copying the data model properties to the form model with the patchValue() and setValue() methods in the next section.

First, refactor the address FormGroup definition as follows:

      
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      this.heroForm = this.fb.group({
  name: ['', Validators.required ],
  address: this.fb.group(new Address()), // <-- a FormGroup with a new address
  power: '',
  sidekick: ''
});
    

Also be sure to update the import from data-model so you can reference the Hero and Address classes:

      
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      import { Address, Hero, states } from '../data-model';
    

linkPopulate the form model with setValue() and patchValue()

Previously, you created a control and initialized its value at the same time. You can also initialize or reset the values later with the setValue() and patchValue() methods.

linksetValue()

With setValue(), you assign every form control value at once by passing in a data object whose properties exactly match the form model behind the FormGroup.

      
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      this.heroForm.setValue({
   name:    this.hero.name,
   address: this.hero.addresses[0] || new Address()
});
    

The setValue() method checks the data object thoroughly before assigning any form control values.

It will not accept a data object that doesn't match the FormGroup structure or is missing values for any control in the group. This way, it can return helpful error messages if you have a typo or if you've nested controls incorrectly. Conversely, patchValue() will fail silently.

Notice that you can almost use the entire hero as the argument to setValue() because its shape is similar to the component's FormGroup structure.

You can only show the hero's first address and you must account for the possibility that the hero has no addresses at all, as in the conditional setting of the address property in the data object argument:

      
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      address: this.hero.addresses[0] || new Address()
    

linkpatchValue()

With patchValue(), you can assign values to specific controls in a FormGroup by supplying an object of key/value pairs for them.

This example sets only the form's name control.

      
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      this.heroForm.patchValue({
  name: this.hero.name
});
    

With patchValue() you have more flexibility to cope with divergent data and form models. But unlike setValue(), patchValue() cannot check for missing control values and doesn't throw helpful errors.

linkCreate the HeroListComponent and HeroService

To demonstrate further reactive forms techniques, it is helpful to add more functionality to the example by adding a HeroListComponent and a HeroService.

The HeroDetailComponent is a nested sub-component of the HeroListComponent in a master/detail view. Together they look like this:

First, add a HeroListComponent with the following command:

      
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      ng generate component HeroList
    

Give the HeroListComponent the following contents:

      
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      import { Component, OnInit } from '@angular/core';
import { Observable }        from 'rxjs/Observable';
import 'rxjs/add/operator/finally';

import { Hero }        from '../data-model';
import { HeroService } from '../hero.service';

@Component({
  selector: 'app-hero-list',
  templateUrl: './hero-list.component.html',
  styleUrls: ['./hero-list.component.css']
})
export class HeroListComponent implements OnInit {
  heroes: Observable<Hero[]>;
  isLoading = false;
  selectedHero: Hero;

  constructor(private heroService: HeroService) { }

  ngOnInit() { this.getHeroes(); }

  getHeroes() {
    this.isLoading = true;
    this.heroes = this.heroService.getHeroes()
                      // TODO: error handling
                      .finally(() => this.isLoading = false);
    this.selectedHero = undefined;
  }

  select(hero: Hero) { this.selectedHero = hero; }
}
    

Next, add a HeroService using the following command:

      
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      ng generate service Hero
    

Then, give it the following contents:

      
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      import { Injectable } from '@angular/core';

import { Observable } from 'rxjs/Observable';
import { of }         from 'rxjs/observable/of';
import 'rxjs/add/operator/delay';

import { Hero, heroes } from './data-model';

@Injectable()
export class HeroService {

  delayMs = 500;

  // Fake server get; assume nothing can go wrong
  getHeroes(): Observable<Hero[]> {
    return of(heroes).delay(this.delayMs); // simulate latency with delay
  }

  // Fake server update; assume nothing can go wrong
  updateHero(hero: Hero): Observable<Hero>  {
    const oldHero = heroes.find(h => h.id === hero.id);
    const newHero = Object.assign(oldHero, hero); // Demo: mutate cached hero
    return of(newHero).delay(this.delayMs); // simulate latency with delay
  }
}
    

The HeroListComponent uses an injected HeroService to retrieve heroes from the server and then presents those heroes to the user as a series of buttons. The HeroService emulates an HTTP service. It returns an Observable of heroes that resolves after a short delay, both to simulate network latency and to indicate visually the necessarily asynchronous nature of the application.

