Angular - controlValueAccessor method writeValue() not being called

Question

Angular Version: 6.1

I'm trying to implement a custom control using what Kara Erickson is calling a Composite ControlValueAccessor as shown in the presentation, Angular Forms – Kara Erickson – AngularConnect 2017. The control's value should be set to one of the two child input values.

The issue is writeValue() seems to only be called initially and not on further value changes.

Here's a stackblitz demo.

form.html

<form #f="ngForm" (ngSubmit)="f.form.valid && Submit(f)">
    <confirm-password name='password' ngModel #pwd='ngModel'></confirm-password>
    <div>{{pwd.status}}</div>
</form>

confirm-password.html

<div [formGroup]='pass'>
  <label>Password:
    <input type="password" formControlName='pwd1' (blur)='onTouched()'>
  </label>
  <div *ngIf="controlDir.touched && controlDir.control.errors?.required" 
         class="error">Password is required.</div>

  <label>Please re-enter password:
    <input type="password" formControlName='pwd2' (blur)='onTouched()'>
  </label>
  <div class='error' *ngIf='controlDir.touched && controlDir.control.errors?.notEqual && 
                            !controlDir.errors?.required'>Passwords do not match.</div>
</div>

confirm-password.ts

import { Component, Self, OnInit, OnDestroy } from '@angular/core';
import { ControlValueAccessor, AbstractControl, NgControl,
         ValidatorFn, Validators, FormGroup, FormControl } from '@angular/forms';
import { Subscription } from 'rxjs';

@Component({
  selector: 'confirm-password',
  templateUrl: './confirm-password.component.html',
  styleUrls: ['./confirm-password.component.css']
})
export class ConfirmPasswordComponent implements ControlValueAccessor, OnInit, OnDestroy
{
  password: string; // the value of the control should be set to this value
  pass = new FormGroup({
    pwd1: new FormControl(),
    pwd2: new FormControl()
  });
  private onChange: (value: string) => void;
  private onTouched: (value: string) => void;
  private valueChanges: Subscription;

  constructor(@Self() public controlDir: NgControl)
  {
    controlDir.valueAccessor = this;
  }

  ngOnInit()
  {
    const control = this.controlDir.control;

    let myValidators = [
            Validators.required, 
            equalValidator(this.pass.controls.pwd1,
                           this.pass.controls.pwd2)
        ]
    // ovoid overwriting any existing validators
    let validators = control.validator
      ? [control.validator, ...myValidators]
      : [...myValidators];
    control.setValidators(validators);
    control.updateValueAndValidity();
  }

  writeValue(val: any)
  {
    /* whether everything inside of this method is commented out or not 
       doesn't seem to affect anything */
    console.log(val);
    //val && this.pass.setValue(val, {emitEvent: false});
    this.password = val;
  }

  registerOnChange(fn: (val: any) => void)
  {
    this.valueChanges = this.pass.valueChanges.subscribe(fn);
  }

  registerOnTouched(fn: () => void)
  {
    this.onTouched = fn;
  }

  setDisabledState(disabled: boolean)
  {
    disabled ? this.pass.disable() : this.pass.enable();
  }

  ngOnDestroy()
  {
    this.valueChanges.unsubscribe();
  }

}

export function equalValidator(el1, el2): ValidatorFn
{
  return (ctrl: AbstractControl): {[key: string]: any} | null => {
    const notEqual = el1.value !== el2.value;

    return notEqual ?  {"notEqual": true} : null;
  };
}

Answer 1

The purpose of writeValue is to inform your component about changes on the outside / the form control on parent component. What you'll usually want to do with it is to bind the change from outside to a local variable.

In order to inform the outside world about changes inside of your component, you need to call the onChange method. You can call it, like you would usually call an event emitter.

I've put together a couple of examples here: https://www.tsmean.com/articles/angular/angular-control-value-accessor-example/

Answer 2

writeValue is called when you are changing control value (eg by setting backing ngModel property or controll value) not by user input.

https://stackblitz.com/edit/angular-hu2bfs?file=src/app/app.module.ts

I have removed errors caused by misused ngModel

As you can see, if you type into extra input new value and push the button, writeValue is called.

to wrap it up - writeValue is called when controller changes model value, and writeValue purpose is to do required stuff to reflect model changes by custom control.