Angular Material custom form field doesn't show mat-error

Question

I thought I have almost the same example but somehow the control tricks me :-/

<form [formGroup]="form">
    <app-ref-urlcheck [maxLen]="20" formControlName="url"></app-ref-urlcheck>
</form>

and the template looks like

<mat-form-field>
  <input matInput #inUrl="ngModel" [(ngModel)]="value" type="url" [attr.maxlength]="maxLen" [errorStateMatcher]="errorStateMatcher"
    (input)="changeInput(inUrl.value)" [disabled]="isDisabled" [value]="strUrl" 
    placeholder="Homepage" />
  <mat-error>test error</mat-error> <!-- doesn't show up - neither the next -->
  <mat-error *ngIf="(inUrl.touched && inUrl.invalid)">This field is required</mat-error>
</mat-form-field>

and the main content

import { Component, HostListener, Input, OnInit } from '@angular/core';
import { AbstractControl, ControlValueAccessor, FormControl, NgControl, NG_VALIDATORS, NG_VALUE_ACCESSOR, ValidationErrors, Validator } from '@angular/forms';
import { ErrorStateMatcher } from '@angular/material/core';
import { MatFormFieldControl } from '@angular/material/form-field';
import { Observable } from 'rxjs';

@Component({
  selector: 'app-ref-urlcheck',
  templateUrl: './ref-urlcheck.component.html',
  styleUrls: ['./ref-urlcheck.component.scss'],
  providers: [
    {
      provide: NG_VALUE_ACCESSOR,
      multi: true,
      useExisting: RefURLcheckComponent
    },
    {
      provide: NG_VALIDATORS,
      multi: true,
      useExisting: RefURLcheckComponent
    }
  ]
})
export class RefURLcheckComponent implements OnInit, ControlValueAccessor, MatFormFieldControl<any>, Validator {
  @Input() maxLen = 254;
  strUrl: string;

  onChange = (changedUrl) => { };
  onTouched = () => { };
  isDisabled = false;
  touched = false;

  @HostListener('focusin', ['$event.target.value']) onFocusIn;

  constructor() { }
  onContainerClick(event: MouseEvent): void {
    throw new Error('Method not implemented.');
  }
  setDescribedByIds(ids: string[]): void {
    throw new Error('Method not implemented.');
  }
  userAriaDescribedBy?: string;
  autofilled?: boolean;
  controlType?: string;
  errorState: boolean;
  disabled: boolean;
  required: boolean;
  shouldLabelFloat: boolean;
  empty: boolean;
  focused: boolean;
  ngControl: NgControl;
  placeholder: string;
  id: string;
  stateChanges: Observable<void>;
  value: any;

  ngOnInit(): void {
  }

  setDisabledState?(isDisabled: boolean): void {
    this.isDisabled = isDisabled;
  }
  registerOnTouched(onTouched: () => {}): void {
    this.onTouched = onTouched;
  }
  registerOnChange(onChange: (changedValue: string) => {}): void {
    this.onChange = onChange;

    this.onFocusIn = (inputVal) => {
      console.log('focus in', inputVal);
      this.markAsTouched();
    };


  }

  writeValue(value: string): void {
    this.strUrl = value;
  }

  markAsTouched() {
    if (!this.touched) {
      this.onTouched();
      this.touched = true;
    }
  }

  changeInput(inVal: string) {
    this.onChange(inVal);
    this.markAsTouched();
  }



  readonly errorStateMatcher: ErrorStateMatcher = {
    isErrorState: (ctrl: FormControl) => {
      console.log('errorStateMatch...')
      this.errorState = true;
      return (ctrl && ctrl.invalid);
    }
  };

  validate(control: AbstractControl): ValidationErrors | null {
    if (control?.value.length <= 5) {
      this.errorState = true;
      return {
        tooShort: true
      };
    }
    this.errorState = false;
    return null;
  }
}

Same question as in the referred example: How to display <mat-error>? It doesn't even show up anyhow.

Answer 1

Reused the attached code and suspect that the FormControl didn't update with an error when the validation is failed.

When the validation fails, should set the error to FormControl as below:

this.inUrl.control.setErrors({ tooShort: true });

import { ViewChild } from '@angular/core';

export class RefURLcheckComponent
  implements OnInit, ControlValueAccessor, MatFormFieldControl<any>, Validator
{
  @ViewChild('inUrl', { static: true }) inUrl: NgControl;

  ...

  validate(control: AbstractControl): ValidationErrors | null {
    if (control?.value?.length <= 5) {
      this.errorState = true;
      this.inUrl.control.setErrors({ tooShort: true });
      return {
        tooShort: true,
      };
    }

    this.errorState = false;
    this.inUrl.control.setErrors(null);
    return null;
  }
}

Sample Demo on StackBlitz