When the user clicks on a hero, the component sets its selectedHero property which is bound to the hero @Input() property of the HeroDetailComponent. The HeroDetailComponent detects the changed hero and resets its form with that hero's data values.

A refresh button clears the hero list and the current selected hero before refetching the heroes.

Notice that hero-list.component.ts imports Observable and finally while hero.service.ts imports Observable, of, and delay from rxjs.

The remaining HeroListComponent and HeroService implementation details are beyond the scope of this tutorial. However, the techniques involved are covered elsewhere in the documentation, including the Tour of Heroes here and here.

To use the HeroService, import it into AppModule and add it to the providers array. To use the HeroListComponent, import it, declare it, and export it:

      
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      // add JavaScript imports
import { HeroListComponent }   from './hero-list/hero-list.component';
import { HeroService }         from './hero.service';

@NgModule({
  declarations: [
    AppComponent,
    HeroDetailComponent,
    HeroListComponent // <--declare HeroListComponent
  ],
  // ...
  exports: [
    AppComponent,
    HeroDetailComponent,
    HeroListComponent // <-- export HeroListComponent
  ],
  providers: [ HeroService ], // <-- provide HeroService
    

Next, update the HeroListComponent template with the following:

      
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      <h3 *ngIf="isLoading"><i>Loading heroes ... </i></h3>
<h3 *ngIf="!isLoading">Select a hero:</h3>

<nav>
  <button (click)="getHeroes()" class="btn btn-primary">Refresh</button>
  <a *ngFor="let hero of heroes | async" (click)="select(hero)">{{hero.name}}</a>
</nav>

<div *ngIf="selectedHero">
  <hr>
  <h2>Hero Detail</h2>
  <h3>Editing: {{selectedHero.name}}</h3>
  <app-hero-detail [hero]="selectedHero"></app-hero-detail>
</div>
    

These changes need to be reflected in the AppComponent template. Replace the contents of app.component.html with updated markup to use the HeroListComponent, instead of the HeroDetailComponent:

      
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      <div class="container">
  <h1>Reactive Forms</h1>
  <app-hero-list></app-hero-list>
</div>
    

Finally, add an @Input() property to the HeroDetailComponent so HeroDetailComponent can receive the data from HeroListComponent. Remember to add the Input symbol to the @angular/core import statement in the list of JavaScript imports too.

      
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      @Input() hero: Hero;
    

Now you should be able to click on a button for a hero and a form renders.

linkWhen to set form model values (ngOnChanges)

When to set form model values depends upon when the component gets the data model values.

The HeroListComponent displays hero names to the user. When the user clicks on a hero, the HeroListComponent passes the selected hero into the HeroDetailComponent by binding to its hero @Input() property.

      
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      <nav>
  <a *ngFor="let hero of heroes | async" (click)="select(hero)">{{hero.name}}</a>
</nav>

<div *ngIf="selectedHero">
  <app-hero-detail [hero]="selectedHero"></app-hero-detail>
</div>
    

In this approach, the value of hero in the HeroDetailComponent changes every time the user selects a new hero. You can call setValue() using the ngOnChanges lifecycle hook, which Angular calls whenever the @Input() hero property changes.

linkReset the form

First, import the OnChanges symbol in hero-detail.component.ts.

