How to dynamically add additional formControls via template outlet?

Question

So, I have the issue that I have a base component that has a form. But I would like to be able to add another field to it. Why? Because I have four pages, with a very similar structure, that differ only in one field at most. I've got a stackblitz that should get the idea across as well. The rendering part is working nice and well.

But to also give a general idea:

@Component({
  selector: 'app-base-form',
  standalone: true,
  imports: [ReactiveFormsModule, CommonModule],
  template: `<form
  style="display: flex; flex-direction: column; gap: 1rem"
  [formGroup]="form"
>
  <input formControlName="search" type="text" placeholder="Search something" />

  <ng-template
    [ngTemplateOutletContext]="{ $implicit: form }"
    [ngTemplateOutlet]="seachOptionalField()!"
  ></ng-template>
</form>`,
})
export class BaseFormComponent {
  seachOptionalField = input<TemplateRef<any>>();
  fb = inject(UntypedFormBuilder);
  form = this.fb.group({
    search: this.fb.control(['']),
  });
}

The idea was initially to pass the form via the outlet context, but programmatically adding a control didn't really impact the original form. Also, there was the question on how to apply the rendered (to be used) form control.

@Component({
  selector: 'app-root',
  standalone: true,
  imports: [CommonModule, BaseFormComponent],
  template: `
    <app-base-form [seachOptionalField]="searchOptionalField"></app-base-form>
    <ng-template #searchOptionalField let-form>
    <input type="text" placeholder="We just added this" />
    {{form.value | json}}
    </ng-template>
  `,
})
export class App {}

Answer 1

You need to provide a corresponding formControlName on the new controls, also wrap them on a form element inside the template.

You can also add an extra parameter, which specifies the new form controls configuration on a object which we can destructure into our form group when initializing.

Working example below with stackblitz!

Full Code:

main.ts

import { Component, inject } from '@angular/core';
import { bootstrapApplication } from '@angular/platform-browser';
import { CommonModule } from '@angular/common';
import 'zone.js';
import { BaseFormComponent } from './app/base-form/base-form.component';
import { ReactiveFormsModule, UntypedFormBuilder } from '@angular/forms';

@Component({
  selector: 'app-root',
  standalone: true,
  imports: [CommonModule, BaseFormComponent, ReactiveFormsModule],
  template: `
    <app-base-form [seachOptionalField]="searchOptionalField" [extraFormParams]="extraFormParams"></app-base-form>
    <ng-template #searchOptionalField let-form>
      <form [formGroup]="form">
        <input type="text" placeholder="We just added this" formControlName="search2"/>
    </form>
    {{form.value | json}}
    </ng-template>
  `,
})
export class App {
  fb = inject(UntypedFormBuilder);
  extraFormParams = {
    search2: this.fb.control(['']),
  };
}

bootstrapApplication(App);

base-form.ts

import { CommonModule } from '@angular/common';
import { Component, TemplateRef, inject, input } from '@angular/core';
import {
  FormGroup,
  ReactiveFormsModule,
  UntypedFormBuilder,
} from '@angular/forms';

@Component({
  selector: 'app-base-form',
  standalone: true,
  imports: [ReactiveFormsModule, CommonModule],
  template: `<form
  style="display: flex; flex-direction: column; gap: 1rem"
  [formGroup]="form"
>
  <input formControlName="search" type="text" placeholder="Search something" />

  <ng-template
    [ngTemplateOutletContext]="{ $implicit: form }"
    [ngTemplateOutlet]="seachOptionalField()!"
  ></ng-template>
</form>`,
})
export class BaseFormComponent {
  seachOptionalField = input<TemplateRef<any>>();
  extraFormParams = input<Object>({});
  fb = inject(UntypedFormBuilder);
  form!: FormGroup;

  ngOnInit() {
    this.form = this.fb.group({
      search: this.fb.control(['']),
      ...this.extraFormParams(),
    });
  }
}

Stackblitz Demo