angular - Create a reusable FormGroup

Question

I am aware of creating custom controls as components, but I can't figure out how to create custom groups.

The same we can do this by implementing ControlValueAccessor and using a custom component like <my-cmp formControlName="foo"></my-cmp>, how can we achieve this effect for a group?

<my-cmp formGroupName="aGroup"></my-cmp>

Two very common use-cases would be (a) separting a long form into steps, each step in a separate component and (b) encapsulating a group of fields which appear across multiple forms, such as address (group of country, state, city, address, building number) or date of birth (year, month, date).

---

Example usage (not actual working code)

Parent has the following form built with FormBuilder:

// parent model
form = this.fb.group({
  username: '',
  fullName: '',
  password: '',
  address: this.fb.group({
    country: '',
    state: '',
    city: '',
    street: '',
    building: '',
  })
})

Parent template (inaccessible and non-semantic for brevity):

<!-- parent template -->
<form [groupName]="form">
  <input formControlName="username">
  <input formControlName="fullName">
  <input formControlName="password">
  <address-form-group formGroup="address"></address-form-group>
</form>

Now this AddressFormGroupComponent knows how to handle a group which has these specific controls inside of it.

<!-- child template -->
<input formControlName="country">
<input formControlName="state">
<input formControlName="city">
<input formControlName="street">
<input formControlName="building">

Answer 1

The piece I was missing was mentioned in rusev's answer, and that is injecting the ControlContainer.

Turns out that if you place formGroupName on a component, and if that component injects ControlContainer, you get a reference to the container which contains that form. It's easy from here.

We create a sub-form component.

@Component({
  selector: 'sub-form',
  template: `
    <ng-container [formGroup]="controlContainer.control">
      <input type=text formControlName=foo>
      <input type=text formControlName=bar>
    </ng-container>
  `,
})
export class SubFormComponent {
  constructor(public controlContainer: ControlContainer) {
  }
}

Notice how we need a wrapper for the inputs. We don't want a form because this will already be inside a form. So we use an ng-container. This will be striped away from the final DOM so there's no unnecessary element.

Now we can just use this component.

@Component({
  selector: 'my-app',
  template: `
    <form [formGroup]=form>
      <sub-form formGroupName=group></sub-form>
      <input type=text formControlName=baz>
    </form>
  `,
})
export class AppComponent  {
  form = this.fb.group({
    group: this.fb.group({
      foo: 'foo',
      bar: 'bar',
    }),
    baz: 'baz',
  })

  constructor(private fb: FormBuilder) {}
}

You can see a live demo on StackBlitz.

---

This is an improvement over rusev's answer in a few aspects:

• no custom groupName input; instead we use the formGroupName provided by Angular
• no need for @SkipSelf decorator, since we're not injecting the parent control, but the one we need
• no awkward group.control.get(groupName) which is going to the parent in order to grab itself.