Form in form. Can there be inheritance of form controls?

Update:

Indeed there is an easier way:

import { FormsModule, ControlContainer, NgForm, NgModel } from '@angular/forms';

@Component({
  ...
  viewProviders: [ { provide: ControlContainer, useExisting: NgForm } ]
})
export class ChildComponent {}

See also

• Angular2 nested template driven form

Previous version:

I would say it's possible. For example you could add the following code to your

child.component.ts

import { NgForm, NgModel } from '@angular/forms';

@Component({
  selector: 'child-component',
  template: `
      <input type="text" name="thirdControl" [(ngModel)]="thirdControl" />
      <input type="text" name="fourthControl" [(ngModel)]="fourthControl" />
    `
})
export class ChildComponent {
  @ViewChildren(NgModel) ngModels: QueryList<NgModel>;

  constructor(@Optional() private ngForm: NgForm) {}

  ngAfterViewInit() {
    if (this.ngForm) {
      this.ngModels.forEach(model => this.ngForm.addControl(model));
    }
  }
}

Plunker Example

Angular DI system gives us the opportunity to get reference to parent NgForm instance because angular dependency resolution algorithm starts with current node and goes up through tree of elements. In my example we can imagine the follwing tree

              @NgModule providers
                    |
                  my-app
                    |
                   form
          /         |       \
   input[text] input[text] child-component

So when we are requiring NgForm token angular will search it in the next order

child-component
     ||
     \/
    form
     ||
     \/
   my-app
     ||
     \/
  @NgModule

On form element NgForm directive is placed so when can get it. Also we can get any token that was declared on NgForm directive within providers array. And this rule applies to any node.

See also Angular 2 - How does ng-bootstrap provide the NgbRadioGroup and NgbButtonLabel to their NgbRadio directive?

Then i just added child NgModel directives manually to NgForm so they should work together.

Angular has two kinds of forms, Template Driven and Reactive. What you want to do isn't possible with Template Driven forms, you'll need to look into Reactive Forms

In general it works by creating a form object in the component, and using it in the template. Angular says this is actually more flexible and easier to test, but I think the syntax is a lot harder to wrap your head around.

You'll want this in your parent component:

  myForm: FormGroup; 

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

  createForm() {
    this.myForm = this.fb.group({
      firstControl: '',
      secondControl: '',
      thirdControl: '',
      fourthControl: ''
    });
  }

Your parent template would look something like this:

<form [formGroup]="myForm" novalidate>
<input type="text"formControlName="firstControl" />
     <input type="text" formControlName="secondControl" />
     <child-component [parentFormGroup]="myForm">
</form>

And your child will accept the form group as an input, and the template will look something like this:

<div class="form-group" [formGroup]="parentFormGroup">
  <input type="text" formControlName="thirdControl" />
  <input type="text" formControlName="fourthControl" />
</div>