Why isn't Angular calling registerOnChange in my implementation of ControlValueAccessor?

Question

I'm trying to implement a custom form control component inside a template-drive form in Angular 4. Because I want it to integrate nicely with the parent form, I am trying to implement it as a ControlValueAccessor. I have followed the guides I could find but Angular is not cooperating. According to the documentation, it should be calling my implementation of registerOnChange(), but it doesn't. Why is that?

/* editor-clause.component.ts */
import { ControlValueAccessor, NG_VALUE_ACCESSOR } from '@angular/forms';
import { QueryClause } from '../../../../models/query-clause';
import { Input, Component, EventEmitter, Output } from '@angular/core';

@Component({
  moduleId: module.id,
  providers: [{
    provide: NG_VALUE_ACCESSOR,
    useExisting: EditorClauseComponent,
    multi: true,
  }],
  selector: 'editor-clause',
  templateUrl: './editor-clause.component.html',
  styleUrls: ['./editor-clause.component.css']
})
export class EditorClauseComponent implements ControlValueAccessor {
  @Input('clause') clause: QueryClause;
  parentOnChange: any;

  writeValue(_obj: any): void {
    return;
  }

  registerOnChange(fn: any): void {
    // This never gets printed and I don't know why
    console.log('Registering parent change tracking');
    this.parentOnChange = () => {
      console.log('Saw a change. Invoking parent function');
      fn();
    };
  }

  registerOnTouched(_fn: any): void {
    return;
  }

}

The containing parent form looks something like:

<form #queryBuilderForm="ngForm">

    <p>Form instructions here</p>

    <editor-clause *ngFor="let clause of clauses" [clause]="clause"></editor-clause>

    <button (click)="addClause()" id="add-query-clause-button">Add clause</button>
</form>

Answer 1

As explained in the article Never again be confused when implementing ControlValueAccessor in Angular forms the ControlValueAccessor is an intermediary between native control and Angular's form control:

The Angular form control can be created automatically by applying NgModel directive like this:

<editor-clause ngModel *ngFor="let clause of clauses" [clause]="clause"></editor-clause>

Or manually in the component and then bound to a control using formControl directive:

export class EditorClauseComponent ... {
    myControl = new FormControl();

Template:

<editor-clause formControl="myControl" *ngFor="let clause of clauses" [clause]="clause"></editor-clause>