      
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      import { Component, Input, OnChanges }             from '@angular/core';
    

Next, let Angular know that the HeroDetailComponent implements OnChanges:

      
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      export class HeroDetailComponent implements OnChanges {
    

Add the ngOnChanges method to the class as follows:

      
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      ngOnChanges() {
  this.rebuildForm();
}
    

Notice that it calls rebuildForm(), which is a method where you can set the values. You can name rebuildForm() anything that makes sense to you. It isn't built into Angular, but is a method you create to effectively leverage the ngOnChanges lifecycle hook.

      
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      rebuildForm() {
  this.heroForm.reset({
    name: this.hero.name,
    address: this.hero.addresses[0] || new Address()
  });
}
    

The rebuildForm() method does two things; resets the hero's name and the address.

linkUse FormArray to present an array of FormGroups

A FormGroup is a named object whose property values are FormControls and other FormGroups.

Sometimes you need to present an arbitrary number of controls or groups. For example, a hero may have zero, one, or any number of addresses.

The Hero.addresses property is an array of Address instances. An address FormGroup can display one Address. An Angular FormArray can display an array of address FormGroups.

To get access to the FormArray class, import it into hero-detail.component.ts:

      
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      import { Component, Input, OnChanges }                   from '@angular/core';
import { FormArray, FormBuilder, FormGroup, Validators } from '@angular/forms';

import { Address, Hero, states } from '../data-model';
    

To work with a FormArray do the following:

1. Define the items in the array; that is, FormControls or FormGroups.

2. Initialize the array with items created from data in the data model.

3. Add and remove items as the user requires.

Define a FormArray for Hero.addresses and let the user add or modify addresses.

You’ll need to redefine the form model in the HeroDetailComponent createForm() method, which currently only displays the first hero address in an address FormGroup:

      
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      this.heroForm = this.fb.group({
  name: ['', Validators.required ],
  address: this.fb.group(new Address()), // <-- a FormGroup with a new address
  power: '',
  sidekick: ''
});
    

linkFrom address to secretLairs

From the user's point of view, heroes don't have addresses. Addresses are for mere mortals. Heroes have secret lairs! Replace the address FormGroup definition with a secretLairs FormArray definition:

      
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      this.heroForm = this.fb.group({
  name: ['', Validators.required ],
  secretLairs: this.fb.array([]), // <-- secretLairs as an empty FormArray
  power: '',
  sidekick: ''
});
    

In hero-detail.component.html change formArrayName="address" to formArrayName="secretLairs".

      
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      <div formArrayName="secretLairs" class="well well-lg">
    

linkInitialize the secretLairs FormArray

The default form displays a nameless hero with no addresses.

You need a method to populate (or repopulate) the secretLairs with actual hero addresses whenever the parent HeroListComponent sets the HeroDetailComponent.hero @Input() property to a new Hero.

The following setAddresses() method replaces the secretLairs FormArray with a new FormArray, initialized by an array of hero address FormGroups. Add this to the HeroDetailComponent class:

      
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      setAddresses(addresses: Address[]) {
  const addressFGs = addresses.map(address => this.fb.group(address));
  const addressFormArray = this.fb.array(addressFGs);
  this.heroForm.setControl('secretLairs', addressFormArray);
}
    

Notice that you replace the previous FormArray with the FormGroup.setControl() method, not with setValue(). You're replacing a control, not the value of a control.

Notice also that the secretLairs FormArray contains FormGroups, not Addresses.

Next, call setAddresses() from within rebuildForm():

      
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      rebuildForm() {
  this.heroForm.reset({
    name: this.hero.name
  });
  this.setAddresses(this.hero.addresses);
}
    

linkGet the FormArray

The HeroDetailComponent should be able to display, add, and remove items from the secretLairs FormArray.

Use the FormGroup.get() method to acquire a reference to that FormArray. Wrap the expression in a secretLairs convenience property for clarity and re-use. Add the following to HeroDetailComponent.

      
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      get secretLairs(): FormArray {
  return this.heroForm.get('secretLairs') as FormArray;
};
    

linkDisplay the FormArray

The current HTML template displays a single address FormGroup. Revise it to display zero, one, or more of the hero's address FormGroups.

This is mostly a matter of wrapping the previous template HTML for an address in a <div> and repeating that <div> with *ngFor.

There are three key points when writing the *ngFor:

1. Add another wrapping <div>, around the <div> with *ngFor, and set its formArrayName directive to "secretLairs". This step establishes the secretLairs FormArray as the context for form controls in the inner, repeated HTML template.

2. The source of the repeated items is the FormArray.controls, not the FormArray itself. Each control is an address FormGroup, exactly what the previous (now repeated) template HTML expected.

3. Each repeated FormGroup needs a unique formGroupName, which must be the index of the FormGroup in the FormArray. You'll re-use that index to compose a unique label for each address.

Here's the skeleton for the secret lairs section of the HTML template:

      
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      <div formArrayName="secretLairs" class="well well-lg">
  <div *ngFor="let address of secretLairs.controls; let i=index" [formGroupName]="i" >
    <!-- The repeated address template -->
  </div>
</div>
    

Here's the complete template for the secret lairs section. Add this to HeroDetailComponent template, replacing the forGroupName=address <div>:

      
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      <div formArrayName="secretLairs" class="well well-lg">
  <div *ngFor="let address of secretLairs.controls; let i=index" [formGroupName]="i" >
    <!-- The repeated address template -->
    <h4>Address #{{i + 1}}</h4>
    <div style="margin-left: 1em;">
      <div class="form-group">
        <label class="center-block">Street:
          <input class="form-control" formControlName="street">
        </label>
      </div>
      <div class="form-group">
        <label class="center-block">City:
          <input class="form-control" formControlName="city">
        </label>
      </div>
      <div class="form-group">
        <label class="center-block">State:
          <select class="form-control" formControlName="state">
            <option *ngFor="let state of states" [value]="state">{{state}}</option>
          </select>
        </label>
      </div>
      <div class="form-group">
        <label class="center-block">Zip Code:
          <input class="form-control" formControlName="zip">
        </label>
      </div>
    </div>
    <br>
    <!-- End of the repeated address template -->
  </div>
</div>
    

linkAdd a new lair to the FormArray

Add an addLair() method that gets the secretLairs FormArray and appends a new address FormGroup to it.

      
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      addLair() {
  this.secretLairs.push(this.fb.group(new Address()));
}
    

Place a button on the form so the user can add a new secret lair and wire it to the component's addLair() method. Put it just before the closing </div> of the secretLairs FormArray.

      
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      <button (click)="addLair()" type="button">Add a Secret Lair</button>
    

linkTry it!

Back in the browser, select the hero named "Magneta". "Magneta" doesn't have an address, as you can see in the diagnostic JSON at the bottom of the form.

Click the "Add a Secret Lair" button. A new address section appears. Well done!

linkRemove a lair

This example can add addresses but it can't remove them. For extra credit, write a removeLair method and wire it to a button on the repeating address HTML.

linkObserve control changes

Angular calls ngOnChanges() when the user picks a hero in the parent HeroListComponent. Picking a hero changes the HeroDetailComponent.hero @Input() property.

Angular does not call ngOnChanges() when the user modifies the hero's name or secretLairs. Fortunately, you can learn about such changes by subscribing to one of the FormControl properties that raises a change event.

These are properties, such as valueChanges, that return an RxJS Observable. You don't need to know much about RxJS Observable to monitor form control values.

Add the following method to log changes to the value of the name FormControl.

      
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      nameChangeLog: string[] = [];
logNameChange() {
  const nameControl = this.heroForm.get('name');
  nameControl.valueChanges.forEach(
    (value: string) => this.nameChangeLog.push(value)
  );
}
    

Call it in the constructor, after createForm().

      
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      constructor(private fb: FormBuilder) {
  this.createForm();
  this.logNameChange();
}
    

The logNameChange() method pushes name-change values into a nameChangeLog array. Display that array at the bottom of the component template with this *ngFor binding:

      
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      <h4>Name change log</h4>
<div *ngFor="let name of nameChangeLog">{{name}}</div>
    

Return to the browser, select a hero; for example, Magneta, and start typing in the name <input>. You should see a new name in the log after each keystroke.

linkWhen to use it

An interpolation binding is the easier way to display a name change. Subscribing to an observable FormControl property is handy for triggering application logic within the component class.

linkSave form data

The HeroDetailComponent captures user input but it doesn't do anything with it. In a real app, you'd probably save those hero changes, revert unsaved changes, and resume editing. After you implement both features in this section, the form will look like this:

linkSave

When the user submits the form, the HeroDetailComponent will pass an instance of the hero data model to a save method on the injected HeroService. Add the following to HeroDetailComponent.

      
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      onSubmit() {
  this.hero = this.prepareSaveHero();
  this.heroService.updateHero(this.hero).subscribe(/* error handling */);
  this.rebuildForm();
}
    

This original hero had the pre-save values. The user's changes are still in the form model. So you create a new hero from a combination of original hero values (the hero.id) and deep copies of the changed form model values, using the prepareSaveHero() helper.

      
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      prepareSaveHero(): Hero {
  const formModel = this.heroForm.value;

  // deep copy of form model lairs
  const secretLairsDeepCopy: Address[] = formModel.secretLairs.map(
    (address: Address) => Object.assign({}, address)
  );

  // return new `Hero` object containing a combination of original hero value(s)
  // and deep copies of changed form model values
  const saveHero: Hero = {
    id: this.hero.id,
    name: formModel.name as string,
    // addresses: formModel.secretLairs // <-- bad!
    addresses: secretLairsDeepCopy
  };
  return saveHero;
}
    

Make sure to import HeroService and add it to the constructor:

      
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      import { HeroService }           from '../hero.service';
    

      
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      constructor(
  private fb: FormBuilder,
  private heroService: HeroService) {

  this.createForm();
  this.logNameChange();
}
    

linkRevert (cancel changes)

The user cancels changes and reverts the form to the original state by pressing the Revert button.

Reverting is easy. Simply re-execute the rebuildForm() method that built the form model from the original, unchanged hero data model.

      
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      revert() { this.rebuildForm(); }
    

linkButtons

Add the "Save" and "Revert" buttons near the top of the component's template:

      
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      <form [formGroup]="heroForm" (ngSubmit)="onSubmit()">
  <div style="margin-bottom: 1em">
    <button type="submit"
            [disabled]="heroForm.pristine" class="btn btn-success">Save</button> &nbsp;
    <button type="button" (click)="revert()"
            [disabled]="heroForm.pristine" class="btn btn-danger">Revert</button>
  </div>

  <!-- Hero Detail Controls -->
  <div class="form-group radio">
    <h4>Super power:</h4>
    <label class="center-block"><input type="radio" formControlName="power" value="flight">Flight</label>
    <label class="center-block"><input type="radio" formControlName="power" value="x-ray vision">X-ray vision</label>
    <label class="center-block"><input type="radio" formControlName="power" value="strength">Strength</label>
  </div>
  <div class="checkbox">
    <label class="center-block">
      <input type="checkbox" formControlName="sidekick">I have a sidekick.
    </label>
  </div>
</form>
    

The buttons are disabled until the user "dirties" the form by changing a value in any of its form controls (heroForm.dirty).

Clicking a button of type "submit" triggers the ngSubmit event which calls the component's onSubmit method. Clicking the revert button triggers a call to the component's revert method. Users now can save or revert changes.

Try the Reactive Forms (final) in Stackblitz / download example.

The key files of the final version are as follows:

      
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      <div class="container">
  <h1>Reactive Forms</h1>
  <app-hero-list></app-hero-list>
</div>
    

You can download the complete source for all steps in this guide from the Reactive Forms Demo / download example live example.